Atmospheric chemistry Day 1 Structure of the atmosphere Photochemistry and Chemical Kinetics

Temperature and pressure variations in the atmosphere Heating by exothermic photochemical reactions Convective heating from surface. Absorption of ir (and some vis-uv) radiation Barometric equation p = p 0 exp(-z/H s ) z

Variation of pressure with altitude Rearranging and integrating we obtain the hydrostatic equation: p = p 0 exp(-z/H s ) Barometric equation where H s = (k B T/mg) = (RT/Mg), H s is termed the scale height and is the height gain over which the pressure falls by a factor of 1/e NB:- Assumes T is constant - compare with Boltzmann distribution - Average M R = H s = 6 km for T = 210 K; and 8,5 km for T = 290 K. Consider section dz in a column of air, cross sectional area A. The density of the air is  = mN/V = mN A (p/RT). where m is the molecular mass Equating forces gives dp = -g  dz 1. = -gm(p/k B T)dz z p+dp p A dz

Sea breeze Reverses at night: sea cools more slowly than land

Convective mixing in the troposphere: Dry adiabatic lapse rate Consider a packet of air rising in the troposphere. Assume process is adiabatic, so temperature of the air packet decreases as z increases 1 st law of TD: dU = dq + dw; dw = -pdV adiabatic so dq=0, p work only. Now dH = dU + pdV + Vdp = Vdp But dH = C p dT so C p dT = VdP = -V  gdZ(from eq 1) For unit mass of gas, this molar equation is changed and C p becomes c p, the heat capacity of 1 kg of gas, and  = 1/V, so the dry adiabatic lapse rate =  d = -dT/dz = g/c p On earth,  d is 10.7 K km -1. If the actual atmospheric temperature gradient, -(dT/dz) atm  d then convection occurs. The presence of condensable vapour affects the calculation

Adiabatic vs atmospheric temperature profiles

Boundary layer (BL) Height = 500 – 3000 m. Mixing near the surface is always fast because of turbulence During the day, the earth heats the surface layer by conduction and then convection mixes the region above in the convective mixed layer. There is usually a small T inversion (dT/dz >0) above this which marks the top of the BL. This slows transfer from the BL to free troposphere (FT). Traps pollutants. Night – surface cools, dT/dz > 0 in surface layer – surface inversion. Confines pollutants to surface layer. Can get extreme inversions in the surface layer in winter that can lead to severe pollution episodes. High build up of pollutants.

Atmospheric transport Random motion – mixing –Molecular diffusion is slow, diffusion coefficient D ~ 2x10 -5 m 2 s -1 –Average distance travelled in one dimension in time t is ~  (2Dt). –In the troposphere, eddy diffusion is more important: –K z ~ 20 m 2 s -1. Molecular diffusion more important at v high altitudes, low p. Takes ~ month for vertical mixing (~10 km). Implications for short and long-lived species. Directed motion –Advection – winds, e.g. plume from power station. –Occurs on Local (e.g. offshore winds – see earlier) Regional (weather events) Global (Hadley circulation)

Winds due to weather patterns As air moves from high to low pressure on the surface of the rotating Earth, it is deflected by the Coriolis force.

Global circulation – Hadley Cells Intertropical conversion zone (ITCZ) – rapid vertical transport near the equator.

Horizontal transport timescales

Photochemistry and kinetics

O 2  O( 3 P) + O( 3 P) Threshold = 242 nm O 2  O( 3 P) + O( 1 D) Threshold = 176 nm Absorption spectra and photodissociation

Measurement of rate constants Laser flash photolysis with laser induced fluorescence Nd:YAG Laser Doubled 532 nm Dye Laser 283 nm KrF Excimer Laser 248 nm Computer Boxcar PMT OH precursor reactant He/N 2 To pump Pulse generator Vary time delay between two pulses and build up decay profile for the radical

Data from a Flash Photolysis Experiment OH + X  products; [X] >> [OH] (pseudo 1 st order conditions) d[OH]/dt = - k[OH][X] = -k’[OH] (k’ = k[X]) [OH] = [OH] 0 exp(-k’t) Analyse exponential decay to obtain k’. Vary [X] Plot k’ vs [X] to obtain k.

Pressure dependent results OH + C 2 H 2 Plot 1 shows the pressure dependence vs T, mainly in He. Note that the reaction is quite close to the high pressure limit at 210 K and 1 bar. Plot 2 shows the a comparison between Leeds and other room T data. Physical Chemistry Chemical Physics, 2006, 48,

Evaluation of kinetic data ( kinetic.ch.cam.ac.uk) Database of evaluated kinetic data. Recommendations from a panel of experts who assess the available experimental data. e.g. Summary of Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry Section I – Ox, HOx, NOx and SOx Reactions IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry Also covers organic compounds, halogens, sulfur, photolysis (cross sections, quantum yields). Some data on accommodation coefficients. Includes ~ 600 reactions. Example of evaluation: HO + CH 4 → H 2 O + CH 3 k 298 = 6.4 x cm 3 molecule -1 s -1 Δlog k 298 = ±0.08 k(T) = 1.85 x exp(-1690/T) cm 3 molecule -1 s -1 for T = K Δ(E/R)/K = ±100 Based mainly on experimental data from three labs