1. ATS 621 Fall 2012
Lecture 2

2. Reactive species in the atmosphere

3. O(1D)
O(3P)

7. Stoichiometry

26. Chain Reaction Example
“M” is any other
molecule available for collisions

27. UNITS

28. Mixing ratio or mole fraction CX [mol mol-1]
remains constant when air density changes
e robust measure of atmospheric composition
GAS
MIXING RATIO
(dry air)
[mol mol-1]
Nitrogen (N2)
0.78
Oxygen (O2)
0.21
Argon (Ar)
0.0093
Carbon dioxide (CO2)
365x10-6
Neon (Ne)
18x10-6
Ozone (O3)
( )x10-6
Helium (He)
5.2x10-6
Methane (CH4)
1.7x10-6
Krypton (Kr)
1.1x10-6
Air also contains variable H2O vapor ( mol mol-1) and aerosol particles
Trace gas concentration units:
1 ppmv = 1x10-6 mol mol-1
1 ppbv = 1x10-9 mol mol-1
1 pptv = 1x10-12 mol mol-1
Trace
gases
Slide courtesy Colette Heald

29. "Perfect Gas" Law PV = N' kT or PV = N RT
N' = number of molecules in the air parcel
N = number of moles; N' = N x Av
k = Boltzmann constant
R = Universal Gas Constant; R = k x Av
=================================
(In meteorology texts: P = r RT - different "R“ = 287 J/kg/K)
k = × m2 kg s-2 K-1
R = 8.31 J mole-1 K-1
Slide courtesy Colette Heald

30. Number density nX [molecules cm-3]
Proper measure for
reaction rates
optical properties of atmosphere
Proper measure for absorption of radiation by atmosphere
nX and CX are related by the ideal gas law:
na = air density
Av = Avogadro’s number
P = pressure [Pa]
R = Universal gas constant
= Av k k=Boltzmann cnst
T = temperature [K]
MX= molecular weight of X [g/mol]
Also define the mass concentration (g cm-3):
Slide courtesy Colette Heald

31. Partial pressure Px [Pa]
Dalton’s law:
Proper measure for phase change
(such as condensation of water vapour)
Evaporation of liquid water from a pan:
No lid: water molecules escape from pan to atmosphere (evaporation)
Add a lid:
escaping water molecules collide on lid and return to surface; collision rate measures PH2O
eventually, flux escaping = flux returning :
saturation (PH2O,SAT)
cloud formation in atmosphere requires
PH2O > PH2O,SAT
T e PH2O,SAT
Slide courtesy Colette Heald

32. CLAUSIUS-CLAPEYRON EQUATION
A = 6.11 hPa (= Pvap at 0C)
B = 5310 K
To = 273 K
Vapour pressure increases sharply with temperature, due to the large latent heat.
PH2O,SAT (hPa)
T (K)
Slide courtesy Colette Heald

33. PHASE DIAGRAM FOR WATER
triple point of water
(n=0)
gas-liquid
metastable
equilibrium
Dew point: Temperature Td such that
PH2O = PH2O,SAT(Td)
Slide courtesy Colette Heald

34. RUNAWAY GREENHOUSE EFFECT ON VENUS
due to accumulation of water vapor from volcanic outgassing early in its history
…did not happen on Earth because farther from Sun; as water accumulated it reached saturation and precipitated, forming the oceans
EARTH
VENUS
Slide courtesy Colette Heald

38. REGIONS OF THE ATMOSPHERE
Troposphere:
generally homogeneous, characterized by strong mixing
decreasing T with increasing altitude from heat-radiating surface
near surface boundary layer exists (over the oceans ~1km depth), BL often cloud topped and can trap emissions
Tropopause:
serves as a “barrier” that causes water
vapor to condense to ice
“tropopause folding” where strat air intrudes
into lower levels  exchange mechanism
Stratosphere:
increasing T with altitude due to OZONE
causing heating from absorption of UV
Mesosphere:
absence of high levels of radiation absorbing
species and thus a T decrease
upper mesosphere and higher defines the
exosphere from which molecules and ions
can escape the atmosphere
Thermosphere:
rarified gases reach temperatures as high as 1200C by absorption of high energy radiation