QUESTIONS

1
QUESTIONS
1. How does the thinning of the stratospheric
ozone layer affect the source of OH in the
troposphere? 2. Chemical production of ozone in
the troposphere is much faster in summer than
winter. Explain. 3. Why dont reactions of
hydrocarbons deplete (or titrate!) all the OH in
the troposphere? 4. Methylperoxy radicals
produced from methane oxidation can self-react to
form methanol    CH3O2 CH3O2 g CH3OH CH2O
O2 What is the effect of this reaction on OH
levels?
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CHAIN MECHANISM FOR O3 PRODUCTION CH4 OXIDATION
Initiation source of HOx (OH production) Propogat
ion CH4 OH ? CH3 H2O CH3 O2 M ? CH3O2
M CH3O2 HO2 ? CH3OOH O2 CH3O2 NO ?
CH3O NO2 CH3OOH OH ? CH2O OH H2O
CH3OOH OH ? CH3O2 H2O CH3OOH hv ?
CH3O OH CH3O O2 ? CH2O HO2 CH2O
OH ? CHO H2O CH2O hv O2 ? CHO HO2
O2 CH2O hv ? CO H2 CHO O2 ? CO
HO2 (then CO oxidation)


Oxidation from C(-IV) in CH4 through to C(IV) in
CO2



O2
hn
O3
hn
NO2
NO
O3
hn, H2O
OH
HO2
H2O2
Deposition
CO, CH4
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METHANE OXIDATION SCHEME
HO2
NO2
HO2
HO2
In clean troposphere, 70 of OH reacts with CO,
30 with CH4
4
NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE
Zeldovich Mechanism combustion and lightning At
high T (2000K) oxygen thermolyzes O2 ?? O O O
N2 ?? NO N N O2 ?? NO O
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LIGHTNING FLASHES SEEN FROM SPACE (2000)
DJF
JJA
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USING SATELLITE OBSERVATIONS OF NO2 TO MONITOR
NOx EMISSIONS
SCIAMACHY data. May-Oct 2004 (R.V. Martin,
Dalhousie U.)
detection limit
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NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH
2006)
March 2006
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NOX CYCLING
Example of PAN formation from acetaldehyde CH3CHO
OH ? CH3CO H2O CH3CO O2 M ? CH3C(O)OO
M CH3C(O)OONO2 M ? CH3C(O)OONO2 M
PAN
carbonyl oxidation
T
NO3
O3
O3
O2
M
hn
N2O5
NO2
NO
Combustion lightning
H2O
OH, M
HO2
HNO3
O3
? 1 day
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PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR
LONG-RANGE TRANSPORT OF NOx
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DECOMPOSITION OF PAN LEADING TO OZONE PRODUCTION
Chemistry Changing NOy speciation
Origin Warm Conveyor Belt over Asia
ECMWF
Evolution Split by blocking high pressure
O3
CO
Heald et al., 2004
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1996-2005 NOx EMISSION TREND SEEN FROM SPACE
Van der A et al., 2008
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GLOBAL BUDGET OF TROPOSPHERIC OZONE
Present-day Preindustrial
O2
hn
O3
STRATOSPHERE
8-18 km
TROPOSPHERE
hn
NO2
NO
O3
hn, H2O
OH
HO2
H2O2
Deposition
CO, VOC
NOperoxy radicals is rate-limiting, so
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GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE
Climatology of observed ozone at 400 hPa in July
from ozonesondes and MOZAIC aircraft (circles)
and corresponding GEOS-CHEM model results for
1997 (contours).
GEOS-Chem tropospheric ozone columns for July
1997.
Li et al. 2001
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Tropospheric ozone Is the third
anthropogenic greenhouse gas
IPCC 2007
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IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC
OZONE (0.35 W m-2) RELIES ON GLOBAL MODELS
but these underestimate the observed rise in
ozone over the 20th century
Fitting to observations would imply a radiative
forcing of 0.8 W m-2
Preindustrial ozone models

Observations at mountain sites in Europe
Marenco et al., 1994
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RECENT TRENDS IN TROPOSPHERIC OHinferred from
methylchloroform observations