OZONE.Space Vision

Dr. Fred Ortenberg

OZONE: SPACE VISION (Space monitoring of Earth Atmospheric Ozone)

Haifa, 2002

Ozone: Space Vision (Space monitoring of Earth Atmospheric Ozone)

Copyright © 2002

By Dr. Fred Ortenberg, ASRI, Technion November 2002

Printed in Israel

Design by Graphic Touch, Ltd (Haifa)

Asher Space Research Institute, Technion Technion City, Haifa, 32000, Israel http://www.technion.ac.il/ASRI

FOREWORD

“Space Monitoring of Earth Atmospheric Ozone” was written as part of the activities

of the Asher Space Research Institute (ASRI), Technion, Israel Institute of

Technology, intended for public education. It grew as a result of ASRI involvement in

the development of an ozone-meter on board the Gurwin-Techsat Technion satellite,

launched in July 1998 and still active in space.

The main mission of the ASRI is to advance science, technology and education in all

space-related fields. The ASRI operates with a broad national perspective. It fosters

interdisciplinary work and collaboration between Israeli researchers from all

universities and agencies as well as industry. The ASRI has also established

collaborative projects with other countries.

The ASRI was established in 1984. Its members are professors in several Technion

faculties, and it has a technical staff of scientific experts in a variety of space-related

fields. ASRI is the leading space research center in Israel and is also involved in the

development of space systems based on advanced and innovative technologies.

As part of its activities, ASRI developed a technology demonstration micro satellite

called Gurwin-Techsat. This project covered the conception, design, development,

construction, test, launch and operation of a low Earth orbit satellite carrying a

number of experimental scientific payloads. The purpose of this program was to

demonstrate that smaller, high technology satellites could be rapidly developed and

implemented for a fraction of the cost of heavier and more complicated satellites, and

still be able to perform valuable space research. This project was based on a unique

collaboration between Israel’s leading industrial organizations and ASRI.

Being designed as a multi-purpose platform suitable for space research, Gurwin-

Techsat was equipped with six different devices to carry out on-board scientific

experiments. All of the experiments have been repeatedly run during more than four

years of satellite flight history. Of these six payload devices, the operation of the

ozone-meter was one of the most successful. The ozone-meter was intended for

measuring atmospheric ozone concentration. The ozone-meter flight test has enhanced

the prospect of developing reliable, low-cost instruments for small satellites, necessary

for carrying out such vital tasks as continuous ozone monitoring.

The study of ozone is important because of the effects of UV radiation on humans

(e.g., skin cancer, eye cataracts, and deterioration of the immune system) and on

agricultural systems (e.g., slower plant growth and reduced crop yield). Each spring,

when the Sun rises over the Antarctic, chemical reactions involving man-made

chlorine and bromine compounds occur in the stratosphere and destroy ozone, causing

the "ozone hole". Total recovery of the ozone layer to levels observed before 1980

will take at least 50 years, and expected changes in climate, including a cooler

stratosphere, could delay this process.

Here is the story:

The World Meteorological Organization in Scientific Assessment of Ozone Depletion

published in 1998 a truly "global" document, reflecting the thinking of the

international scientific community. This document discussed the general questions

most frequently asked by students, the general public, and leaders in industry and

government:

How can chlorofluorocarbons (CFCs) get to the stratosphere if they're heavier than

air?

What is the evidence that stratospheric ozone is destroyed by chlorine and bromine?

Does most of the chlorine in the stratosphere come from human or natural sources?

Can natural changes such as the Sun's output and volcanic eruptions be responsible for

the observed changes in ozone?

When did the Antarctic ozone hole first appear?

Why has an ozone hole appeared over Antarctica when CFCs and Halons are released

mainly in the Northern Hemisphere?

Is there an ozone hole over the Arctic?

Is the depletion of the ozone layer leading to an increase in ground-level ultraviolet

radiation?

Does ozone depletion cause climate change?

How severe is the ozone depletion now?

Is the ozone layer expected to recover? If so, when?

In this book you can find answers to all the above questions. This book focuses on the

observation of the stratospheric ozone layer from space, and humanity's attempts to

protect it. A study of ozone variations in the Earth’s atmosphere is a very complicated

dynamic phenomenon. Methods of ozone monitoring from space are also very

complex. This book tries to present this very important and multifaceted technical

subject as simply as possible, with minimal formulae and without recourse to

complicated mathematics.

Funding of ASRI’s research and educational activities is partly met by different

national and international organizations, but it is the Asher family’s support that

provides the principal source of ASRI’s financial stability, including the publication of

this book. We wish to express our thanks to the Asher family whose commitment has

enabled the advancement of space research in Israel on a firm basis.

Professor Moshe Guelman

Head, Asher Space Research Institute,

Technion, Israel Institute of Technology

Haifa, September 2002

Content

Introduction ...............................................................................................5

1. General Properties of Ozone .............................................................7

2. Optical Properties of Ozone ........................................................... 12

3. Ozone in the Atmosphere ................................................................ 16

4. Ozone Control, Physical Concepts and Methods ......................... 23

5. Solar Backscattering Ultraviolet Method ..................................... 31

6. The Emission Method ...................................................................... 39

7. Limb Methods .................................................................................. 44

8. Lidar Sounding of Atmospheric Ozone ......................................... 51

9. Monitoring Instruments Present State and Trends ..................... 53

10. The Antarctic Ozone Anomaly ....................................................... 62

11. Conclusion ........................................................................................ 73

12. A Certain Philosophical Epilogue .................................................. 75

13. Timeline of Atmosphere Ozone History ........................................ 78

Ozone Devoted References and Internet Web Sates .......................... 80

Glossary of Ozone and Space related terms ....................................... 81

Introduction

The outcome of ozone depletion is one of the disasters facing humanity, alongside

such threats as drastic changes in the world climate, exhaustion of soil and water

resources, progressing deforestization and the uncontrolled expansion of deserts.

Possibly, the destruction of the ozone layer over the Antarctic is a precursor to

rigorous changes of the ozonosphere on a global scale.

Scientists, worldwide, are involved in studies of the complex processes going on in

atmospheric ozone. Theoretical and experimental research of the ozonosphere is being

conducted on an unprecedented scale. However, the problem is far from being

unravelled; many important happenings related to the enigma are yet waiting to be

resolved, especially those concerning the impact of natural and anthropogenic issues

on the precious ozone layer. Consistent monitoring of the environment on a broad

basis is an essential requisite for exhaustive analysis and far-reaching conclusions.

Scientific research leading to credible forecasts of changes in the Earth's ozonosphere,

both on a global and local scales, necessitates regular measurements of ozone

concentrations and other characteristics with the help of existing devices, as well as

new and emerging methods and means of ozone observation.

Atmospheric ozone is a product of dynamic equilibrium of numerous processes going

on both in the atmosphere and beyond; that is why optical methods for remote sensing

in the different layers of the atmosphere are becoming increasingly effective. Space

based systems for the control of the ozone layer in the Earth's atmosphere have been

developed and Research and Development in this area is continuing in many

countries. The main objective of space monitoring is the quantitative determination of

ozone in specified layers of the ozonosphere on a global scale, providing periodicity,

precision and special resolution unattainable by other means.

How are space-based ozonometric devices equipped and arranged?

What space methods are used for measuring ozone concentrations and what are their

advantages over other methods?

What perspectives are envisioned in the improvement of space-based apparatus?

What impact on the whole endeavour has been brought about by satellite data

acquisition and analysis?

How has our vision of the Earth's atmosphere changed as a result of ozone space

research program implementation?

What results have been obtained in this area during the last few years?

How are Israeli scientists involved in operative ozonosphere monitoring?

This is the range of questions that will be answered in this book. However, before

addressing sophisticated problems of global ozone monitoring with the help of space

apparatus we will introduce some basic conceptions of physics, chemistry,

meteorology and atmospheric optics, essential to the understanding of the phenomena.

In the first chapters of the book, chemical and physical properties of ozone and basics

of ozone spectroscopy will be discussed. After that, we will characterize our

knowledge of the ozonosphere and relevant methods of research.

This interdisciplinary book has been written for students of the aerospace faculty,

physics, chemistry, electrical and civil engineering as well as for readers interested in

problem of ecology, space exploration, and science progress. The publication is

intended for public education, its main purpose is raising the level of public awareness

of issues dealing with global environmental changes. It includes an advanced insight

into the Earth's ozonosphere. Satellite research methods of atmospheric ozone are

outlined; basic principles and architecture of remote probing apparatus are described.

This publication could become a basis for designing a course of lectures for students

of different faculties associated with state-of-the-art achievements in space research

and related high-tech disciplines.

First of all, the author would like to appreciate the continual and valuable support of

Prof. M. Guelman during the preparation of this book. The author also would like to

thank Drs. A. Shiryaev, A. Livne and O. Tublin for their assistance with the editing,

proofreading and formatting of the book, as well as A. Volfovsky, B. Kimelman and

D. Rosenberg for the graphics.

1. General Properties of Ozone

The principal player in our story is ozone. A molecule of ozone O3 is a relatively

stable combination comprising three atoms of oxygen. It is well known that the

specific smell of air after a thunderstorm is explained by the presence of ozone in the

atmosphere. This peculiar smell has been mentioned in many works of literature,

beginning from ancient Iliad. The German chemist Christian Frederick Shenbein who

is said to have discovered ozone in 1840 coined the new substance "ozone" (ozone-

smelling). Later it was shown that ozone is a modification of oxygen, and that the

ozone molecule is actually a trivalent atom (Fig. 1).

Fig. 1. Molecule Ozone Structure

At room temperature ozone is a gas of a light-blue color; at lower temperatures it

transforms into a liquid of indigo-light blue color, and has a boiling point of 119.9°C;

in the solid state, ozone forms needle-like crystals of O3 and molecular oxygen O2; to

coexist in all three states of matter is one of its exceptional features. Pure ozone in all

these three states is of an explosive nature.

A molecule of ozone is nonlinear and has a triangular structure with an obtuse angle at

the apex and equal inter-nuclei distances (Fig. 1). The process of ozone formation

from oxygen can be depicted in the following way:

Exothermic reaction

2 O3 = 3 O2 + 68 Kcal

→

←

Endothermic reaction

The formation of ozone is accompanied by heat absorption, whereas decomposition is

associated with release of heat. At normal temperature and pressure the reaction

proceeds very slowly. This is due to the important role played by atomic oxygen in the

reaction:

O2 + O + M = O3 + M, (1)

where M is any particle necessary for the removal of energy from the molecule of

ozone being formed.

At high temperatures ozone disintegrates, the equilibrium of the reaction shifts to the

left in the direction of high concentration atomic oxygen formation; at low

temperatures the equilibrium shifts to the right in the direction of ozone formation;

however, atomic oxygen concentration at such temperatures is low, and, therefore,

again, there is no output of ozone. This is why the most favorable conditions for the

formation of ozone are relatively low temperatures and the presence of an additional

quantity of unstable atomic oxygen. The source of such oxygen could be the

dissociation of oxygen molecules due to the impact of a particle stream,

electromagnetic radiation, electric discharge, etc. Essentially these principles underlie

the operation of ozonizer, used to produce ozone for practical purposes. For example,

the electrosynthesis of ozone in a barrier discharge is based on the dissociation of

oxygen molecules under the influence of electric energy discharge in an electric gap.

Atomic oxygen formed in the course of such dissociation, combines with an oxygen

molecule in the presence of any particle (oxygen, nitrogen) and is converted into

ozone, which, in its turn, reacts with oxygen atoms and is converted into molecular

oxygen. This establishes mobile equilibrium in the formation and disintegration of

ozone, limiting the yield of ozone in such ozonizers to 5-7%. We should note that this

description of ozone synthesis is of a schematic character. In reality, obtaining ozone

is accompanied by a number of additional chemical processes, and depends on such

factors as temperature, humidity, supply rate of oxygen or air, as well as specific

features of the apparatus being used.

A molecule of ozone is essentially stable, i.e. it does not dissociate on its own. Low

concentration ozone, void of impurities, dissociates relatively slowly. However, as

temperature rises and the quantity of admixed gases increases (e.g. NO, Cl2, Br2, I2

and others), under the influence of radiation and particle flow, ozone dissociation

speed substantially increases. Thus ozone is unstable at the presence of admixed

gases, and this is one of its main properties. It is also a very strong oxidant (second to

F). Its high activity as an oxidizer and its capacity to react with many substances

greatly increases its applicability. In addition, it has a number of beneficial properties

as a disinfectant and odorizing agent. The following applications of ozone are well

established; purification and disinfections of drinking water, industrial water, and

drainage, decolarization, neutralization of harmful and toxic substances, eliminating

unpleasant odors, cleansing industrial exhausts, ozonizing in air conditioners,

processing and storing foodstuffs and forage, sterilizing bandaging materials, as well

as in therapy and disease prophylaxis.

Ozone in the atmosphere is generated mainly by processes

accompanying the absorption of light: the photochemical

reaction bringing about the formation of ozone consists of a

sequence of happenings, beginning with the absorption of

light by a molecule of oxygen and ending with the

formation of stable molecules. This comprises primary and secondary events. The

primary events include the initial act of light absorption by a molecule, bringing it to a

state of excitation, followed by its destruction; and thus the end products of the

primary events are two atoms of oxygen. Atoms and molecules are known to exist

only in specific energetic states determined by laws of quantum mechanics. Thus, an

atom of oxygen can exist in states designated 3P, 1D, 1S, where O(3P) is the normal

state of the atom, whereas O(1D) and O(1S) are the excited states. The energy bonding

atoms in an oxygen molecule comprises 5.115 eV. To "cleave" an oxygen molecule, a

light quantum is necessary which must have energy equal to the bonding energy of

atoms in a molecule. While absorbing such a quantum, an oxygen molecule

dissociates into two normal atoms. Under the influence of light having a lower

wavelength (respectively larger quantum energy) the molecule O2 will be dissociated

into excited oxygen atoms. The threshold wavelengths of the radiation absorbed in the

course of molecular oxygen dissociation, can be presented as:

O2→ O(3P) + O(3P) - 242.4 nm,

O2→ O(3P) + O(1D) - 175.0 nm,

O2→ O(3P) + O(1S) - 133.2 nm.

Thus, irradiation of gaseous oxygen by ultraviolet radiation, can generate substantial

quantities of highly concentrated atomic oxygen. In addition to the appearance of

oxygen atoms, as a result of irradiation, there also emerge excited molecules of

oxygen. All these active particles take part in secondary events (analogous to reactions

described by formula (1) producing the end product-ozone). The principle described is

effected in photochemical ozonators, in which the dissociation of oxygen is brought

about by ultraviolet radiation, generated by a special discharge lamp. Due to the

reversible character of reaction (1), formation of ozone runs alongside with its

destruction. Ozone concentration reached in such ozonizers, does not exceed 1-2% in

volume. We should remind the reader that the amount of ozone in the atmosphere is

extremely low ("traces" in volumetric estimates 10-6-10-5 %).

In this book we will frequently deal with values showing ozone concentration in

mixtures of gases. In addition to the volumetric percentage mentioned above,

atmospheric chemistry employs the term volumetric concentration expressed as the

number of parts per million (ppm), which is equivalent to 1cm3 of ozone in 1m3 of air.

The partial pressure of a gas depends on the fraction of this gas in a mixture of gases.

Thus, on the Earth's surface where the pressure is 1 ATM, an ozone concentration of

1ppm corresponds to a pressure of 10-6 ATM. The usage of volumetric units is really

handy because there is a direct connection between the volume (or partial pressure of

ozone) and the number of its molecules; for instance, if the amount of ozone in the air

constitutes 1ppm, then, on an average, every millionth molecule would be a molecule

of ozone. When the amount of gas in the air is minute, it is appropriate to use the

number of molecules in a unit of volume for defining concentration. In normal

conditions the number of molecules in 1cm3 of air is 2.69 . 1019. An ozone

concentration equal to 1ppm would be equivalent to 2.69 . 10 13 cm-3. Sometimes

ozone concentration is described in units of mass (micrograms) per unit of air volume

(usually, microgram/ m3). For translating millionths of a part into ozone density, one

can use the following relationship: 1ppm = 2140 µg/m3. The term "ozone density" is

also used to denote the thickness of the ozone layer contained in a 1km-thick layer of

the atmosphere referred to normal pressure and temperature.

In addition to the above-mentioned unit, the term "total column amount" is widely

used in metrology. Total ozone X denotes the amount of ozone in a vertical column of

the atmosphere; it is numerically equal to the thickness of the ozone-gas layer in this

column in normal conditions, and is expressed in ATM⋅cm. The value X=10-3

ATM⋅cm. is often called the Dobson unit after the English scientist who conducted

research in the field of atmospheric ozone and in 1924 created one of the first optical

devices for measuring Total Ozone (TO). At this point it is appropriate to emphasize

the very small content of ozone in the atmosphere. If all Earth's atmospheric ozone

were concentrated in a single layer on the Earth's surface, the thickness of such a layer

of pure ozone would be only 3mm. At the same time, the total ozone mass in the

atmosphere is 3.109 ton.

Having revealed some basic properties of such an exotic substance as ozone, we will

mention its toxic impact on humans, animals and vegetation. Not going into details,

we will just point out that at certain concentrations, ozone is capable of poisoning a

human being, causing death of experimental mice, birds; it has a destructive impact on

forests and plants even at low concentrations observed in natural surroundings. This

"bad ozone" is a powerful photochemical oxidant that damages rubber, plastic, and all

plant and animal life. It also reacts with hydrocarbons from automobile exhaust and

evaporated gasoline to form secondary organic pollutant such as aldehydes and

ketones. Nevertheless, the term ozonizing is often referred to as a beneficial process

for purification or refreshing.

2. Optical Properties of Ozone

The energy of a molecule can be depicted as the sum of three parts - electronic,

vibrational and rotational energy. It is well known that energy states change in a

discrete manner. Sets of energetic states are of a specific (individual) nature for every

molecule. Transitions of molecules from a certain energy state to another are

accompanied by the absorption or radiation of a quantum of electromagnetic energy.

Spectra, originating with such transitions, depend on the molecular constants of the

radiating or absorbing molecule and are a sort of visiting card ("signature") of the

specific molecule. In ozone, transitions from one electronic state to another occur on

instants of radiation or absorption of light in the visible, ultraviolet (UV) and so-called

vacuum UV (lower than 200.0nm) ranges of the spectrum. Every electronic transition

is accompanied by relatively small changes in the energy of vibrational-rotational

states of the molecule; due to this, the electronic, vibrational, and rotational spectrum

of the molecule is essentially a system of bands closely spaced out. If a molecule, as a

result of light absorption, reaches an excited state, possessing adequate energy to

destroy the weak link in a molecule, the latter will dissociate. In an ozone molecule

the energy of the (O-O2) bond is equal to 1.05 eV; the rupture of this bond leads to the

disintegration of ozone into molecular and atomic oxygen. The most important ozone

absorption bands are located in the 200.0-300.0 nm wavelength range (Fig. 2). The

capacity of a gas to absorb light is characterized quantitatively by the coefficient of

absorption k(ν) in the Lambert - Beer law

I(ν, x) = I (ν, 0) • 10 - k(ν) x (2)

Here, I (ν, 0) is the intensity of a monochromatic beam, of a frequency ν, entering the

window of a vessel of length x, filled with gas at a given pressure; I(ν, x)- the

intensity of light after passing through the gas in the vessel. Sometimes the base e of a

natural logarithm is used instead of the figure 10 in (2). The value of the coefficient of

absorption k(ν) measured in cm-1 in Hartley's bands of absorption for ozone is

calculated according to formula (3); this is shown in Fig. 2 as a dependence of the

coefficient k(ν) on the incident radiation wavelength. Just as many other bands of

absorption in molecular spectroscopy, these bands are named after scientists who

discovered them. From formula (2) we can see that in the point of maximum

absorption k=135 cm-1, at a thickness of the ozone layer equal to 0.3cm, the factor

I(ν, 0)/I(ν, x) would be equal to 10.40 ! This means that the Earth's layer of ozone will

weaken the incident radiation on this wavelength 1040 times; i.e. it will absorb

practically all of it.

