Atmospheric chemistryDay 1

Structure of the atmosphere

Photochemistry and Chemical Kinetics

Textbooks

R.P.Wayne, “Chemistry of Atmospheres”, Chapter 4

(3rd Edition, OUP, 2000)

D. J. Jacob, “Introduction to Atmospheric Chemistry”,

Princeton University Press, 1999..

G.P. Brasseur, J.J. Orlando and G.S. Tyndall,

“Atmospheric Chemistry and Global Change” (OUP,

1999),

J.H. Seinfeld and S.N. Pandis, “Atmospheric Chemistry

and Physics : From Air Pollution to Climate Change”,

(Wiley, 1998),

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 = p0exp(-z/Hs)

z

Variation of pressure with altitude

• 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 = mN/V = mNA(p/RT).

where m is the molecular mass• Equating forces gives

dp = -gdz 1.

= -gm(p/kBT)dz

z p+dp

p

A

dz

Sea breeze

Reverses at night: sea coolsmore 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

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 = -VgdZ (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 = 1/V, so

the dry adiabatic lapse rate = d = -dT/dz = g/cp

On earth, d is 10.7 K km-1.

If the actual atmospheric temperature gradient, -(dT/dz)atm < d 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 > 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 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)

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

O2 O(3P) + O(3P) Threshold = 242 nm

O2 O(3P) + O(1D) Threshold = 176 nm

Absorption spectra and photodissociation

Measurement of rate constantsLaser flash photolysis

with laser induced fluorescence

Nd:YAG LaserDoubled 532 nm

Dye Laser283 nm

KrF Excimer Laser248 nm

Computer

Boxcar

PMT

OH precursor

reactantHe/N2

To pump

Pulse generator

Vary time delay between two pulsesand build up decayprofile for the radical

Data from a Flash Photolysis Experiment

0.0 2.0x1015 4.0x1015 6.0x1015 8.0x1015 1.0x10160

500

1000

1500

2000

2500

3000

3500

4000

k obs /

s-1

[C2H

2] / molecule cm-3

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.

0 500 1000 1500 2000

0.0

0.3

0.6

0.9

1.2

1.5

Flu

ores

cenc

e In

tens

ity /

Arb

itrar

y U

nits

time / s

Pressure dependent results OH + C2H2

• 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

0.0 5.0x1018 1.0x1019 1.5x1019 2.0x1019 2.5x1019-1.0x10-13

0.0

1.0x10-13

2.0x10-13

3.0x10-13

4.0x10-13

5.0x10-13

6.0x10-13

7.0x10-13

8.0x10-13

9.0x10-13

k (c

m3 m

olec

ules

-1 s

-1)

[M] molecules cm-3

373K, He 298K, He 253K, He 253K, N

2

233K, He 210K, He

0.0 5.0x1018 1.0x1019 1.5x1019 2.0x1019 2.5x10191.0x10-13

2.0x10-13

3.0x10-13

4.0x10-13

5.0x10-13

6.0x10-13

7.0x10-13

8.0x10-13

9.0x10-13

k / c

m3 m

olec

ule

-1 s

-1

[M] / molecules cm-3

This Work, CRDS [N2]

This Work, FP-LIF [He] This Work, CRDS [SF

6]

Sorenson et al. 2003 [N2/O

2]

Michael et al. 1980 [Ar] Perry et al. 1982 [Ar] Wahner and Zetzsch 1985 [N

2]

1 2

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