Post on 13-Jan-2016
description
The Atmosphere as aChemical Reactor
OutputsInputs
Chemistry
Radiation (energy)
Biogeochemical Cycling
Temperature Structure of the Atmosphere
from: http://www.uwsp.edu/geo/faculty/ritter/images/atmosphere/structure_composition/atmospheric_structure.jpg
Pressure Structure of the Atmosphere
from: http://okfirst.ocs.ou.edu/train/meteorology/graphics/VertTP.gif
H
Z
oePP
km4.7Mg
RT H
Mixing Ratio vs. Concentration• Mixing Ratio – the fraction of air by volume that
is composed of a certain compound.– The fraction of N2 is 0.80 or 80%– The fraction of N2O is 3.6x10-7 or 3.6x10-5% or 360
ppbv.
• Concentration (number density) – number of molecules of a specified compound contained in a fixed volume.– [N2O] at 1010 mbar = 8.86x1012 molecules/cm3
– [N2O] at 500 mbar = 4.38x1012 molecules/cm3
The atmosphere has a density of
2.69x1019 molecules/cm3 at sea level
Chemical Composition of the Atmosphere
Compound Mixing Ratio
N2 0.78
O2 0.21
Ar 0.0093
CO2 3.6x10-4
Ne 8x10-6
He 5.2x10-6
CH4 1.7x10-6
Kr 1.1x10-6
H2 5.0x10-7
N2O 3.6x10-7
Xe 9.0x10-8
O3 1 – 10x10-8
H2O
Vertical Mixing of the Troposphere
• Boundary Layer – hours to days
• Free Troposphere – days to weeks
• Tropopause - years
Horizontal Mixing of the Troposphere
from: Jacob
Horizontal Mixing of the Troposphere
Kinetics
B A AB 1k
[AB]k[AB][B][A]
1
dt
d
dt
d
dt
d
D C B A 2k
[A][B]k[D][C][B][A]
2
dt
d
dt
d
dt
d
dt
d
Kinetics
22k
24 CO O2H 2O CH
224
4 ]][Ok[CH][CH
dt
d
Only true for elementary reactions!
Lifetime, Pseudo First Order • First order reaction:
• Second order reaction:
1k
1 AB])([
B][k
1
k
1 A])([
2
D C B A 2k
B A AB 1k
B][kk where[A];k[A][B]k[A]
22 dt
d
Lifetime• Some compounds have multiple sinks
• Methane
chemical loss in troposphere by OH ≈ 12yrs
physical deposition to the soil ≈ 100 yrs
chemical loss in stratosphere ≈ 40 yrs
Overall lifetime: 40
1
100
1
12
11111
stratsoilOHtot
yrs5.8tot
Rate ConstantsEffect of Temperature on Rate Constant
1.0E-16
1.1E-15
2.1E-15
3.1E-15
4.1E-15
5.1E-15
6.1E-15
7.1E-15
200 220 240 260 280 300
Temperature (Kelvin)
k (c
m3/
mo
lecu
les
s)
RT
Ea
A k
e
J Values
• Photochemistry
320nm D);O( O O 12
j3 h
][O][O
33 j
dt
d
max
min
)()()( o
dj
Radical Chemistry
• Radicals do the work of the chemistry in the atmosphere.
• Produced by photochemical reactions and reactions by other radicals
• O(3P) is a bi-radical
• O(1D) is not a radical, but it is very reactive
The OH Radical
OH2OHD)O(
320nm D);O( O O2
1
k2
1
12
j3
h
M P)O(MD)O( 3k1 3
Production of OH Radical
O]D)][H[O(k2[OH]
21
2dt
d
Assume steady-state for O(1D)
D)][M][O(kO]D)][H[O(k]O[ 132
123 j
Production Loss
Production of OH RadicalSolve for [O(1D)]
[M]kO][Hk
]O[D)][O(
322
31
j
Substitute into the production of OH
M][kO]H[k
O]][H[Ok2[OH]
322
2321
j
dt
d
Oxidation of CO
MOMO O
ONOhNO
OHNONOHO
MHOMO H
HCOOH CO
32
2
22
22
2
32h
2 OCO2O CO :reactionnet
The Chemical Family NOx
22k
3
3k
2
J2
ONONOO
MOMOO
ONOh NO
2
1
1
3h
2 ONONO
The Chemical Family NOx
NO][k
][NOj]O[
]O[k
j
][NO
[NO]
0[NO]
NO]][O[k-][NOj[NO]
2
213
32
1
2
3221
dt
d
dt
d
Coupling of NOx and HOx
3h
2 ONO NO
2HO OHCO
NOslow
fast
Sinks for NOx and HOx
OHROHRH
rainoutOOHHOHO
rainoutHNO NOOH
2
22222
32
2222
22
HOOHOHOH
2OHhOH
O3 Production
formed O No ONONOO
OOHHOHO :NO Low
production O OHNONOHO :NOHigh
3223
22222x
322x
Oxidation of Formaldehyde
COHOOHCO
OHHCO OHHCHO
COH
HCOH hHCHO
22
2
2
Maximum O3 production = 3 molecules (high NOx)Maximum HOx production = 2 molecules (high NOx)
Minimum O3 production = 0 molecules (low NOx)Minimum HOx producion = -2 molecules (low NOx)
Oxidation of Methane
2223
33
2233
223
23223
2323
2323
234
HOOCHOOCH
OH OCHhOOHCH
OHOCHOHOOHCH
OHOH OCHOHOOHCH
OOOHCHHOOCH
NOOCHNOOCH
MOCHMOCH
OHCH OHCH
Can oxidize NOto NO2 just like HO2
formaldehyde
O3 Production by Methane
• High NOx (maximum O3 and HOx production):
• Low NOx (minimum O3and HOx production):
2OH5OOHCO O10CH 32224
22224 HOOH3CO 2OOH3CH