Fig. 2. Absorption in Ozone bands

At wavelengths longer than 300.0 nm adjacent to Hartley's bands, weaker absorption

bands of Haggins and Shalon-Lefevr can also be observed (Fig. 2). Coefficients of

absorption in these bands are several orders of magnitude lower than in Hartley's

bands. Some closely spaced bands in these systems have easily discernible sharp

maximums and minimums. Further, in the visible part of the spectrum, there is

Shappuis'es broadband, with which the blue color of ozone is associated. Very strong

absorption of ozone can also be observed in the range of vacuum ultraviolet (100.0-

200.0 nm). Together with the absorption in Hartley's bands, this absorption brings to

an abrupt end the Sun's spectrum on the Earth's surface at wavelengths less than 290.0

nm; this is very important in terms of protecting life on our planet from short wave

irradiation. It should be noted that values of coefficients of absorption change

substantially with temperature (the coefficients of absorption referred to in the graphs

were measured at 0°C).

The bands relevant to the vibrational-rotational transitions in an ozone molecule are

located in the infrared part of the spectrum (3-15µm). Absorption coefficients in these

bands vary greatly. There are several bands of high absorption (4.75; 9.57 and

14.2µm) among which, the λ=9.57µm band is the most interesting one, consisting of a

number of closely spaced spectral lines. This band is situated in a "window" free of

absorption by water vapor and carbon dioxide and, therefore, its role in atmospheric

dynamics is exceptionally important. Furthermore, rotational ozone spectra, just as in

most cases of nonlinear polyatomic molecules, are observed in the microwave spectral

band (1-10cm).

Such a detailed description of the absorption spectra is required for the two following

reasons: first of all, investigation of absorption spectra not only allows the

identification of a specific gas in a mixture of gases, but also the means for measuring

quantitative values. That is why this method is widely used in ozonometry and will be

described in the following sections; the second reason is that when light is absorbed

by ozone, there ensues a chemical transformation, and chemical changes can be

brought about only by light absorbed by a molecule.

The initial stage of photochemical reaction is molecule dissociation. The end products

of the photochemical reaction can differ with the absorption bands in which

photodissociation takes place. Specifically, when ozone is disintegrated by light into

molecular and atomic oxygen, an atom and molecule of oxygen can be found to be

both in ground and excited states, depending on the energy of the absorbed quantum

(wavelength of the absorbed light). We have already mentioned quantum states of an

oxygen atom 3P, 1D, 1S. Analogous symbols are used in molecular spectroscopy to

designate electronic energy states of an oxygen molecule in the ground state (3Σ-g) and

excited states (1∆g, 1Σ+g, 3Σ+

u, 3Σ-u). To understand the secondary processes occurring

after the decomposition of ozone into an atom and molecule of oxygen, it is very

important to know in what energy states they appear as a result of the initial

absorption of light.

The reactive capacity of atoms and molecules in an excited state is substantially

different from their reactive capacity in the ground state. As an example, we will

consider the process of photodissociation in the excited state relevant to the Shappui

bands. When red light (~600.0nm) is absorbed, the initial process ends up in the

decomposition of ozone into an atom and molecule of ozone in ground states:

O3 → O(3P) + O2 (3Σ)

A secondary process of atomic oxygen and ozone interaction follows this

O(3P) + O3 → 2 O2 Consequently, the absorption of one light quantum led to the destruction of two ozone

molecules. In this case, it is said that in the Shappui bands, the quantum yield of ozone

(O3) disintegration is equal to 2. One of the products of photo-disintegration, as a

result of excitation in the Haggins-bands, is an excited oxygen molecule, e.g. O2(1Σ):

O3 → O(3P) + O2 (1Σ),

followed by the processes

O(3P) + O3 → 2 O2

O2 (1Σ) + O3 → 2 O2 + O(3P)

O(3P) + O3 → 2 O2

In accordance with this mechanism, the total yield of O3-disintegration is equal to 4.

In the same way, it can be shown that the quantum yield of ozone - photo dissociation

in the case of excitation in Hartley's bands can be equal to 6.

Two important conclusions can be made on the basis of the above-mentioned

interaction of light and ozone: 1) molecules of ozone absorb light in a broad band -

from vacuum ultraviolet up to the microwave frequencies, the most intensive

absorption taking place on wavelengths shorter than 300.0nm; 2) when ozone absorbs

light in either the ultraviolet or the visible light bands of the spectrum, a molecule of

oxygen is created as a result of the photochemical destruction of ozone.

3. Ozone in the Atmosphere

Due to its peculiar properties, atmospheric ozone is a regulator of radiated energy flow

reaching the Earth's surface. The evolution of ozone on Earth can be described in the

following way. Conversion of methane, water and ammonia in the initial atmosphere

of the Earth into a "broth" of organic compounds, in which life originated, occurred in

the presence of intensive ultraviolet radiation. However, the latter is very dangerous to

the sensitive equilibrium of chemical reactions in live cells, and probably, the first

organisms survived only because they evolved under a layer of water, thick enough to

protect them from ultraviolet light. As a result of photosynthetic disintegration of the

water molecule, the Earth's atmosphere obtained its free oxygen. Only after the advent

of oxygen and ozone in succession, did the intensity of ultraviolet radiation at the

Earth's surface decrease to a level, allowing live organisms to abandon the protection

provided by water, and to begin settling on land. Prolonged existence of life on land

became possible, thanks to the ozone layer, a guardian, which in itself, was a product

of life.

Fig. 3. Vertical structure of Earth atmosphere and ozonosphere

In the process of evolution, oxygen and nitrogen became the basic components of the

atmosphere. Fig. 3 depicts the Earth's atmosphere profile. One can see that pressure

diminishes with height smoothly and monotonously, and does not display any specific

structure. At the same time, interaction of radiation with atmospheric matter brings

about the development of a substantially complex thermal configuration. In terms of

temperature changes with height, the atmosphere can be considered as divided into

several layers. The area of the atmosphere adjacent to the Earth's surface (the

troposphere) is characterized by a lowering of the air's temperature with height equal

to 6.5°C/km. In the next layer (the stratosphere) the temperature somewhat increases

(approximately by 1°C per km) due to the absorption by ozone of ultraviolet (UV)

solar radiation. In the mesosphere, the temperature consistently decreases with height

(2-3°C/km). Higher up stretches the thermosphere, in which air temperature again

increases with height, due to the absorption of the short-wave UV solar radiation by

molecular oxygen, accompanied by dissociation of the latter. The boundaries between

the mentioned layers are called tropo-, strato- and mezo-pauses. Proportions of gas

components in the atmosphere are different in different layers. The mean dependence

of partial ozone pressure on the height above the tropics is depicted in Fig. 3 (bold

line). From the graph we can see that ozone distribution looks like a two-layered pie,

corresponding to two layers of the atmosphere. The concentration of ozone in the

troposphere is low (1-4mPa) and its spread with altitude increases relatively smoothly,

but in the stratosphere it grows sharply, reaching a peak value, and then rapidly

decreasing. Usually, when the height of the ozone layer is mentioned, it refers to the

zone of its maximum concentration. The height of the ozone layer depends on the

latitude of the locality and of the season. The limiting positions of ozone layer on the

Equator and the Pole, and the seasonal layer maximal and minimal positions from

summer to winter are shown in Fig. 3 by two lower dotted lines.

Here, one can also see the height of ozone concentration maximum in the polar

region. Characteristic of the vertical Ozone Profile (OP) in the atmosphere is its

instability with time. Because the build-up of ozone proceeds mainly as a result of

photochemical reactions in the stratosphere, the mass of ozone is concentrated in the

latter (about 85-89% of the Total Ozone (TO) in the atmosphere). The layer of

enhanced concentration in the stratosphere serves as a shield, preventing the

ultraviolet spectral "wing" of the Sun's radiation to reach the Earth's surface. TO

content in a column of atmospheric air varies greatly with latitude. Thus, e.g., the TO

content above the Earth's polar regions is approximately twice as large as over the

equator. In addition, TO undergoes daily, seasonal, perennial and long-term

variations; the latter are associated with the cyclic nature of solar activity. The

thickness of the ozone layer, observed in nature, varies substantially: from 70 to 760

Dobson Units. Variations of ozone concentration round the clock could amount to

25% of its average value. Both TO and OP are determined by the concentration of

ozone in each layer of the atmosphere and depend on the process intensity of build-up

and destruction.

The above described mechanism of ozone formation and destruction was based on the

assumption that the process is initiated via the absorption of UV radiation by oxygen,

whereas ozone destruction occurs under the influence of sunlight in the visible and

UV spectral ranges, as a result of collisions with oxygen atoms. Computation for such

an (oxygen) atmosphere showed vertical profiles much the same as those in reality,

although actual concentration proved to be substantially higher. It was revealed that in

the stratosphere approximately 80% of the ozone build-up in sunlight is eventually

destroyed through mechanisms, taking into account interaction of ozone with many of

the "small" atmospheric components. Hundreds of reactions between ozone and the

other gases- components of the atmosphere were investigated; it was shown that a

major role in the destruction of ozone (in the catalytic cycle) is played by nitrogen

oxide:

NO + O3 → NO2 + O2,

NO2 + O → NO + O2.

Such reactions can be displayed for other substances such as clorine:

Cl + O3 → ClO + O2

ClO + O → Cl + O2

The combined effect of each pair of reactions brings about the disappearance of ozone

and atomic oxygen, whereas nitrogen oxide and atomic oxygen are consistently

reduced, i.e. each molecule or atom of these substances is responsible for the

destruction of a large number of ozone molecules. When the above mentioned

reactions are included in the ozone layer model, the calculated values become

substantially more realistic. Now we understand why implantation of contaminating

substances into the stratosphere is considered to be so significant. Actually, a single

molecule of a contaminating substance can initiate a sequence of reactions, bringing

about the dissipation of many ozone molecules.

At this point it seems appropriate to answer the question: in essence, why is the ozone

shield so vital? It is well known that ultraviolet radiation in small doses increases the

generation of vitamin D in humans and animals, thereby enhancing assimilation of

phosphorus, and bone formation. Medical and prophylactic effects of ultraviolet

radiation are well known. Life on Earth has adjusted itself to solar radiation,

transparent to ozone (~290.0nm), and is very sensitive to shorter wavelengths. The

depletion of the ozone layer leads to an increase of UV radiation reaching the Earth's

surface, and a change in the vertical profile of ozone alters atmosphere heating and,

consequently, the climate. UV radiation decomposes the chromatin of the cell nucleus

and deters cell reproduction; it also damages the DNA molecule that contains genetic

code. Superfluous UV, associated with ozone layer depletion can bring about growth

of skin cancer and could decrease the effectiveness of the human immune system.

Even a small decrease in the total thickness of the ozone shield (e.g. for inhabitants of

mountainous areas) would substantially increase the probability of these diseases; a

two-fold thickness decrease would be threatening to the Earth's genofund.

Higher doses of UV radiation have a direct impact on the health of people and lead to

the increase of infectious diseases, impairment of eyes (e.g. cataract), probability

increase of skin cancer. UV radiation limits the growth of some plants; larger doses of

radiation can also prompt the lowering of agricultural output. UV radiation also has a

negative impact on water organisms, specifically on phyto- and zooplankton, fish roe.

Radiation also affects non-biological objects: it causes destruction of many sorts of

plastic materials, enhances dangers in the aftermath of air pollution in cities and

industrial regions, etc. At the same time, an increase of ozone concentration has been

distinctly observed at heights of up to 10 km, especially above industrial locations of

Europe and the USA. We have already mentioned the toxicological impact of

increased ozone content on the biosphere. A negative effect of ozone on humans has

been observed even in the stratosphere, during flights of modern aircraft. At specific

altitudes, concentrated ozone, detrimental to the health of the crew and passengers, is

pumped into the cabin together with stratospheric air.

To assess possible scales of ozonosphere distortion, we will consider the sources from

which substances, catalyzing the destruction of ozone, enter the atmosphere. The main

source of nitrogen oxide (NO) is N2O, which forms in the course of bacterial

processes on the Earth's surface. Gradually penetrating the stratosphere, nitrogen

dioxide reacts with atomic oxygen (which appears during ozone photolysis, or, even

higher - during molecular oxygen photolysis) with the formation of NO. Another

direct source of nitrogen oxides in the stratosphere is high-flying aircraft. . Several

researchers suspected that the reactive nitrogen compounds from the supersonic

transport exhaust might accelerate the natural chemical destruction of ozone, causing

ozone levels to drop.

The main natural source of chlorine in the stratosphere is methyl chloride formed from

algae, which provides but a part of all the chloride currently transported through the

tropopause. In addition to methyl chloride, there are other natural sources of chlorine,

transported into the stratosphere-nitrous acid in volcanic emissions, chlorides in sea

compounds. Anthropogenic sources include perchlorate in hard fuels used in rockets.

Nevertheless, in comparison with the chlorofluorocarbons (CFC), all these sources

contribute a relatively small share.

At the beginning of the 70s it was decisively established that CFC's used in

refrigerators and aerosol containers became a substantial constituent of the

atmosphere. There is no effective mechanism of destroying these extremely stable

substances in the lower atmosphere, and they are transferred into the stratosphere. The

most important of chlorofluorocarbons-freons, CFCl, CF2Cl2, etc.- absorb ultraviolet

radiation; as a result, photochemical reactions with the release of Cl are effected.

Without the breakdown of manufactured chlorofluorocarbons, there would be almost

no chlorine in the stratosphere. CFC-12 concentrations were less than 100 part per

trillion by volume when they were first measured in the 1960s. Between 1975 and

1987, concentrations more than doubled from less than 200 parts per trillion by

volume to more than 400 parts per trillion by volume. The amount of chlorine in

stratosphere increased by a factor of 2 to3.

In parallel with the catalytic destruction of ozone, NO and Cl take part in reactions

with molecules and radicals containing oxygen and hydrogen; the end products of

these reactions (specifically HNO3, HCl) are conveyed into the troposphere and then

flushed by rain. If these reactions providing an effective exit of NO and Cl out of the

atmosphere, are not taken into account in model computations, the impact of nitrogen

oxide and chlorine on the ozone layer could be overestimated. The fact that ozone

emerges as a result of photochemical processes implies that its build-up depends on

the intensivity of sunlight. Many instances were observed when variations in TO

correlated with the Sun's activity. As a result of the latter, there could also be an

increase in the amount of nitrogen oxides, leading to the decrease of ozone.

Seasonal variations of ozone concentration brought about by changes of atmosphere

circulation are extensive in the higher altitudes (Fig. 3). Natural temporary variations

could be quite large in comparison with values brought about by expected

anthropogenic changes. For instance, at an average global content of ozone in the

Earth's atmosphere equal to 297 Dobson's Units (D.U.), average monthly values of TO

are subject to three-fold seasonal and territorial variations during the year. All these

factors up till now did not allow the registration or reliable assessment of the impact

of human activities on the ozone layer, although there is no doubt that this influence is

substantial. The first evaluations were based on a simplified model, which includes

only several reactions. More complicated models are being developed, and there is a

growing feeling that present scales of emissions do not justify the credibility of those

preliminary assessments, which predict a catastrophic outcome.

Nevertheless, the ozone layer may prove to be extremely sensitive to different

influences. Large amounts of nitrogen oxides could enter the atmosphere when the

Earth passes through meteoric showers. It has been calculated that the Tungus

meteorite which landed in Siberia in 1908 brought about the cumulation of 30 million

tons of nitrogen oxide at an altitude of 10-100km, and this is about 5 times as much as

the total amount of nitrogen in all the stratosphere. Such a huge amount of nitrogen

oxides affected the ozone layer; this was substantiated by the decrease of atmosphere

transparency to radiation in the UV range, which during 1909-1911slowly returned to

its initial state. An explosion of supernovae, close to the solar system, could also

cause a similar effect.

Nuclear explosions also pose a threat to the ozone layer. A small decrease of ozone in

the beginning of the sixties was caused by an increase of nitrogen oxides, due to

armament testing. After nuclear tests came to an end, TO again increased. This proved

that nuclear explosions are capable of influencing TO through photochemical

reactions of the nitrogen cycle, leading to the destruction of ozone.

4. Ozone Control, Physical Concepts and Methods

A valid question evolves: how was the information about the ozonosphere, its

structure and variability obtained? What methods and means can be used to determine

ozone concentration in the "thick" of the atmosphere in different geographical regions

of our globe? Clearly, the laws and patterns of ozone distribution in the atmosphere

are not just fruits of scientists' imagination; they are the product of countless

measurements of ozone content. Such measurements have been conducted

systematically beginning from the thirties, both on the ground, and with the help of

equipment positioned on high-flying airplanes, scientific rockets and balloons.

Measuring methods, in terms of instrumentation interaction with atmospheric ozone,

can be divided into three groups:

♦ taking air samples from specific parts of the atmosphere and their successive

laboratory analysis;

♦ contact measuring, whereby the instrument interacts with the air in situ and

during the metrical process;

♦ remote methods, based on measurement interpretation of different parameters,

characterizing electromagnetic radiation caused by the presence of ozone in

the atmosphere.

The development of associated apparatus and its application in scientific research

grew into an independent area of scientific knowledge - ozonometry. We will not

overburden the reader with the description of devices, meant for ozone control from

earth, and in airborne, rocket, and balloon conditions, keeping in mind, too, that the

reader will get acquainted with similar space-based equipment. To give the reader an

idea about the scale of ozone control, we will just note that the world net of ground-

based ozonometric stations alone, a decade ago, exceeded seventy. Apparatus based

on the latter provides a large part of climatological information about atmospheric

ozone. Remote methods having high selectivity, sensitivity, and precision are

preferred when mass measurements of ozone are necessary for providing instant

global depiction of ozonosphere conditions. The practicality of installing optical

instrumentation aboard satellites gave new impetus to remote methods. Measurements

in space began in August 1967 on board the OGO-4 satellite and were followed in the

USA on Nimbus-3,-4,-6,-7, Tiros-4, Atmospheric Explore-5, and on board former

Soviet Union satellites of the Meteor and Cosmos series, as well as orbital stations. A

large selection of ozonometric devices was developed and tested for the acquisition of

data relevant to the time- and space- state of the ozonosphere, over vast territories,

with good periodicity.

