1. Greenhouse gases/ Atmospheric Structure
Chapter 3—Part 3
Greenhouse gases/
Atmospheric Structure

2. Te4 = So(1-A) 4 Review from last time…
We used the balance of incoming and outgoing energy to establish an equation for the effective radiating temperature of the earth:
Te4 = So(1-A) 4
This is the temperature that Earth’s surface would have if it did not have an atmosphere (but if the albedo was the same as today)

3. Te4 = So(1-A) 4 Dividing both sides of the equation by 4 gives:
Te4 = (1370 W/m2)(1-0.3)
4 (5.67 x 10-8 W/m2/K4)
Te4 = 4.23 x 109 (K4)
Te = 255 K (or -18oC)
(which is about 0oF)

4. The observed temperature (Ts) is 288 K,
which is 15oC, or about 59oF.
The difference between observed and expected temperatures (Tg):
Tg = Ts - Te
Tg = 288 K – 255 K
Tg = + 33 K (or 33oC)
We call this warming the greenhouse effect, and is due to absorption of energy by gases in the atmosphere.

5. The greenhouse effect:
Heat is absorbed and
reemitted by gases in the atmosphere. Earth naturally has a greenhouse effect of +33oC.
Often we hear that outgoing
heat energy is “trapped” by
greenhouse gases. This is not really true. The heat eventually does escape.

6. The concern is that the amount of greenhouse warming will increase with the rise of CO2 due to human activity.

7. Warming results from interactions of gases in the atmosphere with incoming and outgoing radiation.
To evaluate how this happens, we will focus on both the composition and structure of the Earth’s atmosphere.

8. Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas concentration (%)
N Major
O constituents
Ar 0.9

9. Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas concentration (%)
N2 78
O2 21
Ar 0.9
H2O variable
CO Minor
CH constituents
N2O O3

10. Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas concentration (%) ppm
N2 78
O2 21
Ar 0.9
H2O variable CO CH
N2O
O to 0.01
(stratosphere-surface)

11. Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas concentration (%)
N2 78
O2 21
Ar 0.9
H2O variable
CO ppm CH
N2O
O to 0.01
(stratosphere-surface)
greenhouse
gases

12. Greenhouse Gases

13. Water
Methane

14. N2 O2 N  N O = O Non-greenhouse Gases
What distinguishes these gases from
greenhouse gases?

15. N  N O = O Non-greenhouse Gases Answer: Symmetry!
(Technically speaking, greenhouse gases
have a dipole moment whereas N2 and O2
don’t)

16. O H H Oxygen has an unfilled outer shell
(−) O H H
(+)
Oxygen has an unfilled outer shell
of electrons (6 out of 8), so it wants
to attract additional electrons. It gets
them from the hydrogen atoms.

17. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.

18. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.
Water H O H
oxygen is more electronegative than hydrogen

19. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.
Water
Electron-poor region H O H
oxygen is more electronegative than hydrogen
Electron-rich region

20. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.
Water
Electron-poor region:
Partial positive charge H O H
oxygen is more electronegative than hydrogen
Electron-rich region:
Partial negative charge

21. Vibration Molecules absorb energy from radiation.
The energy increases the movement of the molecules.
The molecules rotate and vibrate.
stretching
bending
Vibration

22. Approximate absorption regions
H2O O3
CO2
H2O
Radiant energy
Sun
Earth
0.1
1.0 10 15
100
 (m)

23. Thermal IR Spectrum for Earth
H2O pure rotation
H2O vibration/rotation
CO2 (15 m)
(6.3 m)
O3 (9.6 m)
Note that wavelength increases towards the left in this
diagram..
Ref.: K.-N. Liou, Radiation and Cloud Physics Processes in the Atmosphere (1992)

24. Distribution of Gases in the Atmosphere
Most gases are well mixed and distributed evenly
throughout the lowermost 100 km of the atmosphere.
Examples: O2, N2, Ar, CO2, freons
Gases with short lifetimes are not well-mixed.
Example: O3

25. Structure of the Atmosphere
Pressure = force per unit area
(exerted by a gas or liquid
on a surface)
At sea level, P = 1 atmosphere
= bar (or 1013 mbar)
Pressure decreases away from the Earth’s surface. The air becomes “thin” at high elevations.

26. The Barometric Law Pressure declines exponentially with altitude
Thus, it forms a (nearly) straight line when plotted on a log scale 

27. 100 80
Altitude
(km) 60 40 20
Pressure (mbar)

28. 100 80
Pressure decreases away from the Earth’s surface.
Altitude
(km) 60 40 20
Pressure (mbar)

29. 100 80
Altitude
(km) 60 40 20
( oC)
Temperature (K)

30. 100 80
Altitude
(km) 60 40 20
Temperature decreases
Temperature (K)

31. 100 80
Altitude
(km) 60 40 20
Troposphere
0-10 km
Temperature decreases
Temperature (K)

32. 100 80
Altitude
(km) 60
Temperature increases 40 20
Troposphere
0-10 km
Temperature (K)

33. 100 80
Altitude
(km) 60
Temperature increases
Stratosphere
10-50 km 40 20
Troposphere
0-10 km
Temperature (K)

34. 100 80
Mesosphere
50-90 km
Temperature decreases 60
Stratosphere
10-50 km 40 20
Troposphere
0-10 km
Temperature (K)

35. Temperature increasesoC
Thermosphere
90 + km
100
Temperature increases 80
Mesosphere
50-90 km 60
Stratosphere
10-50 km 40 20
Troposphere
0-10 km
Temperature (K)

36. Troposphere
heated by convection
turbulent, mixed
contains all weather (wind, rain, clouds, etc.)
water is important in this region
Stratosphere
not well mixed, or “stratified”
cold at base, warmer in upper region
ozone present
ozone heats upper region by absorbing uv
radiation

37. Stratosphere 10-50 km Troposphere 0-10 kmoC
100 80 60
ozone
Stratosphere
10-50 km 40 20
water
Troposphere
0-10 km
Temperature (K)