Atmosphere

Atmospheric PhysicsStratification and Earth’s Energy Balance

Prashant MehtaAssistant Professor, National Law University, Jodhpur

Introduction

• The atmosphere surrounding the earth is an envelop of gases.• It is believed to extend to a height of about 2000 km.• The structure of the atmosphere and the properties of different atmospheric

layers greatly control the atmospheric chemistry. • The atmosphere forms a thin layer around the earth, with the pressure

decreasing exponentially from the earth upwards.

Earth’s Atmosphere In Relation To Venus and Mars

Earth is situated between Venus and Mars (Sun-Venus-Earth-Mars)

Expectation: Earth’s composition and other properties to lie between Venus and Mars

Fact: This is not true

In Earth’s atmosphere O2 and N2 are unexpectedly high.

Earth’s atmosphere is , therefore, expected to consist primarily of oxidized compounds and O2 is expected to be used in oxidizing gases,

e. g., N2 ---------- NO3- , H2----------H2O, CH4---------- CO2 + H2O

Body/ Surface Temp. CO2, ppm N2, ppm O2, ppm

Venus 955000 35000 >0.3

Earth 350 780840 209460

Mars 953200 27000 1300

Inference

1. There are identifiable sources of the gases.2. The concentrations in atmosphere are fairly constant on smaller time frame3. So to keep the concentrations at a constant level, there must be sinks also.4. Sources and sinks have been used calculating life times of pollutants.5. To a first approximation, Lithosphere, Hydrosphere and Atmosphere constitute a

closed system.6. Total quantity of elements is fixed, although distribution between elements and

combined forms can alter.7. So if a species appears in the atmosphere at a rate, this rate should be equal to

its rate of disappearance.8. Hence elements must be passing through a cycle of chemical and physical

processes.

Atmospheric Layers

The nature of variation in temp. divides the atmosphere in four layers.

1. Troposphere (11 km) – Bottom layer, Continuous temperature decrease, Air for breathing, Prevents mixing with layer above it.

2. Stratosphere( above troposphere to 45 km) - Continuous temperature increase, Drier and less dense with little vertical mixing, contains UV blocking ozone.

3. Mesosphere(above stratosphere to 80 km) - Continuous temperature decrease, extremely low air pressure

4. Thermosphere( above 80 km) - Continuous temperature increase, topmost layer.

Troposphere

• It contain most of the atmospheric gases. about ½ of the total mass of atmospheric is found in lower 5 km.

• By far the troposphere is the most important layer as all weather phenomenon occur in this layer.

• Its height is about 0-11 km which varies with latitude, being lower at poles and higher at equator.

• It contains most of water. • It is different from the layers above as it mixes thoroughly through

convection. • Lapse rate. In general, there is a steady decrease in temperature in

troposphere with increase in altitude. The lapse rate is about 6 K per km.• The exchange of material through tropopause in either direction is slow.• The cooler air of the atmosphere which lies over it act as a lid. The life time

of the materials is of the order of months.

Troposphere: Chemical Composition

All weather phenomena occur in troposphere only.

Mixing time of a given hemisphere is of the order of weeks. Complete exchange between two hemispheres requires months.

Heating is through convection. Solar radiation are not directly absorbed by atmospheric gaseous in troposphere.

Gases Percentage

N2 78.08

O2 20.95

Argon 0.934

CO2 0.350

Ne, Kr, Xenon, He, Methane

Traces

Stratosphere

• The stratosphere is the second lowest layer of the atmosphere, lying between the troposphere and the overlying mesosphere, which ends at approximately 50 km elevation.

• A key feature of the stratosphere is that it contains the majority of earth’s atmospheric ozone. The air is extremely dry, and cloud formation only occurs rarely in the polar regions.

• There is reversal in the temperature gradient and with increase in height there rise in temperature, which rises to 10-20OC at 60 km. The temperature rises from the bottom to the top of the stratosphere, mostly due to heating from the interaction of UV and ozone. Here there is almost no air movement

• Mixing within is owing to horizontal mixing, very little H2O vapors are present. There is a peak in temperature increase is due to absorption of U.V. and infrared radiation by ozone and UV radiation by O2

O2 + hυ (λ<242 nm) O + O

O3 + hυ (280 < λ < 380 nm) O2 + O

Mesosphere

• The height of mesosphere is about 60-80 km.• In mesosphere there is again a temperature decrease with increase in

height and it falls up to -70 0C• This is the coldest layer of the atmosphere, and is where most meteors

burn up.• The most important chemical reaction in this layer is:

O2 + hυ O + O• There is no ozone here.• The mesosphere contains mostly ions of the same molecules that make up

the stratosphere. • Being closer to the sun, these molecules are exposed to even more

intense radiation that has the ability to simply ionize small molecules into positive ions and electrons.

Thermosphere/Ionosphere

• Thermosphere or ionosphere is above 100 km, There is steady rise in temperature and at 200 km the temperature is > 500 0C and at 700-800 km the temperature is more than 10000C.

• The increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen and nitrogen.

• The high energy UV-C radiation ionize the air present here. • Some reactions are listed below (Atkins, 1998).