We will examine, in more detail, remote methods of probing with the help of

satellites. These methods are of the passive and active types. Passive methods are used

on board satellites for measurements of the spectral distribution of outgoing

electromagnetic radiation of the Earth, so as to determine the composition of the latter,

on the basis of the data obtained. Active methods are applied for determining

atmospheric parameters, using spectroscopic measurements of electromagnetic

radiation transmitted from a satellite and reflected by the atmosphere. To understand

the processes going on in the atmosphere, we shall analyze the interaction of

electromagnetic radiation with the substance, in an arbitrary uniform layer of

thickness l. Let the lower-lying layers' radiation reach the boundary of the above-

mentioned layer with intensity I (ν, 0).

The absorption spectrum, i.e. dark lines or stripes against a bright backdrop of

incoming radiation is observed in rays, outgoing from the layer, if the decrease of light

flux because of absorption is more than the contribution of the layer's own thermal

radiation (spontaneous and compelled). If medium domination prevails, then the

emission spectrum will be observed, i.e. bright stripes and lines on a weaker backdrop

of incoming radiation. The character of ray propagation in a medium largely depends

on the optical thickness along the distance passed by the ray l, generally defined as

τ (ν, l) = ∫ k(ν, x) dx,

where k(ν, x) -volumetric spectral absorption coefficient of the medium. For a

homogenous medium τ (ν) = k(ν) l. If k(ν) l ≤ 1, then the medium is considered

optically thin; if k(ν) l > 1, the medium is said to be optically thick.

In the case of cold medium (no thermal radiation of layer molecules) radiation

propagation in a layer follows the previously mentioned Beer-Lambert Law. For an

optically thin medium and a continuous external source spectrum, the spectral

absorption coefficient graph of a substance is "reproduced" without distortion in the

spectrum of radiation after the latter has penetrated the layer. Moreover, the decrease

of radiation intensity is proportional to the optical thickness, and, therefore, to the

number of absorbing molecules in the layer. The maximum in the absorption band

becomes less evident with the increase of optical thickness. A similar situation

evolves in the case of a hot (self-radiating) medium, when no radiation from an

external source enters the layer. If the medium is optically thin in the frequency range

of interest, then the intensity of the radiation observed will be characterized by a

distribution, proportional to the spectral form of radiating component lines. As the

optical thickness of the hot medium grows, so the spectral distribution of the radiation

being observed begins to deviate from the spectral form of the radiation component

line, and, expanding, reaches saturation. Because absorption lines are usually

"stronger" than lines of excitation, and are prone to easier observation, absorption

spectra are used in remote probing of atmospheric components (including the

"smaller" ones).

The Earth's atmosphere can be envisaged as consisting of layers having approximately

the same density of matter, and insignificantly changing temperature and pressure with

layer height. Transport of radiation from the lower layers to the upper ones is

essentially a process much more complicated than that considered above for radiation

propagation in a uniform layer. However, the simplest case allows us to come to

important conclusions relevant to radiation behavior in such a complex system as the

Earth's atmosphere. Estimations have shown that separate layers, and the atmosphere

as a whole, are optically thick media in respect to practically all substances present in

the atmosphere. That is why in the outgoing Earth radiation the absorption band of the

component being studied, has a deformed shape.

Radiation of external sources interacts with substances in the atmosphere and forms

the outgoing radiation; these sources are, specifically the Earth, the lower layers of the

atmosphere in respect to the higher layers, and also celestial bodies (the Sun, the

Moon and the stars). The thermal radiation of the Earth and the lower layers of the

atmosphere depend on the temperature and the radiation properties of its surface, the

temperature of the surrounding air and also on the cloudiness. For some average

temperature of the Earth's surface (~290K) the Earth's maximal radiation comes from

within the waveband 10-12 µm. The Earth's own thermal radiation exceeds radiation

of all the other sources beginning with λ = 4 µm.

Fig. 4. Spectrum of Earth Thermal IR-radiance recorded from space: a - Desert Sahara; b - Mediterranean Sea; c - Antarctic Region

Thermal radiation spectra registered on board the "Nimbus-4" satellite from an

altitude of 800km are shown in Fig. 4. (The satellite was launched by the USA in

1974). The spectral distribution curve (a) was obtained above a hot, sandy surface at a

temperature of ~50°C. In the waveband 12.8 - 13,7µm there is a clear manifestation of

thermal flux absorption by atmospheric carbon dioxide CO2. Abrupt, shallow

declivities at wavelengths 16 - 25 and 6.7 - 8 µm, are the result of absorption by water

vapor, H20.The ozone absorption band close to λ = 9.6 µm is clearly identified,

whereas close to λ = 7.7 µm there is a relatively weak absorption band of methane

CH4.

In the spectral radiation curve (b) obtained above the Mediterranean Sea, where the

temperature of the water is ~20°C, absorption bands relevant to gases mentioned

above, are less distinct, whereas above Antarctica (curve c) in the region of absorption

bands of CO2 and O3 the radiation flux exceeds the level relevant to the thermal

emission of the Antarctic surface. The spectral distribution curves clearly demonstrate

the process of thermal radiation transport from the Earth's surface into the upper layers

of the atmosphere. Above the hot regions of the Earth mainly the thermal radiation of

the Earth cover and the lower layers of the atmosphere build up the outgoing

radiation. This radiation interacts with the atmosphere throughout its depth and is

substantially weakened in spectral areas enclosing absorption bands of atmospheric

gases. Characteristic of a cold Earth surface and low levels of the atmosphere, is that

the outgoing radiation depends mainly on the radiation of the upper layers, where

there is but a low presence of absorbing atmospheric gases and, therefore, an

insignificant weakening of penetrating radiation flows.

Fig. 5. Solar spectra and absorption bands of atmospheric gases

Sunrays interacting with the Earth's atmosphere and its surface produce the short-

wave part of the outgoing radiation. Fig. 5 shows the energy spectrum of solar

radiation before it enters the atmosphere and at sea level, in the absence of clouds. The

energy spectrum of blackbody radiation at Sun surface temperature ~5900 K is also

shown in Fig. 5. We can see that as the energy spectrum of solar radiation penetrates

the atmosphere, it changes significantly: there is a general weakening, and abrupt

declivities appear; these are a result of selective absorption by atmospheric gases -

water vapor, carbon dioxide, oxygen, ozone and others. Specifically, absorption of

solar radiation by ozone is observed in all the previously mentioned bands of its

molecules. The general weakening of the solar radiation flux depends on the

wavelength insignificantly; it is brought about by the scattering of electromagnetic

waves caused by optical irregularities of the atmosphere (due to density fluctuations)

and by solid and liquid particles suspended in the atmosphere, which shape aerosol

and water-vapor clouds. The short-wave part of the outgoing radiation is composed of

solar radiation, scattered by the atmosphere and reflected from the surface of the Earth

and clouds.

There are two methods of identifying atmospheric gas components on the basis of

outgoing spectral radiation characteristics. The first method demands a solution of the

direct radiation transportation problem, making use of data relevant to the distribution

of density and temperature with altitude, as well as characteristics describing the

interaction of electromagnetic waves and matter. The solution of this problem yields a

spectral distribution of the outgoing Earth radiation. By changing the distribution of

the unknown component quantity, relative to altitude, and also its total amount, one

can, in principle, reach a state when the calculated and measured spectral distributions

in the outgoing radiation will be in good agreement. Here two problems evolve. The

first one is the lack of certainty: is the density distribution of the unknown component

quantity (versus altitude) the only possible distribution? The second problem lies with

large computational volumes required by numerous possible variants of density

distribution.

This necessitates the development of inverse ways for solving the problem (the second

method) - determination of the unknown component in the atmosphere according to

the spectral characteristics of the outgoing radiation, and avoiding numerous

intermediate calculations. The mathematical basis of these methods is the integral

equation of the following type

I(ν) = ∫ G(ν, x) φ(x) dx,

where I(ν) is the dependence of the outgoing radiation intensity on frequency ν; φ(x) -

the unknown density distribution of substance vs altitude; G(ν, x) - the core of the

integral equation, characterizing the contribution to the outgoing radiation of the

amount of substance found in a layer of atmosphere at an altitude x. This equation is

known as Fredholme's integral equation of the first order, and is notable for the

absence of a single answer. In this sense the solution to the equation is relevant to the

class of incorrect problems, demanding the involvement of statistical methods to

assess the correctness of solutions, by acquisition of data, characterizing the

atmosphere and the parameter being determined.

There are three basic passive methods of remote ozone layer sensing: method of

atmospheric emission, based on measuring the inherent radiation of the Earth and its

atmosphere, measurements of Solar Backscattered Ultra Violet (SBUV) radiation, and

the absorption method involving the measurement of atmosphere transparency in the

following direction: satellite - radiation source (the Sun or the stars). Radiation

measurements in these methods are conducted in the nadir direction, or at different

angles to nadir. Depending on the direction of observation relative to nadir, these

measurements are conditionally divided into limb and nadir measurements.

Fig. 6. Methods of Ozone measurements from space

Charts illustrating these measurements are shown in Fig. 6. In the case SBUV

radiation, the outgoing radiation of the atmosphere is measured in the vertical nadir

direction (1) or at different angles to the vertical line (2). In the case of limb

absorption measurements lines of observation are pointed to the Earth's horizon (3).

Due to the immense length of the ray's path in the atmosphere, these measurements

allow, the determination of other "small" gas components in the upper layers of the

atmosphere, in addition to ozone. Nadir (4) and limb (5) measurements of the

atmosphere inherent thermal radiation are carried out on the night Earth side. In the

case of the limb measurements of SBUV radiation (6), lines of observation are pointed

near the Earth's horizon.

5. Solar Backscattering Ultraviolet Method

It has been mentioned above that the short-wave part of solar radiation undergoes

substantial scattering, caused by molecules and density fluctuations. Radiation

scattering follows Rayleigh's Law, according to which the intensity of scattered

radiation is inversely proportional to the wavelength in the 4th power, and is

distributed in such a way that in the direction of propagation, it is twice as intense as

in the transverse direction. The content of ozone, even at a maximum of its

distribution vs height, is substantially smaller than the concentration of the main

components in the atmosphere. That is why the absorption of sunrays in the ozone

bands is a gradual process, which increases as the rays move "in-depth" into the

atmosphere. Radiation penetrating into the atmosphere brings about a considerable

flux of scattered Ultraviolet Radiation in the opposite direction. Surely, the

penetration depth of solar rays into the atmosphere depends on the absorption

properties of ozone on a given wavelength. In Hartley's main absorption band, solar

radiation is totally absorbed in the ozone layer, and the intensity of the outgoing UV-

radiation in this band of the spectrum depends on the scattering of solar radiation in

the upper layers of the atmosphere and its absorption by ozone.

In the area of weaker absorption (the Higgins band) the Sun's radiation reaches the

ground after having been thinned down. The outgoing radiation in this part of the

spectrum depends on solar radiation scattered by the atmosphere, and reflected from

the Earth's surface and it’s clouding. Designing instrumentation for measuring

backscattered UV solar radiation should take into consideration such an important

factor as the choice of a specific number of spectral ranges in the ozone absorption

bands for conducting measurements.

To accomplish this objective, one can use methods of mathematical modeling to solve

the direct problem of determining the intensity of scattered UV solar radiation. The

following data has to be assigned for this purpose: standard pressure distribution with

altitude, vertical distribution of ozone concentration found out in the course of on-the-

ground and rocket measurements, coefficients of Rayleigh's ozone scattering and

absorption, coefficients of aerosol scattering, the albedo of the underlying surface, the

Sun's zenith angle, etc. The so-called weighting functions constitute the results of such

computations. These functions characterize the contribution of radiation scattered

from different levels of the atmosphere, in respect to the total radiation on a given

wavelength, outgoing beyond the upper borders of the atmosphere, in the nadir

direction.

Calculated weighting functions show that, in the waveband 0.25-0.3µm, UV radiation

from the Sun is absorbed in the ozone layer before reaching the Earth's surface; each

of the wavelengths in this band has a certain depth of scattering and a strongly

manifested, though relatively narrow, layer of effective scattering. The "effective

layer" for shorter wavelength located on more higher atmosphere layers. This part of

the spectrum is used to determine the vertical distribution of ozone (the so-called

Ozone Profile). UV-radiation in the 0.3-0.34µm band penetrates into the lower layers

of the atmosphere. On these wavelengths, scattering envelopes practically all the

dense layers of the atmosphere from the Earth's surface to altitudes of 60km. This

waveband is used to determine the total content of ozone in the atmosphere (the so-

called Total Ozone).

10-3

10-2

10-1

10 0

10 1

2500 3000 3500

3398

2557

Wavelength, Ao

erg

/ cm

2.A

.sr

ado

2 1

10-3

10-2

10-1

10 0

10 1

2500 3000 3500

3398

2557

Wavelength, Ao

erg

/ cm

2.A

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ado

10-3

10-2

10-1

10 0

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2500 3000 3500

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2557

Wavelength, Ao

Wavelength, Ao

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/ cm

2.A

.sr

ado

erg

/ cm

2.A

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ado

2 1

Fig. 7. Backscattered ultraviolet radiation (1), Extraterrestrial Solar irradiance (2)

The total radiation scattered by all the layers determines the intensity of UV solar

radiation scattered by the Earth's atmosphere. The end result of the method is to

describe the distribution of ozone in the atmosphere on the basis of spectral

measurement data, obtained from a satellite. The mathematical premise for solving the

inverse problem - reduction of ozone to its initial content, on the basis of

spectrometric data - is Fredholm's integral equation of the first kind (mentioned

above). In Fig. 7 we can see a typical dependence of the intensity of UV solar

radiation, scattered by the Earth's atmosphere in the absorption band of ozone;

(measurements were conducted on board the Nimbus-4 satellite).

What attracts attention is the extensive range of scattered radiation intensity, which

varies by a factor of 104, from small values in the short-wave band to large values in

the long-wave range, whereas the intensity of direct UV solar radiation increases

smoothly with the growth of wavelength. Estimations have shown that a number of

factors (variations in ozone profile concentrations, albedo of the Earth's surface and

the clouds, Sun zenith distance) taken as a whole, bring about changes in the intensity

of scattered radiation limited to one order (ten times). Hence, the total variation of

atmosphere scattered UV-radiation in the given band of ozone absorption is equivalent

to five orders of magnitude. To establish ozone content in the atmosphere, with a

precision of 3-5% (level of natural variations of ozone), the precision of relative

intensity measurements of scattered radiation should be better than 1% (using the

above-mentioned wavelengths.)

The simplest way of meeting flux measurement precision requirements is to use an

UV spectrometer with sequential spectrum scanning and a single radiation receiver.

Here it seems appropriate to remind the reader that assessing the vertical ozone profile

requires all the abovementioned spectral channels, including the long-wave bands,

whereas for total ozone assessments, long-wave channels alone can provide adequate

information. Henceforth, we will use the terms UV OP-spectrometer and UV TO-

spectrometer to designate apparatus used for determining the vertical ozone profile

and total ozone content, respectively. The probability of flux changes by a factor of

105 leads to serious problems in the design of electronic and optical devices for the

UV OP-spectrometer. To provide a signal-to-noise ratio exceeding 100, the electronic

channel must have highly linear characteristics in all the range of varying input

signals. A change of radiation intensity in the ozone absorption band equivalent to 104

times (at a measurement precision of at least 1%) in all the spectral channels, requires

a drastic rejection of radiation, (dispersed on the elements of the optical system),

reaching the receiver; this rejection should provide a signal as low as 10-6 of the flux

input. At the same time, the contribution of dispersed radiation (not distributed in

terms of wavelengths) to any spectral channel constitutes a value less than 1% of the

flux being measured. Such a degree of dispersed radiation rejection can be achieved

with the help of a double monochromator. In UV OP -spectrometers, developed in the

USA and other countries, the dispersion of light, according to the wavelengths is

effected by diffraction gratings reflecting a fraction (10-3) of the falling radiation flux.

In these spectrometers a double monochromator with two diffraction gratings is used.

2

1

34

5

6

7

8

9

10

11

12

13

Fig. 8. Simplified schema of TOMS

In the UV TO- spectrometer, due to the relatively small change of radiation intensity

in the spectrum-scanning interval, there is no need for a double chromator. The UV

TCO- spectrometer, developed in the USA was coined TOMS (Total Ozone's

Mapping Spectrometer). It has six spectral channels on the following wavelengths

(µm): 0.3125; 0.3175; 0.3312; 0.3398; 0.36 and 0.38. TOMS observes the ozone layer

in a direction perpendicular to the satellite's orbit plane, with a 3° pitch in the angle

range of ± 52.5°, relative to nadir. The first two pairs of spectral channels are used to

determine TCO; two long wavelength channels (in which ozone absorption is

practically nonexistent) are required for albedo control of the underlying surface.

A simplified optomechanical diagram of the TOMS spectrometer is shown in Fig. 8.

Here elements 7 and 9 are a subassembly for scanning the spectrometer view of sight

across the Earth's surface; the rest of the elements, with the exception of the light flux

modulator 10, and the directing mirror 11, constitute the optomechanical UV-

spectrometer of the series spectrum scanning type.

The following is a description of the spectrum scanning principle. The light flux 8,

after passing the input slot of the monochromator 13, and having been reflected from

the collimating mirror 1, is converted into a parallel beam, which throws light on the

diffraction gratings 12. The radiation, diverged in terms of wavelength, is reflected

from the collimating mirror and focused on a stationary array 2 of the output slots.

Light beams, each having one of the mentioned wavelengths, pass through the defined

slots. The corresponding slots are positioned on a selector-disc 3 in such a way, that

when a pair of slots are aligned, the others are overlapped (shut). At the same time,

beams of specific wavelengths pass through the output optics and are focused on the

cathode surface of a photomultiplier 5. Shifting of the selector-disc 3 with the help of

a step motor 6 allows the sequential measurements of signal intensity on each of the

six allocated wavelengths.

The Solar Backscatter Ultraviolet (SBUV) spectrally scanning radiometer is based

also on abovementioned optical schematic diagram (Fig. 8). The SBUV-2 nadir-

viewing sensor measures the spectral solar irradiance and spectral scene radiance

(backscatter solar energy). Sensor can operate in sweep mode (continuous scan over

range 160-400nm) and discrete mode (measures from 252.0-339.8nm in 12 discrete

bands with 1nm bandwidth). The instrument makes measurements from which the

vertical distribution off atmospheric ozone can be determined to an absolute accuracy

of 5%.

In 1978 a set of UV spectrometers was stationed on the NASA satellite Nimbus-7 for

TO mapping and OP measurements above the satellite footprint areas: This set

(TOMS and SBUV) allows TO mapping during a 24 hour period over the whole of the

Earth's surface and at any time of the year. One exception: measurements in winter

over the polar caps are not feasible - during the winter the Sun does not shine over

these areas. Since the satellite launching, massive data has been obtained, relevant to

the Earth's ozone layer, and, specifically, over the South Pole, in the "ozone hole"

area.

