Atmospheric chemistry

1
Atmospheric chemistry

• Day 1
• Structure of the atmosphere
• Photochemistry and Chemical Kinetics

2
(No Transcript)
3
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)
4
Variation of pressure with altitude

• 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

pdp
z
dz
p
A

• 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.

5
Sea breeze
Reverses at night sea cools more slowly than land
6
Convective mixing in the troposphereDry
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 dq0, 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 lt 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 doesnt rise. If
  -(dT/dz)atm gt Gd then convection occurs.
• The presence of condensable vapour affects the
  calculation

7
Adiabatic vs atmospheric temperature profiles
8
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 gt0) 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 gt 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.

9
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)

10
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.
11
Global circulation Hadley Cells

• Intertropical conversion zone (ITCZ) rapid
  vertical transport near the equator.

12
Horizontal transport timescales
13
Photochemistry and kinetics
14
Absorption spectra and photodissociation
O2 ? O(3P) O(3P) Threshold ? 242 nm O2 ?
O(3P) O(1D) Threshold ? 176 nm
15
Measurement of rate constantsLaser flash
photolysiswith laser induced fluorescence
reactant
OH precursor
He/N2
NdYAG 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
16
Data from a Flash Photolysis Experiment

• OH X ? products X gtgt OH (pseudo 1st order
  conditions)
• dOH/dt - kOHX -kOH (k kX)
• OH OH0exp(-kt)
• Analyse exponential decay to obtain k.
• Vary X
• Plot k vs X to obtain k.

17
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

18
Evaluation of kinetic data (http//www.iupac-kinet
ic.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
• ?log k298 0.08
• k(T) 1.85 x 10-12 exp(-1690/T) cm3 molecule-1
  s-1
• for T 200-300 K
• ?(E/R)/K 100
• Based mainly on experimental data from three labs