Download - TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY
TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY
Troposphere
Stratosphere:90% of total
The many faces of atmospheric ozone:
In stratosphere: UV shield
In middle/upper troposphere: greenhouse gas
In lower/middle troposphere: precursor of OH, main atmospheric oxidant
In surface air: toxic to humans and vegetation
THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES
EARTHSURFACE
Emission
Reduced gas Oxidized gas/aerosol
Oxidation
Uptake
Reduction
Atmospheric oxidation is responsible for removal of many pollutants, e.g.• methane (major greenhouse gas)• CO (toxic pollutant)• HCFCs (ClOx sources in stratosphere)
TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970
• “The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygen…Methane and CO are chemically quite inert in the troposphere” [Cadle and Allen, Atmospheric Photochemistry, Science, 1970]
• Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern about global CO pollution from increasing car emissions [Robbins and Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric Pollutants, SRI report, 1967]
FIRST BREAKTHROUGH:
• Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969]
SECOND BREAKTHROUGH:• Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in
tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, Science, 1971, J. Geophys. Res. 1973]
THIRD BREAKTHROUGH:• Methylchlroform observations provide indirect evidence for OH at levels
of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977]
WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(1D) in troposphere takes place in narrow band [290-320 nm]
solar flux I
ozone absorptioncross-section s
O(1D)quantumyield f
fsI
~tropopause
10 ppmv
40 ppbv
TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHERE
Stratospheric ozone mechanism doesn’t apply to troposphere
2
2 3
13 2
1
2
( )
( )
O h O OO O M O M
O h O O D
O D M O MXO O X O
O2+hvO3+hv
By contrast, in troposphere:
• no photons < 240 nm no oxygen photolysis;
• neglible O atom conc.gno XO + O loss
In stratosphere:
• Estimate ozone flux FO3 across tropopause (strat-trop exchange)– Total O3 col = 5x1013 moles– 10% of that is in troposphere– Res. time of air in strat = 1.4 yr
• Estimate CH4 source SCH4:– Mean concentration = 1.7 ppmv– Lifetime = 9 years
• Estimate CO source SCO:– Mean concentration = 100 ppbv– Lifetime = 2 months
UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work.
FO3 = 3x1013 moles yr-1
SCH4 = 3x1013 moles yr-1
SCO = 9.7x1013moles yr-1
SCO+ SCH4 > 2FO3 e OH would be titrated!
OZONE PRODUCTION IN TROPOSPHEREPhotochemical oxidation of CO and volatile organic compounds (VOCs)
catalyzed by HOx and NOx
HOx ≡ H + OH + HO2 + RO + RO2
NOx ≡ NO + NO2
Oxidation of CO:
2
2 2
2 2
2
2 3
2 2 3Net: 2
CO OH CO HH O M HO MHO NO OH NONO h NO OO O M O M
CO O CO O
2
2
2 2
2 2
2 3
2 2
2 2
2 3 2
'
Net: 4 ' 2
O
RH OH R H OR O M RO MRO NO RO NO
NO h NO ORO O R CHO HOHO NO OH NO
RH O R CHO O H O
Oxidation of VOC:
RO can also decompose or isomerize; range of carbonyl products
Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)
OH can also add to double bonds of unsaturated VOCs
GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)
O3
O2 h
O3
OH HO2
h, H2O
Deposition
NO
H2O2
CO, VOC
NO2
h
STRATOSPHERE
TROPOSPHERE8-18 km
Chem prod in troposphere,Tg y-1
43001600
Chem loss in troposphere,Tg y-1
40001600
Transport from stratosphere,Tg y-1
400400
Deposition,Tg y-1
700400
Burden, Tg 360230
Lifetime, days 2842
Present-day Preindustrial
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSBox model calculation
NOx-saturatedregime
NOx-limited regime Ridge
SATELLITE OBSERVATIONS OF TROPOSPHERIC NO2
SCIAMACHY data. May-Oct 2004(R.V. Martin, Dalhousie U.)