The Shuttle Solar Backscatter Ultraviolet (SSBUV) is also instrument using UV

backscatter in nadir to measure vertical profiles of ozone in the stratosphere and in the

lower mesosphere in spectral range from 200 to 405nm. The SSBUV design is based

upon the above described technology. The objective is to fly the SSBUV payload on

numerous Shuttle missions to provide complementary calibration data for long-term

satellite ozone data sets. The first flight with SSBUV instrumentation occurred on

October 1989 on the Shuttle Atlantis (STS-34). Throughout this Shuttle flight

coincident observations were taken with the SBUV on Nimbus-7 satellite and SBUV-

2 on NOAA-9 and NOAA-11 satellites. The similar experiments with SSBUV were

continued on following Shuttle flights of Atlantis, Discovery, Columbia, Endeavour

until 1996.

Other countries for measuring TO and OP are also using UV spectrometers.

International efforts allow the monitoring of ozone over our common home - the

Earth. An early example of International Cooperation is the agreement between the

Goddard Center (NASA, USA) and the former USSR, according to which TOMS was

installed on board the space apparatus of the Meteor-3 type. The launching of such an

ecological orbital patrol system led to investigations, which solved numerous puzzles

of the ozonosphere. TOMS has helped revolutionize our understanding of the Earth's

ozone layer and associated systems.

QuikTOMS is the latest mission to carry the TOMS instrument. Its diagrammatic

drawing and real view on the testbed are presented on Fig. 9 and Fig. 10

correspondingly. QuikTOMS will follow in the footsteps of previous TOMS

instrument based satellites like Earth Probe, Nimbus 7, Meteor 3, and ADEOS.

QuikTOMS is a secondary payload, it shares its delivery system (Taurus rocket) with

the Orbview-4 mission. QuikTOMS uses innovative MicroStar satellite platform,

which supports payloads up to 68 kg and provides a three-to five-year mission life.

The instrument records daily global measurements of the Ozone, Aerosols, Erythemal

UV exposure, and Reflectivity. QuikTOMS main objectives are:

♦ Determination of long term change in global total ozone level;

♦ Understanding the processes related to the "ozone hole" formation and local

anomalies in the equatorial region;

♦ Improved understanding of processes that govern the generation, depletion,

and distribution of global total ozone;

Unfortunately, the much waited for NASA QuikTOMS launch on Sept. 21, 2001 has

ended in failure following problems with the Taurus vehicle's second stage. Another

such instrument is slated for launch in 2003 on the Earth Observing System Aura

satellite.

Fig. 9. QuikTOMS view of the instrument diagram with the component call-outs [23].

Fig. 10. QuikTOMS during ground testing [23]The Global Ozone Monitoring

Experiment (GOME) was launched on April, 1995 on board the second European

Remote Sensing Satellite (ERS-2). This instrument can measure a range of

atmospheric trace constituents, with the emphasis on global ozone distributions.

GOME is a nadir-viewing spectrometer that measures the solar radiation scattered by

the. The field of view may be varied in size from 320 km x 40 km to 960 km x 80 km.

GOME can provide complete coverage of the globe at the equator in approximately

three days. The schematic of the spectrometer optics is shown in Fig. 11. As an

instrument, GOME can be described as a double spectrograph, which predisperses

light at a prism and then produces a spectrum using a set of holographic gratings. A

combination of this optical arrangement and the use of four individual linear detector

arrays (each with 1024 detector pixels) enable the simultaneous observation of the

Earth's back-scattered spectrum between 240 and 790 nm (extending from the

ultraviolet into the visible parts of the spectrum). The spectral resolution of GOME is

moderate: between 240 and 400 nm it is approximately 0.2 nm; between 400 and 790

nm it is approximately 0.4 nm. The GOME employs a mirror mechanism, which scans

across the satellite track with a maximum scan angle that can be varied from ground

control. A similar instrument will be flown on the EUMETSAT Metop series of

satellites.

Fig. 11. GOME Spectrometer optics [25]

6. The Emission Method

In contrast to the Solar Backscattered Ultraviolet, the Emission Method involves the

study of the material content of the atmosphere through its inherent (thermal)

radiation. As was mentioned above, ozone has strong vibrational bands on

wavelengths of 4.7; 9.6; and 14.2 µm. The 4.7 µm band is intensively covered by

water vapour bands, and in addition, in this band, reflected radiation still has a

considerable impact on the Earth's outgoing radiation. The 14.2 µm band is

overlapped by absorption bands of carbon dioxides and water vapour. That is why the

4.7 and 14.2 µm bands cannot be readily used for determining ozone content in the

atmosphere. In terms of such an approach, the most suitable is the 9.6 µm ozone band.

In the late sixties and early seventies, in the USA, continuous spectra of outgoing

radiation were identified in a wide band of wavelengths (from 5 to 25 µm), which

allowed appropriate spectral zones to be allotted for probing the Earth's atmosphere.

Consequently, spectral measurements were conducted using the IR spectra-

interferometer IRIS. The spectra registered over different areas of the Earth's surface,

shown in Fig. 4, clearly bring out the 9.6 µm ozone absorption band. The TO was

roughly approximated by measuring the depth of the declivity in the outgoing

radiation on the specific wavelength. In the spectrograms, radiation absorption by

water vapour can be easily observed, and should be taken into account in TO

computations. In general, the retrieval of TO is accomplished on the basis of the

abovementioned formalism of Fredholme's integral equation of the first order, using

data, relevant to the ingredient content of the atmosphere, the vertical distribution

temperature and humidity.

Fig. 12. Simplified schema of infrared interferometer IRIS

Apparatus IRIS is based on Michelsons' interferometer circuit, with a linear movement

of the mirror. The functioning of such an interferometer is explained in Fig. 12. The

atmospheric radiation flux, after being reflected from mirror 1, is directed through

window 2 onto a light-splitting plate 3, on which an amplitudinal division takes place,

and the reflected and propagating parts of the flux are divided in a proportion 1:1. The

radiation fluxes, reflected from the movable mirror 5 and the fixed one 4, after

secondary separation on the plate 3 are directed to the converging mirror 6 and are

focused on the receiver 7 placed in the focus of mirror 6. These fluxes appear at the

radiation receiver with a time lag equal to the doubled value of the distance deviation

between the light separation plate 3 and mirrors 4 and 5. The intensity of

monochromatic radiation entering the light-sensitive surface of the receiver,

depending on the phase difference of the fluxes will change according to cosine law

from zero value to fourfold the intensity of unit flux.

The dependence of the receiver signal value on the position of the movable mirror is

called the interferogram of the flux being studied. With a uniform motion of the

movable mirror, the interferogram is actually a cosinusoid. In the case of a continuous

spectrum the interferogram is a complex time function. Thus, the output of the

interferometer receiver tract is not a spectrum, but is, in essence, some time function,

which, with the help of integral Fourier transform, should be converted into a

spectrum.

To attach the measurements to the wavelength master scale, the following elements

are incorporated into the interferometer: a high-precision channel for measuring the

displacement of the movable mirrors 9 and 10, components of the interferometer 3,4

and 5, the interferometer filter 11, a convex lens 12, and a radiation receiver 13. The

named channel works on the principle of an interferometer, so that any movement of

the mirror at the output of the receiver 13, brings about pulses of voltage having

delays equivalent to λ/2, where λ-the wavelength of the monochromatic source 8. The

mirror 1 is meant to compensate the shift in the field of view of the device, located on

the surface of the Earth, which occurs with the movement of the satellite. For this

purpose, the mirror 1 performs periodical angular movements, so as to aim the device

observation line at a specified footprint zone, during interferogram measurements. In

such a way, the distortion effect brought about by the changing state of the atmosphere

along the footprint line can be eliminated.

Fourier transform spectrometry is a labour-consuming indirect method for generating

spectra. As to the development of the interferometer itself (especially for space-based

apparatus) - the task is a formidable one in terms of scientific and technological

complexity. To mention just one problem: the configuration of the interferometer

should be resistant to vibration and deformations caused by temperature gradients. So

what is so attractive about Fourier Spectroscopy? The main advantage of the method

is that it provides a higher effectiveness in light flux energy consumption, in

comparison with direct spectra measurements (e.g. with the help of a scanning

spectrometer, utilizing diffraction grating and one receiver.). In the latter case,

measurements are conducted sequentially along spectrum stretches equal to the

spectral resolution of the device, whereas the larger part of the spectral interval being

studied does not participate in the measurements. We should note that doubling the

time for measuring a single spectral element is equivalent to taking two independent

measurements and then averaging them. If the error has a stochastic character, then

such averaging will bring about a √2-fold increase in precision. If the time taken for

measuring the specific element increases N times, then the precision with which the

flux intensity is measured increases √N times. All the spectral components of the flux

simultaneously take part in the structuring of the interferogram. This means that with a

certain time for spectrum measurement, the same spectral resolution, and N - the

number of elements being resolved, the precision of flux intensity measurements on a

given wavelength in an interferometer, is √N times higher than that of a spectrometer.

In the interferometer, one radiation receiver, so to say, takes upon itself the

simultaneous measurement of intensity in each of the spectral stretches.

The mass and power consumption of the Fourier- spectrometer IRIS was small; it

provided spectra in the 5 to 25 µm wavebands, recording one interferogram in 11 sec;

the spacial resolution was 100km. The spectrophotometer HIRS was developed in the

USA for operative measurements of ozone and other parameters of the atmosphere. It

is an instrument for high-resolution radiation measurements in the IR band. HIRS is a

20-channel filter spectral device. The channels are in the 0,7 to 15 µm wavebands and

one of them is the central band on a wavelength of 9.6 µm. The spectral data of the

apparatus allow the retrieval of temperature and humidity dependence on altitude; this

data is essential for determining TO on the basis of data in the 9.6 µm channel.

The architecture of HIRS can be depicted as an optical system of a telescopic type

with a scanning mirror at the input end, meant to shift the sighting line in the limits of

the apparatus observation angle. The diameter of the telescope is 15 cm. The light flux

at the output of the telescope is divided by a dichroic splitter into long wave (longer

than 6.4µm) and short wave (lower than 6.4µm) parts. The latter is repeatedly split so

as to single out a flux in the visible part of the light spectrum. The field of view of the

apparatus is formed with the help of two diaphragms - for the short wave and long

wave channels. After passing through the diaphragms, the fluxes reach the bandpass

filters attached to a rotating wheel; as the latter rotates, the respective spectral bands

sequentially reach receivers of the respective radiation fluxes. Two cryo-cooled solid-

state receivers are used in the apparatus: one in the long-wave channels, on the basis

of the triple compound HgCdTe, and the other using InSb in the short-wave channels.

In the visible band, a silicon receiver not requiring cryo-cooling is used. The field of

view of the telescope is 1.8°, the scanning angle is ± 49.5°, and time of scanning

within this angle is 6.4 sec, time for measuring the spectrum - 0.1 sec.

The HIRS spectrometers based on operative meteorological satellites are capable of

producing a global map of TO, with a 30-km geometrical resolution, during 8 hours.

It should be noted that above the Earth's polar ice-caps, TO data precision using this

method is low, especially during the winter period; this is explained by the low

temperature of the Earth's covering and the lower layers of the atmosphere, in the

vicinity of the Poles.

7. Limb Methods

Radiation flux measurements conducted from satellites in the direction of the Earth's

disk brim are called limb evaluations. There are several limb methods for determining

ozone content (and other components of the atmosphere): the sorption method which

infers the measurement of atmosphere transparency in the absorption band of the

component in question, during the instant of satellite setting into the shade, and

emerging from the latter; solar backscattered UV (SBUV) method used in the

direction of the horizon; the emission method. The main advantages of limb methods

for atmosphere probing are: higher sensitivity (60 to 70 times) to the "small" gas

components, brought about by the longer route of the ray in the atmosphere; high

vertical resolution (1-3km) in the altitudinal distribution of atmospheric components;

absence of influence on the part of low layer atmosphere heat radiation, as well as

Earth covers, on measurements.

A schematic of solar ray paths in the case of sorption method measurements is shown

in Fig. 6 (symbol 3). Because of the remoteness of the Sun, the radiation flux can be

roughly perceived as a series of parallel beams in the direction of observation beyond

the Sun at different positions of the satellite in orbit - in the form of parallel lines

through the atmosphere at different distances from the Earth. When the satellite sets

into shade, the lines of observation come closer to the surface of the Earth, when the

satellite emerges out of the shade these lines move away from the surface of the Earth.

The point of smallest withdrawal of observation lines from the surface of the Earth is

called the targeted spot. The normal drawn (perpendicular) from the latter to the Earth

crosses the terminator - the line separating the illuminated part of the surface from that

in the shadow.

The essence of the method is in the measuring of the transparency function of the

atmosphere at different altitudes in the absorption bands of the components being

identified. A measurement of direct solar radiation is taken when the satellite enters

the shadow or emerges from it, and also away from it (in conditions when there is no

influence of the atmosphere). Spectrometric data is used to determine the attenuation

of solar radiation in the atmospheric column along the trajectory of the beam. The

determined total content of the absorbing substance in the atmospheric column takes

into account Rayleigh's absorption and losses due to aerosols. The reciprocal problem

(determining the vertical distribution of the component's density) can be solved, using

the above-mentioned data for different altitudes. The model of the atmosphere is

depicted as a layered spherical structure. The mathematical problem in this case is

considered to be of the correct type, since the vertical distribution being obtained is

the only one possible. The density of the absorbing substance is determined close to

the targeted spots; the altitudinal density distribution is retrieved in the vicinity of the

terminator. The vertical resolution is determined by the field of sight of the apparatus

according to the tilt, whereas the horizontal resolution by the inadequacy of the

mathematical model relevant to the real atmosphere, and comprises approximately

150km.

The Stratosphere Aerosol and Gas Experiment using the sorption method is based on

a spectrometer called SAGE. The instrument vertically scans the limb of the

atmosphere during satellite sunsets and sunrises. The device comprises a tracking

mirror with a two-coordinate electromechanical drive, Sun direction sensors, and a

telescope responding to radiation reflected by the mirror, a diffraction grating

spectrometer, and radiation receivers. The diameter of the telescope is 5.1cm,

spectrometer field of view- 0.5′(elevation) and 2.5′ (azimuth); precision of Solar disk

brim targeting - 0.5′; the diffraction grating includes 1200 lines/mm; radiation

receivers are of the silicon photodiode type.

The instrument has seven spectral channels in the 0.38 to 1.02 µm band; this allows

the identification of the "small" gas components of the atmosphere (H2O, NO2) and

aerosols, in addition to the measurement of ozone content in Shappuis absorption

band. The altitudinal concentration distribution of ozone, nitrogen dioxide and water

vapour can be retrieved on the basis of spectrometric data, with a vertical resolution of

1-3 km, a horizontal resolution of about 150km, and a precision of 10%.

SAGE was launched first aboard the Application Explorer Mission spacecraft and

provided ozone measurements using the solar occultation technique until 1981. SAGE

II began operation with the launch of Earth Radiation Budget Satellite in 1984 and its

observations are making important contributions to studies of the Antarctic ozone

hole.

Occasionally, for investigating ozone with the help of the sorption method, relatively

simple apparatus, usually used in rocket and stratospheric measurements, is positioned

on a satellite. Specifically, these are spectrophotometers with a wide view of sight,

comprising assemblies of several small telescopes, the optical axes of which are

parallel, and each one of which is supplied with a narrow-band filter having a certain

central wavelength. A broad-angled device is placed before the telescope objective so

as to restrict irrelevant light fluxes. Vacuum diodes having stable characteristics were

usually used as radiation receivers. In the spectrophotometer two or three spectral

channels work in the ozone absorption zone (0.26-0.3µm) and one (basic) channel -

beyond this band (on wavelengths 0.34-0.4µm.).

Processing of spectrometric data is based on the method of differential absorption of

UV radiation, which presumes that radiation absorption in an atmospheric column is

described by Beer-Lambert's law. To determine the quantity of absorbed substance the

ratio of radiation intensities on two different wavelengths has to be calculated; one of

these wavelengths is located in the absorption band of the substance, whereas the

other is beyond this band, but relatively close to it. A rough estimation (not taking into

account Rayleigh and aerosol attenuation) shows that the logarithm of the ratio

mentioned, is proportional to the amount of absorbing substance in the atmospheric

column.

The construction of the spectrophotometers described is simple, but they have a

substantial deficiency - the altitudinal distribution of substance density retrieved

according to the spectrometric data has a relatively small vertical resolution (worse

than 5 -10m). This can be explained in the following way: due to the large angular

size of the Sun (33′) large volumes of the atmosphere take part in the formation of the

radiation flux entering the photometer; thus, for example, the Sun's diameter observed

from a height of 800km is equal to 20km in the region of the targeted spot. Despite a

number of advantages of the sorption method, we should mention its limited scope for

measuring the Earth's atmosphere. In the case of typical subpolar orbits, altitudinal

distribution of ozone and other "small" gas components can be determined over some

areas of the Earth's surface in the eightieth Northern and Southern latitudes, where

footprint lines intersect with the terminator. Possibilities of improving the informative

capacities of the method are related to perspectives of utilizing stars having stable UV

radiation, as outer sources irradiating the Earth's atmosphere.

The prospects of the SBUV method (observations in the direction of the horizon,

marked 6 in Fig. 6) are great, in terms of covering the Earth's atmosphere. In contrast

to nadir methods, here the radiation scattered by the atmosphere is measured in a

direction close to normal in respect to falling solar rays.

The emission method in limb measurements is effected in the Infrared (IR) and

Microwave ranges of the spectrum. Investigations of the Earth's atmosphere with the

help of LIMS (Limb IR Monitoring of the Stratosphere) to be the most successful; this

instrument was installed on the satellite Nimbus-7. The apparatus was divided into

two distinct modules - electronics and radiometer sections. The radiometer block in

itself consists of two parts - a solid-state cryo-assembly and optical-mechanical

assembly, both of which are attached to a common plate installed on the satellite. The

solid-state cryo-assembly includes a receiver subassembly and a cryogenic device with

two stages of cooling, the first of which uses solid-state ammonia (NH3) and provides

a temperature of 152K for all the elements of the receiver assembly, with the

exception of the detector itself; the second stage uses solid methane (CH4), and

provides a temperature of 63K necessary for the detector functioning. Photoresistors

made of a triple mixture (Cadmium-Mercury-Tellurium) serve as the sensitivity

elements of the detector.

12

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Fig. 13. Optical schema of LIMS apparatus

The optical-mechanical assembly consists of the optical elements of the telescope and

electric actuators of the scanning mirror and a radiation interrupter. A simplified

optical scheme of the device is shown in Fig. 13. Items 2-6 are parts of the telescope

with a scanning mirror 1. Shown in the figure are: 2,4 – off-axis parabolic mirrors, 3 –

modulator, 5 – aligning plane mirror, 6 - Cadmium –Tellurium lens. Items 7-13 are

part of the radiation receiver assembly. The slit 7 and window 8 having a temperature

of 152K separate the radiation receiver assembly from the telescope, which has the

temperature of the satellite container. After passing the system of parabolic mirrors 9

and a focusing lens 10 the radiation reaches the band filters 11 which slits 12

determined the field of view of the different spectral channels. Separated in space

according to the wavelengths the radiation flux arrives at a strip of independent

radiation receivers 13. Spectral bands of LIMS and a list of "small' gas components of

the atmosphere are listed in the table:

Channel Radiating gas Spectral band, μm

1 NO2 6.14-6.4 2 H2O 6.42-7.3 3 O3 8.77-10.8 4 HNO3 10.9-11.9 5 CO2 13.25-17.3 6 CO2 14.9-15.8

Limb scanning is accomplished in 12 sec., beginning from the altitude 160 km and

down to 40 km lower than the Sun disk brim. During the next 12 sec. the radiometer's

line of observation returns to its initial position. During these 24 sec. of satellite orbit,

the observation instrumentation covers 140 km of the atmospheric extent along the

limb; the vertical and horizontal spatial resolution of the apparatus comprises 2-4 and

20-30 km, respectively. The objective of the LIMS experiment was to map the vertical

profiles of temperature and the concentration of ozone, water vapour, nitrogen

dioxide, and nitric acid in the lower to middle stratosphere range, with extension to

the stratopause for water vapour and into the lower mesosphere for temperature and

ozone.