O + hυ O + e-

N + hυ N + e-

O+ + O2 O2+ + O

O+ + N2 NO+ + N

O2 + hυ O2+ + e-

N2 + hυ N2+ + e-

O2+

+ N2 NO + NO+

N2+ + O N + NO+

N2+ + O2 N2 + O2

+

O2+ + e- O + O

NO+ + e- N + O

Radiation Balance of Earth

The temperature of earth’s surface and atmosphere is maintained by global energy balance between the incoming shortwave radiation from the sun, known as solar radiation, and the outgoing long-wave radiation.

The energy coming from the Sun to the Earth's surface is called solar insolation or shortwave energy.

The amount of light reflected from clouds has a definite effect on the temperature of the earth's surface. The percentage of solar energy that is reflected back to space is called the albedo. Different surfaces have different albedos.

– Separate energy balance also applies to atmosphere alone.– Energy absorbed = Energy emitted.

Global energy budget

• All bodies with heat emit Electro Magnetic radiation. • Insolation can be characterized as shortwave (SW) radiation, whereas radiation

from the atmosphere and earth can be called longwave (LW) radiation.• There is a relatively complex pathway of energy flux once insolation is incident on

the top of the atmosphere. Incoming SW can be scattered (reflected), absorbed, or transmitted by the atmosphere.

• Absorbed radiation can be reradiated, either to space, or to the earth’s surface.• Scattered radiation can be subsequently absorbed in the atmosphere, or reflected

to space…• So – the sun’s energy can be reflected back to space, absorbed by the atmosphere,

or transmitted to the earth’s surface.• The numbers – of the 342 Wm-2 incident from the sun at the top of the

atmosphere, 168 Wm-2 makes it to the surface. This represents 49% of the available energy. 107 Wm-2 is immediately reflected (31%).

Longwave Radiation

• Longwave (LW) radiation can ONLY be emitted from sources that have similar temperatures to the earth. Shortwave radiation cannot be converted to longwave radiation.

• So where does it come from? Remember that all bodies that have a temperature above absolute zero emit EM radiation. Therefore, LW radiation is emitted from sources such as the atmosphere, and the surface of the earth.

• The system therefore works like this: SW radiation is emitted from the sun, and enters the atmosphere. Some of it is reflected, some of it is absorbed, and some of it is transmitted. SW radiation reaching the surface of the earth can be reflected or absorbed. LW radiation is emitted from the atmosphere, and the earth, and can follow the pathways of SW radiation. Vegetation plays a important role in conversion of SW into LW radiation.

The atmospheric energy balanceNote: the incoming energy balances the outgoing energy.

235 (LW out) + 107 (SW out) = 342 (SW in)

Summary of energy balance terms

• Incoming SW radiation:• 168 Wm-2 absorbed by the earth’s surface• 77 Wm-2 reflected by aerosols, atmosphere, clouds• 67 Wm-2 absorbed by the atmosphere• 30 Wm-2 reflected by the surface• Total outgoing SW radiation = 107 Wm-2

• Outgoing LW radiation:• 195 Wm-2 emitted from the atmosphere (clouds responsible for about 30 Wm-2)• 40 Wm-2 emitted from the surface• Total outgoing LW radiation = 235 Wm-2

Summary of energy balance terms

• The atmosphere can also be balanced

• Into the atmosphere:• Shortwave energy absorbed by the atmosphere 67 Wm-2

• Surface radiation absorbed by atmosphere 350 Wm-2 • Thermals (convective heat) and evapotranpiration (latent heat) are absorbed by

the atmosphere in the amounts of 24 and 78 Wm-2 respectively.• Total flux into the atmosphere: 519 Wm-2.

• Losses from the atmosphere:• 324 Wm-2 emitted as back radiation.• 195 Wm-2 emitted to space.• Total flux out of atmosphere 519 Wm-2.

Summary of energy balance terms

• And finally for the surface.• Flux of energy to the surface:• 168 Wm-2 of insolation absorbed by the surface.• 324 Wm-2 reradiated from the atmosphere absorbed by the surface.• Total absorption = 492 Wm-2.

• Losses from the surface:• 390 Wm-2 emitted in surface radiation.• 24 Wm-2 and 78 Wm-2 emitted for convection and latent heat.• Total losses = 492 Wm-2.

Albedo

• The higher albedo is, the more reflective a surface is. • Some examples of albedo values are:

– Sand: 0.20-0.30– Grass: 0.20-0.25– Forest: 0.05-0.10– Water (with sun overhead): 0.03-0.05– Water (with sun near horizon): 0.50-0.80– Fresh snow: 0.80-0.85– Thick cloud: 0.70-0.80…

Residence Time

Gaseous Constituent

Residence time

CO2 3-4 years

N2O 150 years

CH4 9 years

CFC-12 >80 years

CFC-11 ≈ 80 years

CH3Cl 2-3 years

COS ≈ 2 years

O3 100 days

CS2 40 days

CO ≈ 60 days

Gaseous Constituent Residence time

Water vapor 5-10 days

Formaldehyde 1 day

SO2 1 day

NH3+NH4+ 2-10 days

NO2 0.5-2 days

NO 0.5-2 days

HCl 4 days

H2S 1-5 days

H2O2 1.0 day

Dimethyl sulphide 0.7 day

N2 1.6 X 107 years

O2 (3-10) X 103 years