FSN 1500 Week 12 Stratospheric Ozone Depletion, Ground-Level Ozone and the Electromagnetic Spectrum.

FSN 1500 Week 12

Stratospheric Ozone Depletion, Ground-Level Ozone and the

Electromagnetic Spectrum

Introduction

• Although the lay press doesn’t report on stratospheric ozone depletion like it did 5-10 years ago, this issue is still a significant environmental topic with global implications (see slide)

• It’s also important that you realize the difference between stratospheric ozone (beneficial) and ground-level ozone (detrimental)

Background

• Atmospheric scientists partition the Earth’s atmosphere into five layers based on differences in their physical and chemical properties; the two layers closest to the Earth are the troposphere and the stratosphere (see figure)

Troposphere

• Troposphere - extends from the Earth’s surface to an altitude of about 15 kilometers (km); contains about 90% of all the air in the entire atmosphere; with increasing altitude air temperature decreases

Stratosphere

Stratosphere - extends from the top of the troposphere to an altitude of about 50 km; contains about 90% of the atmosphere’s naturally occurring ozone; with increasing altitude the air temperature increases

Why does the air temperature increase with increasing altitude in the stratosphere?

How does stratospheric ozone benefit life on Earth?

Stratospheric Ozone

• Stratospheric ozone benefits humans by absorbing the bulk (~ 95%) of the ultraviolet (UV) radiation released from the Sun

• Ozone - a pale blue, gaseous molecule composed of three atoms (triatomic) of oxygen (O3)

• The average concentration of ozone in the stratosphere is about 10 parts per million (ppm)

Stratospheric Ozone

• Another measure of ozone concentration is the Dobson Unit - defined as a slab of ozone of 0.01 mm thickness that would encircle the Earth; the 10 ppm average ozone concentration is equivalent to 300 Dobson units (DU)

• If the entire stratosphere’s ozone was compressed, a 3 millimeter (mm) thick slab of ozone would encircle the Earth (see slide)

Stratospheric Ozone

• In reality, there is no physical ozone layer - the ozone is distributed throughout the stratosphere with a higher concentration found at 12 - 30 km (commonly termed the ozone layer)

• The phrase “ozone hole” is also misleading; this describes a volume of the stratosphere that is markedly depleted in ozone

• Most studies now recognize an ozone hole as a section of the stratosphere where ozone concentrations lie below 220 Dobson units

Stratospheric Ozone

• Why discuss this topic? A significant body of evidence suggests stratospheric ozone levels are declining.

• Studies suggest a worldwide average 6 - 10% decline in stratospheric ozone levels during the past three decades

• Although first recognized over Antarctica, some degree of stratospheric ozone loss has affected all latitudes (see figures)

South Pole

Source: NASA

Antarctic Ozone

2010: 118

2010: ~21

Stratospheric Ozone

• Be aware; this is another topic that is frequently discussed by the news media - not always correctly!

Stratospheric Ozone

• Possible consequences of continued stratospheric ozone depletion?

• 1) Increase in skin cancer incidence as increased proportions of UV light strike Earth’s surface; the more energetic UV light can penetrate more deeply than visible light and could mutate skin cell DNA

Stratospheric Ozone

• Some dose-response models suggest a 1- 2 percent increase in skin cancer incidence for every 1 percent decline in stratospheric ozone levels

• Worldwide, reported skin cancer cases have been rising at a faster rate than predicted during the last 30 years. What could be some other contributing factors?

Stratospheric Ozone

• 2) Declines in stratospheric ozone may lead to increases in cataracts and other eye damage; the more energetic UV light could also damage the eyes

• As a group optometrists are reporting a higher occurrence frequency of the predicted eye damage

Stratospheric Ozone

• Optometrists and ophthalmologists recommend wearing sunglasses with 100% UV filtration when you venture into daylight

• How could you put yourself at further risk for eye damage if all you wore was a pair of dark sunglasses with no UV protection?