Infrared radiances from the LIMS instrument were stored on a Radiance Archive Tape

(RAT). Archive data was used to derive a series of two products:

♦ Inverted Profile Archival Tape.

♦

This data set contains corrected IR radiance

profiles, inverted daily profiles of temperature, and mixing ratios for ozone,

water vapour, nitric acid, and nitrogen dioxide, all as a function of pressure. In

addition, earth location, time, cloud-top, and housekeeping information are

included.

Map Archival Tape.

This data set contains daily global maps of 6

atmospheric parameters (temperature, ozone, nitrogen dioxide, water

vapour, nitric acid, and geopotential height) derived from radiance

measurements.

The Shuttle Ozone Limb Sounding Experiment (SOLSE) flied on the Space Shuttle

(STS-87, 1997) to demonstrate that vertical ozone profiles can be measured using

light scattered at the earth's "limb" (i.e. the horizon at very high altitudes). The

objective of the SOLSE is to determine the altitude distribution of ozone in an attempt

to understand its behavior so that quantitative changes in the composition of our

atmosphere can be predicted. SOLSE is intended to perform ozone distribution that a

nadir instrument can achieve. This will be performed using Charged Couple Device

(CCD) technology to eliminate moving parts in a simpler, low cost ozone mapping

instrument. Instrumentation includes an Ultraviolet (UV) spectrograph with a CCD

array detector and visible light cameras, calibration lamp, optics and baffling. On orbit

a crewmembers activated SOLSE which perform limb and Earth viewing

observations. Limb observations focus on the region 20 km to 50km altitude above the

horizon for the Earth's surface. SOLSE Earth viewing observations enable to correlate

the data with other nadir viewing, ozone instruments.

The use of the microwave frequencies in limb observations reduces limitations related

to the lower altitudes of atmosphere probing. Such opportunities are explained by

lower attenuation of microwaves in aerosols and water vapour (clouds) in comparison

with other frequency bands. Earth research with the help of limb methods and

development of space based apparatus continue to play a significant role in unraveling

the mysteries of ozone in the atmosphere.

8. Lidar Sounding of Atmospheric Ozone

Nowadays LIDAR has become the synonym of laser radar (locator). It was the advent

of the laser that turned optical radars into such effective instrumentation. Recent

accomplishments in the fields of laser technology, computerization, and methods of

processing signals allowed the development of satellite lidar systems for measuring

concentrations of "small" gas components and ozone in the first place. Active remote

sensing with the help of lidars unveils some unique opportunities, unattainable by

passive systems. One such possibility - improvement of vertical resolution in nadir

sounding, thanks to short pulse duration and precise master time attachments of

reflected signals.

Another peculiarity of lidar systems is relevant to their spectral characteristics. Stable

laser sources emitting close to some lines of absorption are perfect in meeting the

demands necessary, to determine the molecular components of the atmosphere, using

the method of lidar differential absorption, considered to be one of the simplest and

reliable techniques in optical atmospheric research. The method has much in common

with the method of differential absorption of UV radiation described above. It is based

on the independent reception of lidar signals on two close wavelengths, one of which

is in the absorption band of the substance, and the other is not.

Backscattered radiation is used to assess the relative attenuation in the substance with

high distance resolution along the sounding track. Since practically direct calculations

of substance density are used in this case, the precision of the lidar method is

substantially worse than in the case, when reciprocal problems are solved on the basis

of passive spectral nadir measurements.

Fig. 14. Structure of the Lidar system

In Fig. 14 we can see an arrangement of a differential absorbing lidar, designed in the

USA for remote sounding of ozone in the lower atmosphere. The lidar consists of two

large subsystems - for radiating and for reception. The radiating subsystem includes a

pumping laser on a crystal of Yttrium-Aluminium Granate with implanted ions of

Neodymium 2 and dye-based lasers 1; the receiving subsystem comprises a telescope

3, radiation receiver block 4 and an information processing unit 5. One of the dye-

lasers with a frequency multiplier works on a wavelength of 0.286 µm, which is

readily absorbed by ozone; the other emits on a wavelength of 0.3 µm used as a

master-frequency. Dye-lasers on wavelengths of 0.6 and 0.582 µm also emit on basic

harmonics. For sounding the aerosol profile a basic harmonic of a pumping laser

(λ=1.064 µm) is used. Reception of backscattered laser radiation is effected by a

telescope having an input diameter of 30 cm. Photoelectric multipliers are used as

radiation receivers. The field of view of the receiving subsystem is 5′.

The differential absorption lidar system installed on the DC-8 aircraft was repeatedly

used to measure the ozone profiles in the vicinity of the ozone hole over Antarctica.

As early as 1987 ozone profiles were identified for altitudes of 10 to 20 km above sea

level, inside the polar vortex and beyond it. The vertical resolution was 500m.

9. Monitoring Instruments Present State and Trends

Concerns about the atmospheric ozone layer during the past thirty years have led to

the development and application of several satellite-borne sensors for observing the

global distribution of ozone in the stratosphere. Amongst the different techniques

(solar backscattered experiments, limb-scanning apparatus, atmospheric emission

methods, etc.) measurements from space of Solar Backscattered Ultra Violet (SBUV)

radiation in the nadir direction is more widespread. The satellite measurements of

backscattered solar ultraviolet radiation has been used to produce data sets of global

total ozone and ozone profiles from TOMS and SBUV instruments on Nimbus,

NOAA, Meteor satellites, Space Shuttle, GOME on board the ERS satellite. In spite

of all their advantages, these instruments are complicated optic-electronic devices,

bulky, heavy (about 50 kg) and great power consuming (>50W), and they can be

installed and operated only within large space complexes (several tons). Expenses

borne on such ozone experiments amount to dozens of million dollars.

Main instruments intended for measurements of the atmosphere ozone from space are

presented in the following table. There are devices both developed, in flight operation

and future planning. The name of spacecraft, on which the instrument is allocated, its

launch date, and some remarks about instrument performance and experience also

present in this concluding table.

Table

Survey of major spaceborne instruments for ozone monitoring

Instrument Platform Launch date Comment

BUV (Backscatter Ultraviolet Spectrometer)

Nimbus-4, AE-E (Atmosphere Explorer-E)

Apr. 8 1970 Nov. 20 1975

2 Ebert-Fastie-Type monochromators AE-E reentered on June 10, 1981

SSH (Infrared Spectrometer), SSH-2 (Infrared Temperature and Moisture Sounder)

DSMP (Defense Meteorological Satellite Platform) series

Sep 11, 1976 Starting with the F1 satellite

SBUV (Solar Backscatter Ultraviolet) / TOMS (Total Ozone Mapping Spectrometer)

Nimbus-7 Oct. 24, 1978 Nadir viewing Ebert-Fastie spectrometer of TOMS. Swath width of 2700km (scanning). TOMS failed in May 1993. SBUV failed in 1990

SAGE-1 (Stratospheric Aerosol and Gas Experiment)

AEM-2 (Application Explorer Mission-2)

Feb. 18, 1979 The instrument is a sun photometer. Operations continued until Nov. 1981

UVSP (Ultraviolet Spectrometer and Polarimeter)

SMM (Solar Maximum Mission)

Feb. 14, 1980 Occultation measurements make possible a mapping of ozone concentrations at altitudes of 50-75 km until March 1989

UV ozone Experiment, Airglow Instrument, Solar UV Monitor

SME (Solar Mesosphere Explorer)

Oct. 6, 1981 All three instruments are Ebert-Fastie grating spectrometer for mesosphere ozone study. The satellite was operational until April 1989

UV spectrometer EXOS-C (Exospheric Observations-C)

Feb. 14, 1984 Nadir Observation of backscattered UV to obtain ozone profiles in altitude of 25-60 km. The mission ended in 1987

SBUV-2 (Solar Backscatter Ultraviolet-2)

NOAA-9 (National Oceanic and Atmospheric Administration-9), NOAA-11

Dec. 12,1984, Sep. 24, 1988

Satellite service ended Aug. 1995, Satellite service ended Sep. 1994

SSBUV (Shuttle Solar Backscatter Ultraviolet)

Space Transport Systems: STS-34, -41, -43, -45, -56, -62, -66 and STS-72

Oct. 19, 1989 Coincident observations with SBUV-2 on NOAA-9 and NOAA-11

TOMS Meteor 3-6 (Russia) Aug. 15, 1991 TOMS operation until Dec. 1994

HALOE (Halogen Occultation Experiment), MLS (Microwave Limb Sounder), CLAES (Cryogenic Limb Array Etalon Spectrometer), ISAMS (Improved Stratospheric and Mesospheric Sounder)

UARS (Upper Atmospheric Research Satellite)

Sep, 13, 1991 Sun occultation method (HALOE) Heterodyne limb sounder (MLS) Atmospheric Infrared Emission (CLAES) Atmospheric Emission limb-sounding instrument (ISAMS)

POAM-II (Polar Ozone and Aerosol Measurement-II)

SPOT-3 (Systeme Pour l`Observation de la Terre)

Sep.26, 1993 SPOT-3 entered safehold Nov, 97; solar occultation through the Earth's atmospheric limb

GOME (Global Ozone Monitoring Experiment)

ERS-2 (European Remote-Sensing Satellite-2)

Apr. 21, 1995 Differential optical absorption spectroscopy) measurement concept

Ozon-M Priroda Apr, 23. 1996 Module of the MIR station

TOMS TOMS-EP (Earth Probe)

July 2, 1996 Operational as of 2000

TOMS, RIS (Retroreflector in Space)

ADEOS (Advanced Earth Observation Sattellite)

Aug. 17, 1996 ADEOS failed on June 30, 1997

MAHRSI (Middle Atmospheric High Resolution Spectrograph Investigation)

CRISTA-SPAS-2 (Cryogenic Infrared Spectrometer and Telescopes for the Atmosphere)

Aug. 7-19, 1997

Platform took place on Shuttle flight STS-85

POAM-III (Polar Ozone and Aerosol Measurement-III)

SPOT-4 Mar. 24, 1998 Operational

OLME (Ozone Layer Monitoring Experiment)

FASat-Bravo (Fuerza Aerea Satellite)

July 10, 1998 Total column ozone measurements

OM-2 (Ozone Meter-2) Techsat/Gurwin II July 10, 1998 Total ozone, ozone profile measurements

OSIRIS (Optical Spectrograph and Infrared Imaging System)

ODIN Feb. 20, 2001 Detection of aerosols and trace gases

TOMS-5 QuikTOMS Launcher fails in Sep. 2001

GOMOS (Global Ozone Monitoring by Occultation of Stars), SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Cartography)

Envisat (Environmental Satellite)

2002 UV/Visible/Near-infrared limb viewing grating spectrometer; Star occultation measurement method, differential optical absorption spectroscopy and backscatter UV of solar and lunar radiation

SAGE-III (Stratospheric Aerosol and Gas Experiment III)

Meteor-3M-1 Dec. 2001 Self calibrating solar and lunar occultation

GLI (Global Imager), ILAS-II (Improved Limb Atmospheric Spectrometer-II)

ADEOS-II (Advanced Earth Observation Sattellite-II)

2002 Observation from the near UV to the near IR. Solar occultation measurement of polar stratospheric ozone

ACE-FTS (Atmospheric Chemistry Experiment-Fourier Transform Spectrometer)

SCISAT-1/ACE (Science Satellite-1/ Atmospheric Chemistry Experiment)

2002 Instruments is a classical sweeping Michelson interferometer looking at the sun through the atmosphere

HIRDLS (High Resolution Dynamics Limb Sounder), MLS (Microwave Limb Sounder), TES (Tropospheric Emission Spectrometer), OMI (Ozone Monitoring Instrument)

Aura mission EOS/CHEM (Earth Observing System / Chemistry)

2003 Infrared limb sounder, Measurements of limb thermal emission, Infrared imaging Fourier transform, Hyperspectral capabilities

EPIC (Earth Polichromatic Imaging Camera)

Triana 2004 View from Lagrangian point on sunlit Earth from sunrise to sunset at constant scattering angle

GOME-2 (Global Ozone Monitoring Experiment-2), IASI (Improved Atmospheric Sounder Interferometer)

MetOp-1 (Meteorology Operational-1)

2005 Ozone total amounts and profiles , Fourier Transform nadir-viewing imaging interferometer operating in the thermal infrared spectrum

ODUS (Ozone Dynamics Ultraviolet Spectrometer), SOFIS (Solar –Occultation Fourier Transform Spectrometer for Inclined-orbit Satellite)

GCOM-A1 (Global Change Observation Mission-A1)

planned 2006 Hight precision nadir-viewing grating spectrometer, Solar occultation measurements with high vertical resolution

OMPS (Ozone Mapping and Profiler Suite)

NPOESS (National Polar-orbiting Operational Environmental Satellite System)

planned 2006 Instrument includes the nadir wide-field sensor and limb-viewing sensor suite

The current tendency of lowering project costs, on one hand, and miniaturizing space

equipment, on the other hand, has led to the development of microsatellite technology

and the respective devices. Due to their simplicity and low cost, small satellites are

becoming very attractive for Earth observation experiments. However, small satellites

have substantial limitations in terms of incorporating sophisticated, specific payloads.

These restrictions call for a new class of advanced instrumentation and systems,

specifically designed for application in small satellites. For example, NASA is

developing an innovative, relatively lightweight, low-cost ozone instrument for ′s

next-generation objectives in Earth science. This compact ozone-monitoring

instrument optimized for combined absorptive and refractive stellar occultation

techniques. Spectral measurements as the star sets deeper into the atmosphere are

diagnostic the atmospheric composition, and constituent profiles may be determined

from the relative transmission (i.e., the ratio of occulted to unocculted spectra). As a

result, extinctive occultation measurements are self-calibrating and ideal for long term

trend monitoring. Instrument capable flying on a variety of spacecraft platforms for

ozone and climate studies in the 21st century.

The Solar Backscattered Ultra Violet radiation measurement series followed the

launch in July 1998 of the Israeli TechSat microsatellite with the Ozone Meter on

board. This new instrument also corresponds to the above-mentioned cost effective

methodologies. Small mass, low power consumption and acceptable dimensions of

this instrument allowed its incorporation into the microsatellite. The instrument is

designed to measure the vertical distribution of ozone and total ozone content in the

column beneath the satellite.

Fig. 15. Small Ozonometer that successfully operates on the Israeli Techsat

microsatellite

The operating principle of the Ozonometer is based on a filterwheel-photometer that

measures the SBUV radiance in the 252.0-340.0 nm spectral region in 7 specific,

narrow (1nm) wavebands. The instrument's optical head with weight only 1.7kg

(Fig. 15) is mounted so as to look in the nadir direction with a maximum view of

12°x3º. As the satellite moves in its sunsynchronous, retrograde orbit, the

instantaneous field of view traces swaths on the ground approximately 170km wide.

The statistical method was applied to obtain ozone data from both the measured

radiation and a priori information.

Successful flight tests of this "Small SBUV" Ozone Meter: displayed that it can

capture large scale variations of ozone over a broad range of atmospheric conditions

(0.3-100mb) with errors ranging from 10 to 20% for derived profile, and from 5 to

10% for total ozone. Comparison of the Ozone Meter with other space based ozone

instruments shows that instrument has very small weight and size, low power

consumption, and its measurement capabilities (accuracy, spatial resolution, etc.)

approach those of instruments that are more complicated in design.

The FASAT-Bravo microsatellite was launched for the Chilean Air Force. It is a space

science and technology demonstration mission. The primary science instrument is the

Ozone UV Backscatter Instruments (OUBI). The instrument relies on measurement of

UV backscatter in 4 bands over the sunlit part of the orbit. The ratios of backscatter in

bands are a guide to ozone levels in the upper atmosphere, and the photodiode

channels permit regular in-orbit calibration. The instrument is constructed using low

cost techniques based on existing SSTL camera hardware, and simple dye based filters

are employed and applied to conventional array CCD's. The two cameras are tuned to

380 and 313nm and cover an area of 560x400km each at a ground sampling distance

of 1.4km. By processing the differences between the two images, an instantaneous

ozone concentration images can be collected. The instrument objective power is 0.5

W for continuous operation. OUBI was carefully calibrated at NASA facilities and its

measurement results compare closely with the data from the TOMS-EP mission.

Fig. 16. Earth-Satellite-Earth laser beam absorption experiments

Advanced Earth Observing Satellite (ADEOS) is the international space platform

dedicated to Earth environmental research developed and managed by the National

Space Development Agency of Japan (NASDA). The objective of ADEOS is to

contribute to elucidation of phenomena of the earth system through integrated

observation using a number of sensors. One from main observed parameters is three-

dimensional distribution of atmospheric ozone. . Measurements of ozone, CO2, CH4,

CFC-12, etc. are carried out using infrared pulsed lasers on the ground and

retroreflector in space observation concept (Fig. 16). The retroreflector located on the

satellite face panel is used in long-path (Earth-Satellite-Earth) laser absorption

experiment. RIS has a corner-cube structure with an effective diameter of 50 cm,

reflectivity of 0.8, and divergence of reflected beam 60urad. ADEOS was launched

successfully on August 17, 1996. It provided a large volume of data containing

valuable information about our environment atmosphere, ocean and land for about 10

months until it suddenly got out of control because of the structural damage in its solar

array paddle.

The Optical Spectrograph and Infrared Imager System, known as Osiris, is flying on

the Swedish-led Odin small satellite, launched on Feb. 20, 2001, and is providing

scientists with unique set of data on ozone depletion. Osiris produces daily maps that

detail vertical ozone concentrations above Earth’s surface at 1.5 km intervals.

Comparison Osiris’ data with readings taken by other spacecraft and ground-based

instruments confirms high accuracy of Osiris ozone concentration measurements.

The principal goal of the Atmospheric Chemistry Experiment Mission of the Canadian

Space Agency is planned in mid-2002 to measure and to understand the chemical and

dynamical processes that control the distribution of ozone in the upper troposphere

and stratosphere. A comprehensive set of simultaneous measurements of trace gases,

thin clouds, aerosols and temperature will be made from low earth orbit.

Measurements will be taken during solar occultation as the sun's light passes through

the various layers of the atmosphere. The space instrument is a UV and visible array

detector spectrometer designed to measure the attenuation of the solar beam through

the atmosphere as viewed from the spacecraft at sunrise and sunset and make

measurements of solar radiation scattered back into space from the surface of the earth

and the earth's atmosphere. From these measurements in the wavelength range of 285-

1000 nm at a resolution of 1-2 nm, scientific information about the aerosol and ozone

profiles of the atmosphere can be deduced. The concentrations of more than 30

molecules will be measured as a function of altitude.

