Atmosphere Stratification of Atmosphere Earth Radiation Balance

1. Atmospheric Physics
Stratification and Earth’s Energy Balance
Prashant Mehta
Assistant Professor, National Law University, Jodhpur

2. 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.

3. 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

4. Inference
1. There are identifiable sources of the gases.
2. The concentrations in atmosphere are fairly constant on smaller time frame
3. 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.

5. 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.

7. 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.

8. 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

9. 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

11. 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.

13. 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

14. 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.

16. 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%).

17. 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.

18. The atmospheric energy balance
Note: the incoming energy balances the outgoing energy.
235 (LW out) + 107 (SW out) = 342 (SW in)

21. 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

22. 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.

25. 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.

29. 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…

30. 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