Stratospheric Ozone

• Eyeglasses, contact lenses, vehicle windshields, and commercial and residential windows now all available with UV filter options

• 3) Declines in the autoimmune response of humans would be expected if stratospheric ozone levels continue to decline

Stratospheric Ozone

• Sunburn definitely lowers the concentration and function of disease-fighting white blood cells in the body for up to 24 hours after sun exposure

• 4) Plant productivity projected to decline if stratospheric ozone levels continue to decline

Stratospheric Ozone

• The more energetic UV light can damage plant tissue just as easily as human tissue

• 5) Indirectly, declines in stratospheric ozone could lead to an enhanced greenhouse effect and global warming. How? The lower the plant productivity the less CO2 removed from the air; the more air CO2 the more radiated heat can be absorbed.

Stratospheric Ozone

• Studies suggest that ozone is created and destroyed naturally in the stratosphere according to four primary, simultaneously occurring reactions:

• O3 (g) + UV ----> O2 (g) + O + heat

• O2 (g) + O -----> O3 (g)

• O3 (g) + O ------> 2 O2 (g)

• O2 (g) + UV -----> O + O

Stratospheric Ozone

• If the preceding reactions occur normally, stratospheric ozone levels should have a concentration of 300 DU

• Since the mid-1970s, evidence has accumulated linking industrial gas emissions to accelerated rates of stratospheric ozone depletion

Stratospheric Ozone

• Major human impact? Apparently, the release of chlorofluorocarbon (CFC) gases to the atmosphere

• CFCs - a family of gases colloquially known as freons, all of them are composed of various numbers of chlorine, fluorine, and carbon atoms bonded together (see figure)

Stratospheric Ozone

• The CFCs were first produced in large volumes by Dupont chemists in the mid 1930s as a substitute refrigeration gas for the toxic chloromethane and ammonia gases that were used in very small scale at that time

• The CFCs are a perfect refrigeration gas because they’re nontoxic, noncorrosive and highly chemically unreactive

Stratospheric Ozone

• The primary use of CFCs have always been as a refrigerant gas; their wide-scale implementation allowed the US to dominate the world’s economy for close to 50 years

• When CFCs escape from a refrigeration unit, they rise (less dense than air or carried upwards by air currents) toward the stratosphere with virtually no reactivity

Stratospheric Ozone

• Laboratory studies suggest it commonly takes one to two (perhaps as long as ten) years for CFCs to rise to the stratosphere and that some of them may have residence times in the troposphere from 25 – 400 years!

• When the CFCs reach the stratosphere they react with UV light according to the reaction: CFCs + UV ---> Cl + F + C

Stratospheric Ozone

• Apparently it’s the freed Cl and F atoms that disrupt the natural ozone cycle

• How? Cl + O3 ---> ClO + O2 (destroys ozone) and ClO + O ---> Cl + O2 (releases more free Cl)

• These two reactions occur more quickly than the four natural reactions we examined earlier (see slide)

CFCs in Ozone Destruction

Source: Images courtesy NASA.

Stratospheric Ozone

• Lab studies suggest that one Cl atom may destroy as many as 100,000 ozone molecules before it’s naturally purged

• These same studies suggest a batch of CFCs could facilitate ozone depletion for 75 - 100 years!

Stratospheric Ozone

• Predicted outcome? Stratospheric ozone levels would continue to deplete

• The evidence accumulated in the late 1970s and 1980s sparked action via the signing of the Montreal Protocol in 1987 by 43, almost exclusively Western Hemisphere, countries

Stratospheric Ozone

• The signatories agreed to reduce CFC production to one-half their current levels by the end of 1999

• Many less technologically advanced countries (China, India, the USSR) resisted signing, arguing that their minor emissions of CFCs didn’t cause the problem and that they had the right to acquire the West’s living standards

Stratospheric Ozone

• By 1990, more data led to revision of the Montreal Protocol - over 100 nations (exceptions being China, India, Russia) agreed to cease CFC production by the end of 1996 with some exceptions for developing nations

• Later that year an international monetary fund was created to try to lure other countries to sign the agreement

Stratospheric Ozone

• Countries signing the agreement would be given grant monies to research CFC alternatives

• In 1992 the Montreal Protocol was again amended to set timetables for the reduction and/or elimination of the production of other (e.g., halons) substances that can deplete stratospheric ozone (as late as 2010 for some substances and developing nations)

• Currently about 200 nations have agreed to one or more provisions of the Montreal Protocol

Stratospheric Ozone

• The U.S. agreed (1992) to curtail CFC production at the end of 1995 during the waning days of the Bush (senior) Administration? Why?