Science Instruments Triana is the first Earth observing mission to travel to Lagrange-

1, or L1 (the neutral gravity point between the Sun and the Earth). From L1, Triana

will have a continuous view of the Sun-lit side of the Earth at a distance of 1.5 million

kilometers. In order to obtain the same coverage with current Earth-observing

satellites in low Earth orbits and geostationary orbits, scientists must manipulate,

calibrate, and correlate data from four or more independent satellites. The full view of

the Sun-lit disk of the Earth, afforded by the L1 location, has tremendous potential for

Earth science. One from Triana tasks is research ozone hole evolution using ozone

concentration data.

Fig. 17a. EPIC – Earth Polychromatic Imaging Camera [26]

Fig. 17b. Triana Observatory [26]

The Triana contains the science instruments, Spacecraft Bus, and subsystems required

to operate the mission and process the data that it sends and receives (Fig. 17a). Its

Earth Polychromatic Imaging Camera (EPIC) views the entire sunlit Earth, from

sunrise to sunset, in 10 different wavelengths ranging from the Ultraviolet to the near

infrared (see Fig. 17b). The use of 10 precision filters and an accurately calibrated

camera permit us to measure key science products that are of interest to the science

community, the educational institutions, and to the general public. Triana will get total

daily coverage using the 317, 325, and 340 nm channels, measure ozone levels across

the globe with a resolution of 8 km, and monitor the changes in ozone levels through

the course of each day. In addition, Triana has the 605 nm channel (Chappuis ozone

band) that can be combined with the UV channels to produce good data on ozone give

coverage at high solar zenith angles.

10. The Antarctic Ozone Anomaly

Our readers probably expect to find answers in this booklet to exciting

questions relevant to the so-called ozone hole. We shall not disappoint

them, and will, without delay, consider basic experimental data,

confirming the emergence of an ozone deficit, in the South Polar

Region during the last twenty years; we will also discuss the main

hypotheses concerning the origin of this phenomenon.

A drastic fall in the total ozone content (disclosed in the Antarctic after 1979) has

been registered annually in October, i.e. during the Antarctic spring, and some

fluctuations in the effect have been noticed. Ozone specialists did not expect this

phenomenon and, initially, attempts were made to explain it by inadequacy of

measurement precision. However, regular measurements of ozone that followed, both

from satellites and ground based networks, left no doubt about the objectivity of

observations. The expanse with an anomalous low concentration of ozone is

approximately equal to the area experiencing polar night. The total ozone content in

October (annually, beginning from 1979) decreased, and in some places the drop had

reached 50% of the nominal value. At the same time, the boundaries of the ozone hole

shifted and spread beyond the perimeter of the continent in the direction of the

populated areas of Australia, South America, and Africa. Like an infection that grows

more and more virulent, the continent-size hole in Earth's ozone layer keeps getting

bigger and bigger. Its size steadily increased, covering a polar area of several million

square kilometers. Some years saw a relative maximum of the total ozone (TO)

content in the center of the ozone hole, surrounded by an area having lower

concentration of ozone, while other years did not evidence any substantial drop of

concentration during the spring. The dynamics of the hole during October of each year

shows that the hole is, in essence, a huge vortex in the atmosphere. During the months

following October the ozone layer is gradually reinstated and returns to its normal

condition. Thus, speaking about the ozone hole, one implicates a large area over the

Antarctic where the concentration of ozone is diminishing. In same period, for the

Northern Hemisphere a negative anomaly of the TO at the end of the winter months

also was observed. Less dramatic, but still significant, depletion of ozone level has

been recorded around the globe.

Fig. 18. Daily estimates of Antarctic ozone hole area

Image on Fig. 18 shows how change the size and depth of Southern Hemisphere

ozone hole. The ozone hole area is defined as the size of the region with total ozone

below 220 Dobson units. The daily ozone hole area estimates for last years 1999 and

2000 (solid lines) compare with the entire climatology (grey shaded) and with the

climatological mean (white curve). The maximum hole size observed in 2000 is

approximately 28 million square kilometers, while in 1999, the maximum was 25

million square kilometers.

How drastically could the formation of the ozone hole influence our planet? Should

the hole appear in the equatorial region there would emerge a biological effect caused

by the decrease of protection against ultraviolet radiation; what detrimental effect

could this have on live organisms? In the vicinity of the pole, after the polar night,

when the hole appears, the sun's rays reach the Earth's surface under a very low angle,

and are, therefore, less dangerous. It seems that there is no immediate threat to

personnel, taking turns at scientific stations. However, the appearance of the ozone

hole gave an impetus to research dealing with the impact of ultraviolet radiation on

the inhabitants of the ocean and plankton in the Antarctic region. Observations also

showed that there was a strong correlation between the drop of ozone concentration,

especially significant in the 10 to 25 km layer, and the lowering of temperature in this

layer. Such interference in atmospheric thermal processes can lead to climatic

changes.

The ecological situation brought about by short term depletion of the ozone layer in

the Antarctic would not have been so dramatic, had our knowledge been adequate to

understand the laws, regulating the formation and disappearance of the ozone hole,

and thus allowing us to forecast the sequence of expected events. Regrettably, the

annual advent of a spring minimum in the Antarctic ozone content has not yet been

unequivocally explained. The absence of a common understanding of events in the

Antarctic gives rise to alarm and anxiety of the population. Until the reasons behind

the formation of the ozone hole will have been fully understood, it will be impossible

to predict the consequences, both in terms of existence of biological objects on Earth

and the latter's climate. Let us consider the probable mechanisms of ozone formation.

According to one of them, ozone depletion is brought about by the increase of

nitrogen oxides, in its turn, influenced by 11 years cyclic solar activity (sunspot

cycles). During the maximum of solar activity, a 30-60% growth of nitrogen oxide

concentration in the mesosphere was observed in the southern hemisphere. Later on, a

transfer of these oxides into the lower levels of the stratosphere was observed during

the polar winter. As we know, photochemical reactions of the "nitrogen" cycle

together with the contribution of the nitrogen oxides cause the destruction of ozone,

and that leads to the formation of the ozone hole. Such a mechanism can realistically

explain the shaping of the ozone hole. However, variation in UV radiation with

sunspot cycles contributes to ozone production only account for 2-4% of the total

variation in ozone concentrations. There are at least three questions, which remain

unanswered in the framework of this mechanism. In first, why the ozone hole didn't

change with solar activity cyclic, but vary in size year after year? Second, why didn't

an ozone hole appear during previous 11-year cycles of solar activity? Third, why did

the ozone hole evolve only in the Southern hemisphere?

Another possible mechanism associates the formation of the ozone hole with a

"chloride" cycle of anthropogenic origin. One of the photochemical reactions

involving chloride, bringing about the destruction of ozone, was examined in one of

the previous parts of this booklet. The mechanism, related to reactions of the chloride

cycle, presumes an input of chlorine compounds into the polar stratosphere via

atmospheric circulation; compounds destroying ozone are "fed" into the atmosphere

from the Earth's surface by millions of aerosol packages, domestic refrigerators, and

emissions of chemical plants, etc. Despite the fact that anthropogenic activities have

not, as yet, caused a substantial decrease of total ozone content in the atmosphere,

freons could be responsible for the destruction of ozone over the Antarctic - such is

the opinion of a large group of scientists. Here there is also an unanswered question:

why didn't the mechanism driven by anthropogenic activity manifest itself so strongly

in the Northern hemisphere, where the supply of chloride, bromide and other

compounds, destroying ozone, is far more intensive?

Finally, a third possible mechanism - the so-called dynamic mechanism, tries to

explain the formation of the ozone hole purely by circulation processes in the

stratosphere and the mesosphere, and a horizontal redistribution of ozone. Ozone loss

is accelerated over the frozen continent because the Antarctic stratosphere contains

cloud particles not normally present in warmer climes. These icy particles have a

critical effect on the chlorine and bromine pollution floating in the stratosphere.

Normally, the chlorine and bromine are largely locked into "safe" compounds that

cannot harm ozone, but the ice particles transform them into destructive chemicals

that can break apart ozone molecules with amazing efficiency. With very little known

about the Antarctic ozone losses, atmospheric researchers could not tell which theory

was correct.

Despite uncertainty about the Antarctic phenomenon's cause, scientists firmly believed

halocarbons would eventually deplete the global ozone shield. Their certainty and the

jarring unexpectedness of the ozone hole's appearance motivated countries to act. In

September 1987, diplomats from around the world met in Montreal and forged a treaty

unprecedented in the history of international negotiations. Environmental ministers

from 24 nations, representing most of the industrialized world, agreed to set sharp

limits on the use of CFCs and Halons. According to the treaty, by mid-1989 countries

would freeze their production and use of halocarbons at 1986 levels. Then over the

next ten years, they would cut CFC production and use in half. The Montreal Protocol

provides a dramatic example of science in the service of humankind. By quickly

piecing together the ozone puzzle, atmospheric researchers revealed the true danger of

halocarbons, allowing world leaders to take decisive action to protect the ozone layer.

0

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Average within period 7.09-13.10

Vertical lines are minimum & maximum area

Year Fig. 19. Average area of Antarctic ozone hole during 1981-2001 years

Southern Hemisphere Winter Summary of the observed ozone variation during 1980-

2001 years is presented on Fig. 19. The area covered by extremely low total ozone

values of less than 220 Dobson Units, is defined as the "ozone hole area" (prior to the

springtime period in Antarctica, when ozone depletion occurs, the normal ozone

reading is around 275 Dobson units). For each year average in September-October

(07.09-13.10) areas of Antarctic ozone hole are shown together with estimation of

maximum and minimum areas in same period. For some last years (2000-2001) the

size of the ozone hole remained large in September and early October, but rapidly

decreased in size and ended in mid-November, the earliest in the last 10 years. In other

words, last years the ozone hole was of record size, but it formed very early and then

collapsed quickly (Fig. 18). Average October anomalies up to 40 percent below the

1979-1986 base period were observed over the South Atlantic Ocean and Antarctica,

with negative anomalies of more than 10 percent also observed over southern South

America. Vertical profiles of ozone amounts over the South Pole, at the end of

September and early October for last years, showed complete destruction of ozone in

the 15-20 km region, similar to values observed during other recent years. Total

column ozone over the South Pole reached a minimum reading of 100 Dobson units

on Sept. 28, 2001. The minimum total ozone value of 98 Dobson Units, observed on

Sep. 29, 2000, was not as low as the 90 DU value observed in 1999, or the record low

value of 86 DU observed in 1993. Lower stratosphere temperatures over the Antarctic

region in these years were again low. Temperatures lower than -78°C occurred over a

large region, and were sufficiently low for formation of polar stratospheric clouds, and

for enhanced ozone destruction to proceed. The rate of decline in stratospheric ozone

at midlatitudes has slowed during the 1990s. The fact that ozone depletion appears to

have stabilized supports the conclusion that international actions are working well to

reduce the use and release of ozone depleting substances.

2001 satellite data show the area of this year's Antarctic ozone hole peaked at about 26

million square kilometers - roughly the size of North America - making the hole

similar in size to those of the past three years. Researchers have observed a leveling-

off of the hole size and predict a slow recovery. Over the past several years the annual

ozone hole over Antarctica has remained about the same in both its size and in the

thickness of the ozone layer. This is consistent with human-produced chlorine

compounds that destroy ozone reaching their peak concentrations in the atmosphere,

leveling off, and now beginning a very slow decline. However, chemicals already in

the atmosphere are expected to continue ozone depletion for years to come. The

severity of the ozone depletion within the hole reached about the same levels as the

past few years and the highly depleted region filled about three-fourths of the

Antarctic polar vortex. In 2001 the vortex has been more stable and somewhat colder

than average. Year-to-year fluctuations in the geographical size of the polar vortex and

the size of the region with low temperatures will alter the size of the ozone hole over

the next decade during the period that levels of ozone-destroying chemicals in the

atmosphere begin a slow decline In the near future - barring unusual events such as

explosive volcanic eruptions - the severity of the ozone hole will likely remain similar

to what has been seen in recent years, with year-to-year differences associated with

meteorological variability. Over the longer term (30-50 years) the severity of the

ozone hole in Antarctica is expected to decrease as chlorine levels in the atmosphere

decline. Recovery could be expected with international adherence to the Montreal

Protocol and its amendments banning and/or limiting substances that deplete the

ozone layer. Changing atmospheric conditions and natural ozone variability

complicate the task of detecting the start of the ozone layer recovery. Only over the

middle latitudes in both the Northern and Southern Hemispheres has the ozone decline

recently slowed. Based on an analysis of 10 years of South Pole ozone vertical profile

measurements, scientists-optimists estimated that recovery of the Antarctic ozone hole

might be conclusively detected as early as the year 2008. A full explanation of ozone

and temperature anomalies must include all aspects of ozone photochemistry and

meteorological dynamics. Continued monitoring and measurements including total

ozone and its vertical profile are essential toward this end.

Fig. 20. February and March average area of Arctic low zone (<300 DU)

of each year from 1979 to 2001

It has already been mentioned that initial signs of ozone concentration decrease in the

Northern hemisphere have been observed. It should be noted that atmospheric activity

in the stratosphere of the two hemispheres is substantially different. In the Northern

hemisphere the average temperature is higher, and exchange between the polar region

and the mid-latitudes is more extensive. The destruction of the polar vortex occurs

earlier in the Northern polar zone and this poses limitations to the effectiveness of

photochemical reactions taking place in the vortex at low temperatures. Due to the

weaker circulation of the vortex in arctic latitudes, in comparison with the stable

circulation of the vortex surrounding the Antarctic, the amount of impurities entering

the Northern subpolar region with the air currents, is less than that entering the

Southern areas.

Fig. 20 shows the average area, during February and March for each year since 1979,

of low ozone (lower than 300 DU). For 2001, the average area of low total ozone was

smaller than for the previous year, and among the smallest values of all the years. For

the Northern Hemisphere winter and spring of 2000-2001, total ozone values observed

over the Arctic region were substantially higher than average. The conditions in the

Arctic region in year 2001 are in contrast to conditions during 2000, when total ozone

in the Arctic region was below the average. Lower stratosphere ozone destruction is

strong when meteorological conditions of a strong polar vortex and cold polar

temperatures prevail. High total ozone values in the Arctic region in the winter and

spring of 2000-2001 are attributed to meteorological conditions which were not

favorable for ozone destruction, even with the continued presence of ozone destroying

chemicals in the stratosphere. Total ozone declined over mid-latitudes of the Northern

Hemisphere at the rate of about 2 to 4 percent per decade from 1979 to 1993. In recent

years the strong rate of decline of Northern Hemisphere total ozone has not continued,

but current stratospheric ozone amounts continue to be below the amounts measured

before the early 1980s.

New research confirms that giant atmospheric waves, called "planetary-scale waves,"

or "long waves," warm the stratosphere and act to heat polar air. Strong planetary

waves form in the Northern hemisphere by such land features as high mountains

(Himalayan plateau), they warm the Arctic stratosphere and suppress ozone

destruction. Land shapes in the Southern hemisphere also produce planetary waves,

but they tend to be weaker because there are fewer tall mountain ranges and more

open ocean around Antarctica. The scientists long-ago recognized this connection but

it have only now quantified by satellite and meteorological data linking planetary

waves to bursts of warming registered in the Arctic. The planetary waves can warm

the polar region on 5o- 10o C and polar stratosphere temperatures in this case typically

lie in range -73o to -63o C. Of course, as soon as the waves have dissipated, the polar

region begins to cool down again. Polar stratospheric clouds form when temperatures

in the stratosphere become extremely cold (below -78° C). Polar stratospheric clouds

are trouble for ozone since tiny ice crystals and droplets within the clouds provide

surfaces where chlorofluorocarbons are converted into ozone-destroying molecules.

Due to planetary waves the polar stratospheric clouds are common in Antarctica, but a

rare sight in the Arctic.

Fig. 21. Antarctic polar stratospheric clouds those are dangerous for ozone [27]

Indeed, planetary waves in the Northern hemisphere don't always heat the stratosphere

enough to prevent substantial ozone destruction. In 1997, for example, the waves were

weak because of capricious weather. That triggered a rare springtime ozone hole over

the Arctic. Scientists are concerned that according the climate changes we would

expect lower ozone values across the Arctic during this century. On the other hand the

chlorine and other ozone destroying chemicals in the lower atmosphere, for example,

peaked in 1994 and have since declined. Computer simulations show that ozone-

destroying pollutants in the high stratosphere could return to pre-1980 levels in 30 to

50 years. Because climate change occurs on similar time scales, it's difficult to say

which trend would dominate: the cooling of the stratosphere, which would encourage

an Arctic ozone hole, or the decline of ozone-destroying molecules, which would

suppress it. With the aid of Earth-watching satellites and ever-improving computer

climate models, scientists hope to unravel the puzzle of Arctic ozone before it

becomes a problem. After all, one planetary ozone hole is more than enough!

According to last (October, 2002) joint report, which was prepared by United Nations

Organization (Environment Department) and the World Meteorological Organization,

the level of ozone-depleting chlorine in the atmosphere was declining because of the

ban on the use of chlorofluorocarbons (CFCs) in fridges and air conditioners, agreed

under the Montreal Protocol. Under this global protocol, developing countries

committed themselves to halving consumption and production of CFCs by 2005 and

achieving an 85 per cent cut by 2007. The report asserts that after the ban was

adopted, the atmospheric level of chlorine continued to rise, peaking in 2000; since

then, the level had stabilized and was now declining, albeit slowly. Scientists note that

the hole in the ozone layer was first detected nearly 30 years ago, when the hole size

was equal to Australian continent; then hole grew three times the size of Australia.

The researchers declare that now the hole over Antarctica is about to start shrinking

and will close by 2050. They predict that the hole in the ozone layer would contract

steadily from about 2005 and disappear by mid-century, although the ozone would be

vulnerable for a decade.

This optimistic estimation of the ozone layer evolution is agreed with top story on

Antarctic hole’s progression. Scientists from NASA and the Commerce Department's

National Oceanic and Atmospheric Administration (NOAA) have confirmed the

ozone hole over the Antarctic this September is not only much smaller than it was in

2000 and 2001, but has split into two separate "holes" (see Fig. 22).

Estimates for the last two weeks of the size of the Antarctic Ozone Hole (the region

with total column ozone below 220 Dobson Units), from the NASA Earth Probe Total

Ozone Mapping Spectrometer (EPTOMS) and the NOAA-16 Solar Backscatter

Ultraviolet instrument (SBUV/2), are around 15 million square kilometers. The

researchers stressed the smaller hole is due to this year's peculiar stratospheric weather

patterns and that a single year's unusual pattern does not make a long-term trend.

Moreover, the data are not conclusive that the ozone layer is recovering. This year

warmer-than-normal temperatures around the edge of the polar vortex that forms

annually in the stratosphere over Antarctica are responsible for the smaller ozone loss.

Fig. 22. Comparison of the first split ozone hole on record and the Antarctic ozone

hole at the same time one year earlier [28]

In 2001, the ozone layer thinning over Antarctica reached 26.5 million square

kilometers, larger than the size of the entire North American continent. Due to higher

Antarctic winter temperatures, the 2002 hole seems to be about 40% smaller.