detectionlimit
NOx EMISSIONS (Tg N a-1) TO TROPOSPHERE
FOSSIL FUEL 23.1
AIRCRAFT 0.5
BIOFUEL 2.2
BIOMASSBURNING 5.2
SOILS 5.1
LIGHTNING 5.8
STRATOSPHERE 0.2
LIGHTNING FLASHES SEEN FROM SPACE (2000)
DJF
JJA
GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE
Zhang et al. [2010]
TES thermal IR satellite observations for 2006, seasonal means at 500 hPa
• Maximum values at northern mid-latitudes in spring-summer due to anthropogenic pollution;
• High values in tropical regions affected by seasonal biomass burning;
• Minimum values over tropical oceans due to chemical loss
LONDON FOGAerosols a.k.a.particulate matter (PM) from domestic+industrial coal combustion
“Killer fog” of December 1952 resulted in 10,000 excess deaths
Coal combustionTemperature
Altitude
inversion
sulfateorganic carbonblack carbon
particles< 1km
LOS ANGELES SMOGRespiratory problems, vegetation damage due to high surface ozone
troposphere
stratosphere8-18 km
temperature
inversionozone
altitude
Nitrogen oxides (NOx ≡ NO + NO2) Volatile organic compounds (VOCs)
UV radiation Ozone (O3)
vehicles, industry, vegetation
produced by photolysisof oxygen (O2)
AIR POLLUTION IN THE US TODAY:Ozone and fine particulate matter (PM2.5) are the two main pollutants
75 ppb (8-h average)15 mg m-3 (1-y av.)
http://epa.gov/airtrends/2010/
OzonePM2.5
2008 REVISION TO OZONE STANDARDFROM 84 to 75 PPB CAUSED MORE U.S. AREAS TO BE OUT OF COMPLIANCE
0 20 40 60 80 100 120 ppb
Europe AQS(seasonal)
U.S. AQS(8-h avg.)
U.S. AQS(1-h avg.)
Preindustrialozone
background
Present-day ozone background at
northern mid-latitudes
Europe AQS (8-h avg.)
Canadian AQS (8-h avg.)
Mexican AQS(1-h avg.)
…AND INCREASED THE IMPORTANCE OF THE OZONE BACKGROUND
20082014?
Currently proposed 60-70 ppb standard would have extensive non-compliance
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed
NOx-saturated
NOx-limited Ridge
LARGE SUPPLY OF BIOGENIC VOCs – unrecognized until the 1990s
Isoprene (biogenic VOC)Anthropogenic VOCs
Jacob et al., J. Geophys. Res. [1993]
Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime!recognized in Revised Clean Air Act of 1999
MAPPING OF VOC EMISSIONS FROM SPACEusing satellite measurements of formaldehyde
confirms dominance of biogenic over anthropogenic VOCsMillet et al. [2008]
1970-2003 TREND OF U.S. EMISSIONS
Focus until past decade was on VOC emission controls
DECREASE OF POWER PLANT NOx EMISSIONSOVER THE PAST DECADE
Decreasing US NOx emissions from power plants
Growth Trends
0%
100%
200%
300%
1980 1990 2000 2010 2020
Norm
aliz
ed to
198
0
Population
Vehicle Miles
Traveled
Gross State
Product
Emission Trends
0%
50%
100%
150%
1980 1990 2000 2010 2020
Norm
aliz
ed to
198
0 CO2
NOx
SOx
ROG
CO
PM10 Trends
0
100
200
300
Max
24-
hr P
M10
(µg/
m3 )
South Coast
State Standard
San Joaquin Valley
0.0
0.1
0.2
0.3
0.4
0.5
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
Max
1-h
r O
zone
(pp
m)
State Standard
South Coast
San Joaquin Valley
Ozone Trends
Historical Ozone Levels
0
50
100
150
200
250
300
1965 1970 1975 1980 1985 1990 1995 2000 Year
Above Califor niaOzone Standar d
Stage I E pisodes
Stage II Episodes
Num
ber o
f Day
s
Stage II > 350 ppb O3Stage I > 200 ppb O3
SOUTH COAST O3 HISTORY
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000Years
Max
O3 (
ppm
)
1952
– P
rof.
Haa
gen-
Smit
disc
over
s sm
og fo
rmat
ion
1963
– C
lean
Air
Act
(CA
A)
1970
– E
PA c
reat
ed, C
AA
Am
men
d.
1976
– V
olvo
intr
oduc
es fi
rst c
ar to
us
e 3-
way
cat
alys
t
1984
– S
mog
Che
ck P
rogr
am
1990
– C
AA
A
1994
– S
mog
Che
ck II
1971
– C
AR
B a
dopt
s 1s
t aut
o N
Ox
stan
dard
sEP
A pr
omul
gate
s N
AA
QS
NAAQS
Improvement Over Past 20 Years
-80%
-60%
-40%
-20%
0%NitrogenDioxide
SulfurDioxide
CarbonMonoxide
Ozone PM10 Air Toxics(CancerRisk)
Perc
ent
Chan
ge
Approaching StandardsAttained
Standards
T H A N K S ! !