ATS 621 Fall 2012 Lecture 2

Reactive species in the atmosphere

O(1D) O(3P)

Stoichiometry

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

UNITS

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) (0.01-10)x10-6 Helium (He) 5.2x10-6 Methane (CH4) 1.7x10-6 Krypton (Kr) 1.1x10-6 Air also contains variable H2O vapor (10-6-10-2 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

"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 = 1.3806503 × 10-23 m2 kg s-2 K-1 R = 8.31 J mole-1 K-1 Slide courtesy Colette Heald

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

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

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

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

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

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