So, humanity shares the alarm and anxiety related to changes of atmospheric ozone

concentration. The quest for a viable answer to the question posed by nature gave rise

to a spectrum of opinions concerning the mechanism leading to the formation of the

ozone hole, and the outcome facing our planet: beginning with total complacency and

ending with predictions of ozone catastrophe. What actually lies between these

extreme points of view - objective truth or a new problem? Further research should

lead to the correct answer.

11. Conclusion

A variety of satellite methods can be used to obtain substantial masses of information

relevant to ozonosphere characteristics, necessary for understanding a sophisticated

complex of dynamic, photochemical, and radiation processes, analyzing natural and

anthropogenic disturbances, as well as for the investigation of temporary changes in

the state of the ozonosphere. Of course, shifting only the ozonometric apparatus to

space will not in itself solve the ecological problem of ozone; however it can

substantially help in enhancing data adequacy and improving the quality of

ozonosphere observation. In comparison with on-the-ground observations, remote

methods of satellite meteorology provide the coverage of vast expanses of the Earth's

atmosphere with a periodicity and spatial resolution not attainable by other means. In

addition, remote methods do not have any influence on the characteristics of the

atmospheric volume being investigated, even in the case of lidar methods (if the

radiated laser power does not exceed a certain critical value).

In the previous chapters a whole series of satellite methods for ozone control was

considered and their high effectiveness was demonstrated. The following question

could arise: why so many methods of measurement, and such a diversity of

instrumentation? The crux of the matter is that each of the methods described does not

only have indisputable advantages, but also specific drawbacks. These were

characterized in the course of apparatus description. For example, measurements,

using certain methods can be conducted at a specific time, only above the terminator.

Some of the methods cannot be used for control, over Polar Regions during specific

periods of time. Still other methods allow the assessment of TCO only or substantially

smoothed vertical distribution of ozone; Lidar-based apparatus requires excessive

power supply on board the satellite, etc.

One of the principal criteria characterizing the performance of instrumentation is its

concentration measurement precision. According to existing estimations, investigation

and control of the ozone layer by space monitoring systems demands a precision of

TCO better than several percent. Monitoring on a continuous basis, first and foremost,

requires a thorough estimation of on-board measuring equipment quality. That is why

in the course of optimal long-term satellite monitoring system development, a special

effort is being made to investigate the limitations, pertaining to the different methods

of ozonosphere probing. Great attention is also being paid to the impact of methodical

and instrumental errors, and their influence on remote measuring of ozone.

Performance of apparatus in a harsh environment is also being consistently upgraded.

Satellite monitoring system with automatic means of data acquisition, processing and

transmitting of observation data are essential for ozonosphere control; they should

allow the detection of global or local changes in the ozone layer if and when they

occur. Nonetheless, the main purpose of space monitoring of atmospheric ozone is

investigating the formation, transportation and destruction of ozone, and research in

the area of global meteorological, cosmic, anthropogenic impacts on the ozone layer.

All this should lead to the creation of an ozonosphere behavior model, which could be

used for long-term prediction of changes.

12. A Certain Philosophical Epilogue

The remote sensing techniques for atmospheric ozone evaluation are based on

comparison of observed radiance with the calculated radiance values of a modeled

atmosphere. The method is based on measuring the outcoming radiation on a selected

set of wavelengths and finding a solution of the radiative equation in terms of ozone

concentration. Amongst the different techniques (solar backscattered experiments,

limb-scanning apparatus, atmospheric emission methods, etc.) measurements from

space of Solar Backscattered Ultra Violet (SBUV) radiation is more widespread. An

inverse solution of the radiative integral transfer equation is possible due to significant

narrowing of the weighting functions and the use of several of the wavelengths,

backscattered at various levels in the atmosphere. Ozone values are derived from the

ratio of backscattered Earth radiance to incoming solar irradiance. Ozone data are

determinated from these measured albedos using the statistical inversion technique.

The differences between the calculated albedos and measured albedos are then used to

provide new profile values that are more consistent with both measured and calculated

albedos. The computation of theoretical albedo, using successive iterations of the

radiative transfer equation, requires the following a priori information:

♦ ozone absorption coefficient αλ as a function of temperature;

♦ Rayleigh scattering coefficient β λ ;

♦ standard temperature profiles;

♦ standard ozone profiles;

♦ surface pressure at the lower boundary of the atmosphere;

♦ solar zenith angle θ.

The ozone climatology used to construct the a priori information was obtained from

the best available satellite, rocket, airplane and balloon ozone sound measurements.

From possible mathematical solutions it is necessary to select the particular solution

most likely representing physical reality. Programs developed for processing

measurements, performed by the above-mentioned devices, provide adequate means

for solving the problem and the construction of real profile sets and total ozone

content maps, on the basis of statistical analysis of climatological data. The

information accumulated by the space-based instruments, during many years of

operation, is extremely precise and sound. Results of ozone profile and total ozone

measurements filed in archives are a good basis for control of the atmospheric ozone

evolution, also for calibration and validation of novel space instruments being

developed for ozone monitoring.

It is known, that the very existence of the objects, available for observations, as well

as of the observer himself, imposes some limitations both on the feasible set of the

laws of nature, and on their realization. These limitations find their expression in the

so-called Anthropic Principle, which can be briefly formulated as follows:

The Universe exists just to enable man's living in it.

Nowadays, the progress in natural sciences made it clear, that the physical

peculiarities of the Nature (laws, constants) are adjusted in such a way that makes the

life possible. Lately it was noticed that both the physical conditions, and the constants

of the Universe are created such as to be prerequisite for the life in our Solar system.

Yet the exact wording of the Anthropic Principle can be found in the works of the

naturalists of all preceding epochs, even though in a less precise and definite ways,

tinted according to the scientific conceptions of their time. Galileo, for instance, poses

a question, what could happen, if the Moon had been created on the orbit closer or,

conversely, more distant from the Earth. In the former case, it would cause the very

strong tides, fatal to the mankind. In the latter case, the Moon, under the solar

perturbations, would eventually abandon the Earth, what in its turn would lead to the

deathly consequences for the life, due to the stagnant water, lack of the mixing of the

salt and fresh water, etc. In other words, the people by no means can see the Moon on

the orbit substantially different from the actual one. Thus the Moon's orbit, tuned just

to be "correct for life", is a good illustration of the Anthropic Principle.

As another classical example of this principle, the existence of the vital ozone layer

can be given. The processes in the Earth atmosphere, that enabled the creation of the

ozone layer, were considered before. Simultaneous action of a number of physical and

chemical laws resulted in the ozone layer arisen in the stratosphere, which role is to

control the Earth's heat balance and absorb the short-wave radiation of the Sun, lethal

for the life. The protective level of the ozone layer is provided by joint action of a

number of factors, viz.: the contents of the solar irradiation with respect to the

magnitude and the frequency; specific concentrations of the atmosphere basic

constituents; the presence of the trace gazes; physical and chemical properties of the

ozone and oxygen molecules; the dynamics of the atmosphere, etc. Whatever the

variation of the present conditions, it would cause the change of the ozonosphere

characteristics and hence, its protective and heat-regulation properties, which, in the

long run, can lead to the extermination of the life on the Earth. Consequently, in

accordance with the laws, acting in the nature, this unique, life-preserving layer had

emerged, to become a spectacular example of the Anthropic Principle in action.

As we've just mentioned, the tiny changes of the trace gazes abundance, when

accumulated, can bring about the deterioration of the ozone layer. The protective layer

proved to be very sensitive to the change of any of those conditions that prompted its

creation, in compliance with the Anthropic Principle. It is pitiful, that it was the

mankind itself, who provoked the ozone depletion in the atmosphere; but it is

inspiring, that this phenomenon was duly noticed and explained, and the relevant

international activity was institutionalized and set going. Thus, there is a hope, that the

process of the ozone layer deterioration would be stopped.

13. Timeline of Atmosphere Ozone History

600 million years ago: Formation of the ozone layer. Ozone atmospheric

concentration ran the amount required to shield Earth from biologically lethal UV

radiation with wavelengths 200-300nm. The ozone presence enabled organisms to

develop, to come out of the ocean and to live on the land.

1840: The German chemist Christian Frederick Shenbein discovered the new

substance "ozone".

1920: Beginning of study of the ozone concentration in the atmosphere by ground

based instrumentation.

1920: Invention of the class of chemicals known as clorofluocarbons (CFCs), which

contain chlorine, flouorine, and carbon atoms in a stable structure.

1924: English scientist G. M. B. Dobson creats the earliest optical devices

(photoelectric spectrophotometer) for measuring total ozone in atmosphere.

1930: British physicist Sydney Chapman described reactions of ozone dissociation

into molecular oxygen and atomic oxygen, and recombination of free oxygen atoms

with ozone.

1973: First identification of the atmospheric ozone destroying pollutants.

1973: Two scientists from the University of California: F. Sherwood Rowland and

Mario Molina, first discovered that man-made chlorofluorocarbons (CFCs) could play

a major role in the destruction of stratospheric life-protecting ozone.

1978: United States government bans CFCs used in aerosol spray cans.

1978 to 1993: Nimbus-7 longest duration of any satellite mission to measure

atmospheric ozone by three instruments: TOMS (Total Ozone Mapping

Spectrometer), SBUV (Solar Backscatter Ultraviolet), and LIMS (Limb Infrared

Monitor of the Stratosphere).

1980: British team, which had measured ozone levels over the Antarctic coast, first

began noticing the phenomenon of ozone abundances dropped below normal at spring

over the ice-covered continent.

1984: Ozone loss of 40% is detected over Antarctica during austral spring.

1985: Satellite images show existence of an Antarctic ozone hole.

1987: Montreal Protocol, stating that there would be a 50% cut back in CFC

productions by 2000, is signed.

1987: Antarctic studies find chlorine to be primary cause of ozone depletion, ozone

concentrations above Antarctica fell to half their normal levels.

1988: Ozone losses of 1.7 to 3% are measured over Northern Hemisphere.

1990: International delegates meet in London to strengthen the Montreal Protocol and

agree to a complete phaseout of CFCs by 2000.

1991: Upper Atmospheric Research Satellite (UARS), launched from the Space

Shuttle, contains several instruments to measure important trace gases and

meteorological quantities in the earth's stratosphere and mesosphere.

1991 to 1994: Russian meteorological satellite Meteor-3 with TOMS on the board

was launched and operate with success.

1991: Mount Pinatubo eruptions increasing natural levels of atmospheric chlorine.

1991: Airborne Arctic Stratospheric Expedition (AASE I) studies northern vortex.

1992: Record levels of ClO, 1.5 parts per billion, are measured over Bangor, Maine.

Ozone depletion rates of up to 20% are found in the Northern Hemisphere. Maximum

losses of 40 to 45% discovered over Russia.

1992: Parties to the Montreal Protocol meet in Geneva and agree on a 75% reduction

in CFCs by 1994 and overall phaseout by January of 1996. Production grace period, to

supply CFCs for essential purposes and the needs of developing countries, is extended

to 2006.

1995: Professor Paul Crutzen, Professor Mario Molina, and Professor F. Sherwood

Rowland receive the Nobel Prize in Chemistry. Their pioneering science research -

motivated by a desire to understand nature – leads to practical results of immense

societal benefit that could not have been anticipated when the research first began.

1999 to 2001: Antarctic ozone hole's size and ozone layer thickness has stabilized.

The concentrations in the stratosphere of the ozone destroying gases, curtailed under

international agreements, are only now reaching their peak, due to their long

persistence in atmosphere.

Ozone Devoted References and Internet Web Sates

1. Atmospheric Ozone. Edit by C. S. Zerofos and A.Ghazi., Reidel, Hingham, Mass., 1985.

2. Ozone Measuring Instruments for the Stratosphere. Edit by W. B. Grant. Vol.1, Optical Society of America, Washington, D. C., 1989.

3. H.J.Kramer. Observation of the Earth and Its Environment. Survey of Missions and Sensors. Springer, Berlin, 4-th edition, pp.1500, 2002.

4. K.Y.Kondratyev, C.A.Varotsos. Atmospheric Ozone Variability. Praxis Publ. Ltd, 624 pages, 2000.

5. A.P.Cracknell. Remote Sensing and Climate Change. Praxis Publ. Ltd, 336 pages, 2001.

6. M.Guelman, F.Ortenberg, A.Shiryaev, R.Weller. Microsatellites for Science and Technology: Gurvin-Techsat In-flight Experiments Results. Small satellites for Earth Observation, Digest of the 3-rd International Symposium of IAA, Berlin, pp.67-71, 2001.

7. M. Guelman, F. Ortenberg, B. Wolfson. Flight Tests of the Novel TechSat Satellite Ozone Meter. Proceedings of the 40th Israel Annual Conference on Aerospace Sciences, pp. 299- 309, 2000.

8. http://chemistry.beloit.edu/Ozone/index.html 9. http://chemistry.beloit.edu/Ozone/pages/reference.html 10. http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ATM_CHEM/ozone_measurements.html 11. http://earthobservatory.nasa.gov/Library/Ozone/ozone_4.html 12. http://jwocky.gsfc.nasa.gov/ 13. http://oea.larc.nasa.gov/PAIS/HALOE-Ozone.html 14. http://spacelink.nasa.gov/NASA.Projects/Earth.Science/Atmosphere/Ozone.Studies/ 15. http://www.al.noaa.gov/WWWHD/pubdocs/Assessment98.html 16. http://www.ciesin.org/TG/OZ/cfcozn.html 17. http://www.faqs.org/faqs/ozone-depletion/uv/ 18. http://www.esa.int/export/esaCP/Pr_16_2000_p_EN.html 19. http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/earthsci/ozone.htm 20. http://www.solcomhouse.com/OzoneHole.htm 21. http://www.technion.ac.il/ASRI 22. http://wrabbit.gsfc.nasa.gov/ 23. http://toms.gsfc.nasa.gov/ 24. http://auc.dfd.dlr.de/GOME/ 25. http://earth.esrin.esa.it/ 26. http://triana.gsfc.nasa.gov/home/ 27. http://science.nasa.gov/ 28. http://www.gsfc.nasa.gov/topstory/20020926ozonehole.html

Glossary of Ozone and Space related terms

A

absorption - The process by which radiant energy is absorbed and converted into

other forms of energy. A substance that absorbs energy may also be a medium of

refraction, diffraction, or scattering.

absorption coefficient - A measure of the amount of normally incident radiant energy

absorbed through a unit distance or by a unit mass of absorbing medium

action spectrum - Relative effectiveness of radiation in generating a certain

biological response over a range of wavelengths, as erythema (sunburn), changes in

plant growth, or changes in molecular DNA. The action spectrum for DNA

respresents the probability of DNA damage by UV radiation at various wavelengths.

Such DNA damage can lead to skin cancer.

aerosol – (1) Small droplet or particle suspended in the atmosphere, typically

containing sulfur.

Aerosols are emitted naturally (e.g., in volcanic eruptions) and as the result of human

activities (e.g., by burning fossil fuels.

aerosol – (2) A product that relies on a pressurized gas to propel substances out of a

container.

Now most aerosol products use propellants that do not deplete the ozone layer, such as

hydrocarbons and compressed gases.

albedo - Portion of incident electromagnetic radiation that is reflected by by a body to

the amount incident upon it, often expressed as a percentage; e.g., the albedo of Earth

is 34%. The concept is identical with reflectance; however, albedo is more commonly

used in astronomy and meteorology and reflectance in physics. Albedo is sometimes

used to mean the flux of the reflected radiation; e.g., the Earth albedo is 0.64 calorie

per square centimeter. The albedo is to be distinguished from the spectral reflectance,

which refers to one specific wavelength (monochromatic radiation).

aldehyde - Organic chemical compound derived from the oxidation of primary

alcohols, having the common group CHO. Used in manufacturing of dyes, resins, and

organic acids. Atmosphere secondary organic pollutant.

anaerobic - Capable of living in the absence of free oxygen.

anthropogenic - Involving the impact of man on the natural environment.

anticyclone - Extensive wind system, of high barometric pressure, that circulates

clockwise in the Northern hemisphere and counterclockwise in the Southern

hemisphere.

atmosphere - Envelope of air surrounding Earth, which is retained by Earth's

gravitational attraction.

attitude - Orientation of a satellite relative to its direction of movement.

B

backscatter ultraviolet (BUV) technique - One of several remote sensing techniques

used for measuring atmospheric trace gases by satellite. Measurements are made of

solar ultraviolet (UV) light entering the atmosphere (the irradiance) at a particular

wavelength and of the solar UV that is either reflected from the surface or scattered

back from the atmosphere (the radiance) at the same wavelength.

biosphere - Portion of Earth and its atmosphere that supports life, including the living

organisms within it.

blackbody - An ideal emitter that radiates energy at the maximum possible rate per

unit area at each wavelength for any given temperature. The spectral distribution of

blackbody radiation is described by Planck law and by the related radiation laws.

bromine (Br) - Deep red, corrosive, nonmetallic, liquid halogen that gives off an

irritating reddish brown vapor. Element of halons, used in pesticides and fire

extinguishers.

C

calibration - Systematic adjustment by comparison to a standard, such as the

graduated scale of a measuring instrument. May be used in algorithms and models to

remove geometric and radiometric distortions in the data.

carbon dioxide (CO2) - Odorless, colorless, incombustible, nontoxic gas that is

produced during respiration, decomposition of organic material, and combustion.

Important "greenhouse" gas that contributes to global warming by allowing solar

radiation to pass through the atmosphere and trapping radiant heat reflected from

Earth's surface.

carbon monoxide (CO) - Poisonous, odorless, colorless gas, produced by incomplete

combustion of gasoline and diesel fuels.

carbon oxides - Compounds containing carbon and oxygen.

Chapman Reactions - Stratospheric process described by Chapman in which ozone

dissociates into molecular oxygen and atomic oxygen, and the resulting free oxygen

atoms recombine with ozone to form molecular oxygen.

chlorine (Cl) - Heavy, greenish-yellow, irritating gas with a pungent odor. Capable of

reacting with almost all other elements. Catalyst for ozone destruction.

chlorine monoxide (ClO) - Intermediate product of chlorine interaction with ozone.

chlorofluorocarbons (CFCs) - Group of inert, nontoxic, nonflammable compounds

made up of chlorine, fluorine, and carbon; used in cooling, foam insulation and

cleaning agents as well as aerosol propellants. CFCs are very stable in the troposphere.

They are broken down by strong ultraviolet light in the stratosphere and release

chlorine atoms that then deplete the ozone layer. CFCs are commonly used as

refrigerants, solvents, and foam blowing agents. Two primary chlorofluorocarbons

that photolyze at high altitudes to release chlorine atoms are CFC-11 (CFCl3), that

known as trichlorofluorocarbon, used in aerosols and CFC-12 (CF2Cl2) known as

dichlorofluoromethane, used in air conditioning systems as a refrigerant.

chopper disk - Slotted disk that is rotated by an electrical motor. During rotation, the

detector views the target and reference source alternately. The known radiance from

the reference source and the amplitude of the incoming signal enable estimation of the

target's radiance.

climatological ozone profiles - Twenty-three standard profiles derived from a

combination of SBUV measurements taken at altitudes greater than 16-mbar and low

altitude balloon radiosonde data. Yearly averages were developed for three latitude

bands: low (15°), mid (45°), and high (75°).

climatological temperature profiles - Standard atmosphere temperature profiles.

convective - The transfer of heat through motion within the atmosphere, especially

upward directed motion.

