Transcript 3280 – Atmospheric chemistry

Atmospheric chemistry
Day 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
z
Heating by
exothermic
photochemical
reactions
Barometric equation
p = p0exp(-z/Hs)
Convective heating
from surface.
Absorption of ir (and
some vis-uv) radiation
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
p+dp
z
dz • Equating forces gives
p
dp = -grdz
1.
= -gm(p/kBT)dz
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.
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
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
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
Absorption spectra and photodissociation
O2  O(3P) + O(3P)
Threshold  = 242 nm
O2  O(3P) + O(1D)
Threshold  = 176 nm
Measurement of rate constants
Laser flash photolysis
with laser induced fluorescence
OH
precursor
Nd:YAG Laser
Doubled 532 nm
reactant
He/N2
Dye Laser
283 nm
KrF Excimer Laser
248 nm
PMT
To pump
Computer
Pulse
generator
Boxcar
Vary time delay
between two pulses
and build up decay
profile for the
radical
Data from a Flash Photolysis Experiment
4000
1.2
3500
3000
-1
0.9
kobs / s
Fluorescence Intensity / Arbitrary Units
1.5
0.6
2500
2000
1500
0.3
1000
500
0.0
0
500
1000
time / s
1500
2000
0
0.0
2.0x10
15
4.0x10
15
6.0x10
15
8.0x10
15
-3
[C2H2] / molecule cm
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.
1.0x10
16
Pressure dependent results OH + C2H2
1
2
9.0x10
-13
8.0x10
-13
7.0x10
-13
6.0x10
-13
5.0x10
-13
This Work, CRDS [N2]
4.0x10
-13
This Work, FP-LIF [He]
This Work, CRDS [SF6]
3.0x10
-13
Sorenson et al. 2003 [N2/O2]
2.0x10
-13
Michael et al. 1980 [Ar]
Perry et al. 1982 [Ar]
Wahner and Zetzsch 1985 [N2]
1.0x10
-13
3
-13
6.0x10
-13
5.0x10
-13
-1
3
4.0x10
k / cm molecule s
-13
7.0x10
-1
-1
k (cm molecules s )
373K, He
298K, He
253K, He
253K, N2
233K, He
210K, He
-13
8.0x10
-1
-13
9.0x10
-13
3.0x10
-13
2.0x10
-13
1.0x10
0.0
-1.0x10
-13
0.0
5.0x10
18
19
1.0x10
1.5x10
19
2.0x10
-3
[M] molecules cm
19
19
2.5x10
0.0
5.0x10
18
1.0x10
19
1.5x10
19
[M] / molecules cm
2.0x10
19
2.5x10
19
-3
• 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
Evaluation of kinetic data (http://www.iupackinetic.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