• The CFC production ban allowed freons produced before the ban started to be used after January 1, 1996

• The CFC ban temporarily produced another Black Market economy! In 1996 the value of illicit CFCs smuggled into the US was second only to the value of illicit cocaine

Stratospheric Ozone

• What are the CFC substitutes now being used in new air conditioners, refrigerators and other cooling units? Two types: HFCs - hydrofluorocarbons and HCFCs - hydrochlorofluorocarbons

• The slight chemistry differences from CFCs results in substances that are reactive in Earth’s lower atmosphere

Stratospheric Ozone

• If the refrigerant gases react in the lower atmosphere there’s a much lesser chance of their Cl and F components ever reaching the stratosphere

• Are the substitutes safe? Some evidence suggests that the most commonly used CFC substitute may react to form another type of acid rain detrimental to wetlands and is a potent greenhouse gas!

Stratospheric Ozone

• Updates:

• In early October 2006 the area of the Antarctic ozone hole was the largest ever measured and correlated to below average stratospheric temperatures

• Possible global warming connection? (see figures)

Stratospheric Ozone

Source: Science News

Source: NASA

Antarctic Ozone2010: ~21

2010: 118

Stratospheric Ozone

• The figure on the right is from November 7, 2008

• The following link allows you to determine the stratospheric ozone level overhead for anywhere up to two days beforehand

• http://toms.gsfc.nasa.gov/teacher/ozone_overhead_v8.html

Stratospheric Ozone

Overall, these data (hopefully) suggest that stratospheric ozone levels should begin to show modest increases during the next 30-50 years as the Montreal Protocol provisions lower ozone-depleting chemicals in the upper atmosphere

Ground-level Ozone

• Don’t confuse stratospheric ozone with ground-level ozone

• Ground-level ozone - ozone produced in the lower troposphere; a component of photochemical smog

• Photochemical smog is generated by UV light interacting with fossil fuel combustion gases; especially nitrogen oxides (NOx)

Los Angeles, CA photochemical smog

Ground-level Ozone

• NOx + UV ----> N + O; subsequently this reaction, O2 + O ---> O3 , generates ground-level ozone

• You’ve probably smelled ground-level ozone after a nearby lightning strike

• Who cares?

• The American Lung Association cites ground-level ozone as a severe pollutant

Ground-level Ozone

• Studies suggest that prolonged exposure to even seemingly minor amounts (150 parts per billion) of ground-level ozone could cause permanent lung tissue scarring and loss of pulmonary function

• How does this relate to the “Ozone Action Days” we hear about each summer in southeast Michigan?

Ground-level Ozone

• What activities are we asked to curtail or reduce on these days?

• What impact does this issue have for when and where we conduct our outdoor aerobic exercise?

Fairly Recent News

Astronomy

• Astronomers are the most “disadvantaged” natural scientists since they rarely have the opportunity to directly sample their objects of interest

• Most of our astronomical knowledge has been gained indirectly by studying various regions of the electromagnetic spectrum


• Electromagnetic spectrum - refers to a broad band of energies created by the movement of charged particles, mostly electrons

• Origin of name? The charged particle movement generates an electric field and with every electric field there is an associated magnetic field, the name electromagnetic summarizes the electric and magnetic properties


• The term spectrum refers to the broad band of energies resulting from the electron motion (see slide)


• Studies illustrate that all electromagnetic energy has a wave-like component of movement; e.g., think of the movement of a water wave

• All waves have terms used to describe them, including the terms wave crest, wave trough, wave height, wavelength and wave frequency

• See slide

Wave Characteristics

• Wave crest - highest elevation point of wave• Wave trough - lowest elevation point of wave• Wave height - vertical distance between the wave

crest and wave trough• Wavelength - horizontal spacing between two

adjacent wave crests or two adjacent wave troughs


• Wave frequency - the number of wave crests, or troughs, that pass an observation point per unit of time; one wave crest or trough passing a counter every second is defined as a frequency of 1 cycle per second (1 cps) (see figure)

• 1 cps = 1 hertz (Hz)

• How is the term “hertz” familiar to you?