D

deoxyribonucleic acid (DNA) - Self replicating nucleic acid that contains genetic

code within the cell. The primary structure consists of two long nucleotide chains that

are joined by hydrogen bonds and twisted together to form a double helix.

depolarizer - Device that removes the effects of light polarization.

diffuser plate - Plate used for capturing incoming solar radiation for measurement

and intercepting radiation from a mercury-argon calibration lamp.

dissociation - The separation of a complex molecule into constituents by collision

with a second body, or by absorption of a photon.

Dobson spectrophotometer - The earliest instrument that is used to determine ozone

content of the atmosphere from ground station measurements and modern versions

continue to provide data. It compares solar energy at two wavelengths in the

absorption band of ozone by permitting the two radiations to fall alternately onto a

photocell. The stronger radiation is then attenuated by an optical wedge until the

photometer's photoelectric system indicates equality of incident radiation. The ratio of

radiation intensity is obtained by this process, and the ozone content of the atmosphere

is computed from the ratio.

Dobson Unit (DU) - Unit of measurement of total ozone equal to 2.69 x 1016

molecules per square centimeter. An equivalent amount of ozone, at 1 atmosphere and

273° K, would form a layer 0.001 cm thick. Named in honor of the British physicist

G. M. B. Dobson. If 100 DU of ozone were brought to the Earth's surface, it would

form a layer 1 millimeter thick. In the tropics, ozone levels are typically between 250

and 300 DU year-round. In temperate regions, seasonal variations can produce large

swings in ozone levels. For instance, measurements in St. Petersburg, Russia have

recorded ozone levels as high as 475 DU and as low as 300 DU. These variations

occur even in the absence of ozone depletion, but they are well understood. Ozone

depletion refers to reductions in ozone below normal levels after accounting for

seasonal cycles and other natural effects.

E

Ebert-Fastie monochromatic spectrometer - Instrument used to measure energy

intensity within the ultraviolet region of the electromagnetic spectrum.

electromagnetic spectrum (EMS) - Entire range of electromagnetic radiation ranging

from gamma rays, less than 0.03 nanometers, to radio waves, greater than 30

centimeters.

exit slits - Array of holes within a chopper disk that serve as fixed exits during

wavelength calibration.

exosphere - Region of the atmosphere beyond 400 km that fades into interplanetary

space.

F

Fourier transform spectrometer - Spectrometer that consists of a collimator and

beamsplitter, which divides the source beam into two parallel beams with equal

amplitudes.

G

global warming - Rise in global temperature caused by increased amounts of

atmospheric gases that trap heat in Earth's atmosphere by absorbing longwave

radiation.

Global Warming Potential (GWP) - a number that refers to the amount of global

warming caused by a substance. The GWP is the ratio of the warming caused by a

substance to the warming caused by a similar mass of carbon dioxide. Thus, the GWP

of CO2 is defined to be 1.0. Ozone depletion substances CFC-12 has a GWP of 8,500,

while CFC-11 has a GWP of 5,000.

GOME (Global Ozone Monitoring Experiment) - the first European passive remote

sensing instrument operating in the ultraviolet, visible, and near infrared wavelength

regions whose primary objective is the determination of the amounts and distributions

of atmospheric trace constituents.

grating - Surface with parallel grooves or slits that enable diffraction of incoming

light into optical spectra.

greenhouse effect - The phenomenon in which outgoing infrared radiation that would

normally exit from a planet's atmosphere but instead is trapped or reflected because of

the presence of the atmosphere and its components. The best scientific estimates to

date suggest that increasing amounts of greenhouse gases are resulting in higher

temperatures worldwide. These greenhouse gases are water vapor, carbon dioxide,

ozone, nitrous oxide, methane, and chlorofluorocarbons (CFCs).

H

halon - Compound formed when a halogen, such as fluorine (F) or bromine (Br)

attaches to a carbon atom. The halons are used as fire extinguishing agents, both in

built-in systems and in handheld portable fire extinguishers. They couse ozone

depletion because they contain bromine.

hydrogen chloride (HCl) - Important chlorine-containing compound formed from the

breakdown of chlorofluorocarbons. Also produced by volcanic eruptions. Less

reactive than chlorine.

hydrogen fluoride (HF) - Important fluorine-containing compound formed from the

breakdown of chlorofluorocarbons. Also a product of volcanic eruption.

hydrosphere - Aqueous envelope of Earth, including oceans, lakes, soil moisture,

ground water, and atmospheric water vapor.

I

instantaneous field of view (IFOV) - Ground or target area viewed by a sensor at a

given point in time.

infrared radiation - Electromagnetic radiation having a wavelength slightly longer

than visible red light, from 750 nanometers to 1 millimeter. Its lower limit is bounded

by visible radiation, and its upper limit by microwave radiation. Most energy emitted

by Earth and its atmosphere is at infrared wavelength. The triatomic gases, such as

water vapor, carbon dioxide, and ozone, absorb infrared radiation and play important

roles in propagating infrared radiation in the atmosphere. Abbreviated IR; also called

"longwave radiation."

irradiance - Radiant flux per unit area of a surface.

K

ketones - Organic compounds in which the carbon atoms of two hydrocarbon radicals

are linked to a carbonyl group. Generally represented by the formula R(CO)R1, where

R1 and R may be the same.

L

Lambert-Beer Law - A relationship describing the rate of decrease of flux density of

a plane-parallel beam of monochromatic radiation as it penetrates a medium that both

scatters and absorbs at that wavelength.

lidar (light detection and ranging) - A technique for active remote sensing in which

a light source is used to probe the atmosphere. Laser light fired at the atmosphere is

reflected back by the atmospheric molecules to a detector and the attenuation

(reduction) of this light provides information on atmospheric particles and molecules.

limb emission technique - This is one of several remote sensing techniques for

measuring atmospheric trace gases by satellite. Also called limb sounding technique.

Instruments based upon the limb emission technique infer trace gas amounts (such as

ozone) from measurements of longwave radiation (infrared or microwave) thermally

emitted in the atmosphere along the line of sight of the instrument.

lithosphere - Solid mass of Earth composed of rock, soil, and sediment.

M

mercury-argon calibration lamp - Lamp that produces radiation centered at 253.7

nm, which is then diffused from a diffuser plate. Radiation measurements are made at

multiple wavelengths and possible shifts are noted.

mesopause - Transitional atmospheric region between the mesosphere and

thermosphere.

mesosphere - Region of the atmosphere, between approximately 50 to 100 km, in

which temperature decreases with altitude.

Meteor-3 - Third in a series of weather satellites launched by the former Soviet

Union. Launched in August 1991 with a payload that included a Total Ozone Mapping

Spectrometer (TOMS).

methane - Simple combustible hydrocarbon. The major component of natural gas.

microwave radiometer - Sensor that measures the intensity of microwave radiation

(0.3 cm-30 cm) within a specific field of view.

Mie scattering - Developed by Gustav Mie in 1908, this is a complete mathematical-

physical theory of the scattering of electromagnetic radiation by spherical particles. In

contrast to Rayleigh scattering, the Mie theory embraces all possible ratios of diameter

to wavelength, particularly the atmospheric scattering caused by large particles such as

dust, pollen, smoke, and water droplets. More prevalent in the lower atmosphere, from

0 to 5 km.

mixing ratio - Relative number of molecules of a specific type in a given volume of

air.

monochromator - Spectrometer that operates within a narrow range of the

electromagnetic spectrum.

Montreal Protocol (MO) - The international treaty governing the protection of

stratospheric ozone. The Montreal Protocol on substances that deplete the ozone layer

and its amendments control the phaseout of ozone depleting substances production

and use. Under the MP, several international organizations report on the science of

ozone depletion, implement projects to help move away from depleting substances,

and provide a forum for policy discussions. In addition, the Multilateral Fund provides

resources to developing nations to promote the transition to ozone-safe technologies.

N

nadir - Point directly beneath a satellite, opposite the satellite zenith.

nanometer - A distance of one billionth of a meter. The nanometer, or nm, is a

common unit used to describe wavelengths of light or other electromagnetic radiation

such as UV. For example, green light has wavelengths of about 500-550 nm, while

violet light has wavelengths of about 400-450 nm.

Nimbus-7 - Polar orbiting satellite launched on October 24, 1978, as a research and

development satellite to enable multidisciplinary studies of pollution, oceanography,

and meteorology. The following instruments were onboard: coastal zone color scanner

(CZCS), earth radiation budget (ERB), limb infrared monitor of the stratosphere

(LIMS), stratospheric aerosol measurement II (SAM II), stratospheric and

mesospheric sounder (SAMS), solar backscatter ultraviolet explorer total ozone

mapping spectrometer (SBUV TOMS), scanning multichannel microwave radiometer

(SMMR), and temperature humidity infrared radiometer (THIR).

nitrogen - A colorless, odorless, nonmetallic element that occurs as a diatomic gas

and constitutes nearly 80% of the atmosphere by volume.

nitrous oxide (N2O) - Colorless gas, naturally produced through bacteriological

decomposition of organic matter. Also produced anthropogenically and used as a mild

anesthetic.

O

occultation technique - One of several remote sensing techniques for measuring

atmospheric trace gases by satellite. Occultation instruments measure solar, lunar, and

even stellar radiation directly though the limb of the atmosphere during satellite Sun,

Moon, and star rise and set events (depending on which celestial radiator is being used

by the satellite instrument). By measuring the amount of absorption of radiation

through the atmosphere at different wavelengths (e.g., UV, visible, infrared),

occultation instruments can infer the vertical profiles of various trace constituents,

including ozone.

optical spectrum - Portion of the electromagnetic spectrum, from 0.30 to 15

micrometers, that can be reflected and refracted with mirrors and lenses.

oxygen - A nonmetallic element that occurs as a diatomic gas and constitutes 21% of

air by volume, essential for plant and animal respiration, and required for almost all

combustion.

ozone - Gaseous compound of three oxygen atoms that is generated by a photo-electro

process and has a distinct electrical or disinfectant odor.

“ozone” – Greek word meaning “smell”, a reference to ozone‘s distinctively pungent

odor.

ozone absorption coefficients - Variable parameter inputs required for albedo

calculations. Albedo measurements across an entrance slit vary according to ozone

concentrations and temperature. Therefore, an integral of measurements is used in

albedo calculations.

ozone layer, ozone shield - thin ozone layer containing the bulk of atmospheric

ozone. Nearly 90% of the Earth's ozone is in the stratosphere and is referred to as the

ozone layer. This protective layer absorbs harmful, deadly solar ultraviolet radiation.

ozone depletion - Loss of ozone through natural breakdown and anthropogenically

produced chemical reactions. Ozone depletion refers to reductions in ozone below

normal levels after accounting for seasonal cycles and other natural effects.

ozone hole – Popularly known term for region of rapid, dramatic ozone depletion over

Antarctica during the polar spring. Confined to south of 55° latitude. Disperses soon

after temperatures rise above -80° C.

ozone profile - The amounts of ozone at different levels in the atmosphere

represented in a plot of altitude versus ozone amount (measured typically in number

density or partial pressure).

ozonesondes - Balloon-borne instruments used to determine ozone profile

measurements.

P

pair values - Ratio of the albedo value at a longer ozone-insensitive wavelength to the

albedo value at a shorter ozone-sensitive wavelength. Used in the computation of

ozone.

photochemical reaction - A chemical reaction that involves either the absorption or

emission of radiation.

photodissociation - Dissociation (splitting) of a molecule by absorption of a photon.

photolysis - Dissociation process driven by the Sun's radiation.

photometer - An instrument for measuring the intensity of light or the relative

intensity of a pair of lights. If the instrument is designed to measure the intensity of

light as a function of wavelength, it is called a spectrophotometer.

photomultiplier tube (PMT) - Photoemissive detector consisting of a photocathode

and fused silica window that work together to multiply an incoming electron beam.

photosynthesis - Chemical process driven by solar energy in which CO2 and H2O, in

the presence of chlorophyll, are converted to oxygen and carbohydrates. Oxygen and

water vapor are released in the process.

planetary wave - A type of atmospheric wave with a wavelength upward of 10,000

km. These waves are mostly generated by large-scale surface topography like the

Rocky Mountains and the Himalayas-Tibet complex or by land-sea boundaries. Such

geographically forced planetary waves do not propagate horizontally but instead are

stationary. The fact that they are stationary is related to the fact that the topographical

forcing occurs at the same locations. Planetary waves often propagate upward from

the troposphere into the stratosphere.

POAM (Polar Ozone and Aerosol Measurement) - Solar occultation devices that are

designed to measure the vertical distribution and overall stratospheric abundances of

ozone, water vapor, nitrogen dioxide, and various aerosols. Instruments were launched

aboard the SPOT-3 satellite in September 1993 and SPOT-4 satellite in March 1998.

polarization - Uniform and nonrandom elliptical, circular, or linear variation of a

wave, characteristic in light or other radiation.

polar stratospheric clouds - High, thin clouds composed of nitric acid and water that

form in the coldest regions of the stratosphere when temperatures drop below -80°C.

Ice crystal surfaces within these clouds are efficient in converting inert chlorine

reservoirs, such as ClONO2 and HCl, into reactive chlorine compounds.

polar vortex - Wind region around the North or South pole. The southern vortex is a

well formed circular to oblong mass of extremely cold, stagnant air, held in place by

the ocean surrounding the Antarctic land mass and a strong westerly circulation

pattern produced by the coriolis effect. The northern vortex is not as distinct because

the Arctic is a frozen ocean surrounded by rugged land masses, which cause the

circulating winds to encounter a variety of temperatures.

Precambrian - Of or pertaining to the earliest geologic period of history,

approximately 600 million years ago, when Earth atmosphere protective ozone layer

was forming.

R

radiosonde - Balloon borne instrument used to measure and transmit meteorological

data.

Rayleigh scattering - The scattering of light by a body with a particle diameter less

than 0.03 micrometers Dominant form of light scattering in the upper atmosphere,

which produces the blue color of the sky. It is caused by atmospheric particulates that

have very small diameters relative to the wavelength of the light, such as dust particles

or atmospheric gases like nitrogen and oxygen.

reflective spectrum - Portion of the optical spectrum, from approximately 0.38 to 15

micrometers, that defines the direct solar radiation used in remote sensing.

reflectivity - Ratio of intensity of the total radiation reflected from a surface to the

total radiation incident on the surface.

RDCF – (Radiometric Calibration and Development Facility) provides calibration

support for nearly all BUV space-based instruments.

S

satellite zenith angle - Angle between the position of a satellite and the zenith, which

is the point directly over the observed target.

SBUV (Solar Backscatter Ultraviolet) – method and instrument for measurements the

vertical distribution of ozone in the atmosphere.

SAGE (Stratosphere Aerosol and Gas Experiment) – Instrument employing a solar

occultation technique to define the amount of ozone and other trace gases by

measuring sunlight that comes trough the atmosphere at different altitudes.

seasonal cycle - The annual cyclical pattern in any atmospheric variable, whether

temperature or trace gas concentration, caused by the seasons. Also called an annual

cycle.

signal processor - Processor located within the electronics system, that consists of

multiple voltage to frequency converters that are responsible for converting an

incoming signal from optical to digital.

solar cycle - Periodic change in sunspot activity. One cycle is approximately 11.1

years.

solar vector - Direction of an incoming solar radiation beam. Used in conjunction

with the position of a spectrometer's diffuser plate to calculate albedo.

solar zenith angle - Angle between the position of the Sun and the zenith, which is

the point directly over an observed target.

SOLSE (Shuttle Ozone Limb Sounding Experiment) - Space Shuttle (STS-87,

October 1997) instrument that vertical ozone profiles can be measured using light

scattered at the Earth's "limb".

spectrometer - Instrument used to determine the distribution of energy within a

spectrum of wavelengths.

stratopause - Transition layer between the stratosphere and mesosphere. Marks the

maximum altitudinal temperature increase within the stratosphere. It occurs at an

atmosphere height of approximately 50 km; however this depends on latitude.

stratosphere - Portion of the atmosphere between the tropopause, at approximately 8

to 15 km, and 50 km in altitude, depending upon latitude, season, and weather.

sulfur dioxide (SO2) - Chemical compound that absorbs radiation of the same

wavelength absorbed by ozone. Product of large volcanic eruptions.

sulfuric acid (H2SO4) - Heavy, corrosive, oily acid. Vigorous oxidizing agent. Ozone

concentrations may be affected by reactions on the surface of sulfuric acid clouds,

resulting from major volcanic eruptions.

sunspot - Relatively dark, sharply defined region on the Sun associated with an

intense magnetic field.

surface pressure - Pressure at an observation point on Earth's surface.

T

thermopause - Transition layer between the thermosphere and exosphere, located at

approximately 600 km in altitude.

thermosphere - Region of the atmosphere in which temperature increases with

altitude. Located at approximately 100 to 400 km.

TOMS (Total Ozone Mapping Spectrometer) - instruments measure the total amount

of ozone in a vertical column of the atmosphere. A series of four instruments has been

making daily global maps of the earth's ozone field.

total ozone (TO) - Amount of ozone, measured from Earth's surface to the top of the

atmosphere, over a given surface area.

trace gas - A minor constituent of the atmosphere. The most important trace gases

contributing to the greenhouse effect are water vapor, carbon dioxide, ozone, methane,

ammonia, nitric acid, nitrous oxide, ethylene, sulfur dioxide, nitric oxide, CFC-11,

CFC-12, methyl chloride, carbon monoxide, and carbon tetrachloride. Such trace

gases are sometimes referred to as trace species.

tropopause - Boundary between the troposphere and stratosphere, from 8 km in the

polar regions to 15 km in the tropics. Marks the vertical limit of most weather

phenomena.

troposphere - Lowest region of the atmosphere, defined by a steady decrease in

temperature with altitude. Extends to approximately 15 km above Earth's surface. The

troposphere is characterized by decreasing temperature with height, appreciable

vertical wind motion, appreciable water vapor content, and weather.

tropospheric - Having to do with the lowest region of the atmosphere, which extends

to approximately 15 km above Earth's surface.

U

UARS (Upper Atmosphere Research Satellite) - NASA satellite launched in

September, 1991 with instruments to measure temperature, wind, and composition of

the upper atmosphere. Instrument made the first space-based measurements of clorine

monoxide, a principal compound in ozone-depletion chemistry.

ultraviolet (UV) - a portion of the electromagnetic spectrum with wavelengths shorter

than visible light. The sun produces UV radiation, which is commonly split into three

bands: UVA, UVB, and UVC.

UVA (wavelengths 320-400nm) is radiation not absorbed by ozone.

UVB (280-320nm) is mostly absorbed by ozone, although some reaches the Earth, and

has several harmful effects.

UVC (wavelengths shorter than 280nm) is completely absorbed by ozone and normal

oxygen.

V

vibrational energy level - The energy associated with the vibrational motion of an

atoms in molecule.