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Atmospheric chemistry

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. z.

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Atmospheric chemistry

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

  2. Temperature and pressure variations in the atmosphere Heating by exothermic photochemical reactions Convective heating from surface. Absorption of ir (and some vis-uv) radiation z Barometric equation p = p0exp(-z/Hs)

  3. Rearranging and integrating we obtain the hydrostatic equation: p = p0exp(-z/Hs) Barometric equation where Hs = (kBT/mg) = (RT/Mg), Hs 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 MR = 28.8 - Hs = 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 r = mN/V = mNA(p/RT). where m is the molecular mass Equating forces gives dp = -grdz 1. = -gm(p/kBT)dz Variation of pressure with altitude p+dp z dz p A

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

  5. 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 1st law of TD: dU = dq + dw; dw = -pdV adiabatic so dq=0, p work only. Now dH = dU + pdV + Vdp = Vdp But dH = CpdT so CpdT = VdP = -VrgdZ (from eq 1) For unit mass of gas, this molar equation is changed and Cp becomes cp, the heat capacity of 1 kg of gas, and r= 1/V, so the dry adiabatic lapse rate = Gd = -dT/dz = g/cp On earth,Gd is 10.7 K km-1. If the actual atmospheric temperature gradient, -(dT/dz)atm < Gd then the atmosphere is stable – in attempting to rise, the air packet cools by expansion, becomes more dense than its surroundings and so it doesn’t rise. If -(dT/dz)atm > Gd then convection occurs. The presence of condensable vapour affects the calculation

  6. Adiabatic vs atmospheric temperature profiles

  7. 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.

  8. Atmospheric transport • Random motion – mixing • Molecular diffusion is slow, diffusion coefficient D ~ 2x10-5 m2 s-1 • Average distance travelled in one dimension in time t is ~(2Dt). • In the troposphere, eddy diffusion is more important: • Kz ~ 20 m2 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)

  9. 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.

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

  11. Horizontal transport timescales

  12. Photochemistry and kinetics

  13. Absorption spectra and photodissociation O2 O(3P) + O(3P) Threshold  = 242 nm O2 O(3P) + O(1D) Threshold  = 176 nm

  14. Measurement of rate constantsLaser flash photolysiswith laser induced fluorescence reactant OH precursor He/N2 Nd:YAG Laser Doubled 532 nm Dye Laser 283 nm KrF Excimer Laser 248 nm PMT To pump Vary time delay between two pulses and build up decay profile for the radical Computer Boxcar Pulse generator

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

  16. Pressure dependent results OH + C2H2 1 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, 5633-5642

  17. Evaluation of kinetic data (http://www.iupac-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 + CH4 → H2O + CH3 k298 = 6.4 x 10-15 cm3 molecule-1 s-1 Δlog k298 = ±0.08 k(T) = 1.85 x 10-12 exp(-1690/T) cm3 molecule-1 s-1 for T =200-300 K Δ(E/R)/K = ±100 Based mainly on experimental data from three labs

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