• Important relationship: the higher a wave’s frequency, the greater its impact or penetration energy and the shorter its wavelength

• Compare two wave examples drawn on board - which has the higher frequency and therefore the greater energy?


• In our survey of the electromagnetic spectrum, note the great variance in wave frequencies and wavelengths as we move from the higher energy end (e.g., cosmic and X-rays) to the lower energy end (radio waves) (see slide)

• Can you tell me why UV light is potentially more damaging than visible light?


• Within the visible portion (violet, blue, green, yellow orange, red) of the electromagnetic spectrum, each color we see has a specific wavelength and frequency

• See slide


• Notice also that the Earth’s atmosphere is opaque (can’t be effectively penetrated), semi-transparent or transparent (essentially uninhibited penetration) to different energies of the electromagnetic spectrum (see slide)

• In addition, how does a microwave oven cook food?

Astronomy Applications

• Astronomers study portions of the electromagnetic spectrum by using light, radio, X-ray, and microwave telescopes commonly in their research

• Applications?

• Much of our knowledge of the composition of celestial objects (e.g., stars) stems from observations of the light they emit


• Background: you’ve all seen how white light is separated into its spectral colors by a rainbow or prism (see figure)


• Astronomers use special devices called spectrometers to study the light emitted by celestial objects

• Each element (compounds too) has its own light fingerprint

• Use the following slide to discuss why

Element heated or exposed to electricaldischarge until it emits light (glows)

Light emitted by element

Element-specific spectral line colors and spacing


• Why does each element have an unique light fingerprint? Think of the atom structure. A nucleus surrounded by electrons that occupy regions of space called energy level shells

• Electrons can be apparently boosted from the ground (undisturbed) state to a higher energy level (excited state) by the absorption of photons (see slide)

Higher energylevel thanground state


• Photon - a small package of energy (e.g., light, electrical discharge)

• The electrons boosted to a higher energy state are not stable since everything in the universe attempts to reach the lowest energy, highest equilibrium state

• At some point the electrons collapse downward to their original ground state energy level, emitting flashes of light


• Since the number of electrons and the distance between electron energy levels in each atom type (e.g., C, O, Na, etc.) is unique, the emitted light flashes (photons) are also unique

• This light can be separated into its unique spectral colors and spacings by spectrometers

• See slides

(For hydrogen)


• On Earth, we excite electrons in atoms of different elements (compounds too), one at a time, in a laboratory (see slide) and use spectrometers to record their spectral components (characteristic colors and spacings)


• Astronomers use light telescopes outfitted with spectrometers to identify elements (and compounds) in celestial objects that match the lab results

• Today the light telescopes are outfitted with spectrometers that are interfaced with computers; the computers are used to distinguish the chemical composition of objects from the blend of light they emit

• Think about this process when you next gaze upon a star!


• Let’s go to the adjacent room and view a demonstration of the characteristic light spectra of selected elements

• Once we get there, be able to explain to me how “neon” lights work and the origin of the Earth’s aurora (e.g., Aurora Borealis and Aurora Australis)


• So next class we’ll begin our overview of specific astronomy topics; we’ll learn that space probes, satellites and improved telescopes are rapidly increasing our knowledge of astronomy

• I’ll whet your appetite with a short, time-lapse movie/simulation capturing the descent of the Huygens space probe, launched from its parent probe Cassini, to the surface of Saturn’s moon Titan in January 2005 (about 720 million miles from Earth)

An Update

FSN 1500 Week 12 Stratospheric Ozone Depletion, Ground-Level Ozone and the Electromagnetic Spectrum.

Documents

slide slide

ozone concentrations

stratospheric ozone

stratospheric ozone

average concentration

ppm slide

measure of ozone concentration

figures slide