ATMOSPHERIC CHEMICAL SYSTEMS
Source,NASA
ATMOSPHERIC
CHEMISTRY
Climate Change
Regional Air Pollution Acid Deposition
Stratospheric Ozone
IMPORTANCE OF ATMOSPHERIC CHEMISTRY
ATMOSPHERE STRUCTURE & BULK COMPOSITION
? Pressure units
1 atm = 1.013 x 106 g cm-1 s-2
1 mbar = 103 g cm-1 s-2
1 hPa = 103 g cm-1 s-2
? Ideal gas law
p V = ? Rg T
p = (? Rg T)/Ma
Source,Environmental Science,Cunningham,P,W,and B,W,Saigo,2001
ATMOSPHERIC TRANSPORT TIME SCALES
Source,Introduction to Atmospheric Chemistry,Jacob,D,J.,1999
TYPICAL ATMOSPHERIC COMPOSITION
N
2
0,78
O2 0,21
Ar 0,093
H
2
O <0,000 1 - 0,04
CO 2 3 70 ppm
CH
4
1,7 ppm
O
3
1 0 ppb – 10 ppm
CO 5 0 ppb – 300 ppb
NO
x
5 ppb – 1 ppt
OH < 1 pp t
(mole fraction) ? Typical units
Number concentration,molecules cm-3
Mixing ratio,ppm,ppb,ppt
? Molecular weight of air
Ma = 28.96 g mole-1
? Trace chemistry
Reactants are present in trace amounts
OXIDIZING NATURE OF THE ATMOSPHERE
? Earth’s atmosphere is oxidizing due to presence of O2
SO2 --> SO42-; CH4 --> CO; CO --> CO2; NO2 --> HNO3
? Radicals are oxidizing agents in the atmosphere
OH is the cleansing agent of the atmosphere
? Key to understanding atmospheric oxidant chemistry
Understand radical cycling ? but radical cycle is
intimately connected to oxidant chemistry of other
trace compounds including O3
? Start with O3
O3 is important from chemical,climate,and health
perspectives
ATMOSPHERIC O3 – A BRIEF HISTORY
? 1840,Ozone discovered in 1840 by C,F,Sch?nbein
– thought it was made up of oxygen and hydrogen
? 1848,Systematic measurement attempts – curiousity,growing
interest in env.,health effects,‘economy of nature’
? 1861,Odling suggested that ozone was O3
? 1930,Chemical mechanism for O3 layer postulated
? 1952,O3 identified as component of chemical smog
Christian Frederich Sch?nbein
OZONE AND HEALTH
? 90% of O3 is in the stratosphere; O3 layer with max ~ 9 ppm
? Absorption of ? = 200-320 nm (UV-B and UV-C) by strat,O3
Source,Stratospheric Ozone,NASA/GSFC
OZONE AND STRATOSPHERIC TEMPERATURE
? Local heating of the stratosphere due to UV absorption by O3
? Tropospheric O3 is also an important greenhouse gas
Source,Stratospheric Ozone,NASA/GSFCSource,Environmental Science,Cunningham,P,W,and B,W,Saigo,2001
O3
O(1D)
O(3P)
2OH
Solar radiation,
wavelength 290-320 nm
H2OO2,N2
O2
OZONE AND ATMOSPHERIC CHEMISTRY
? O3 is the primary source of tropospheric OH
? OH is atmospheric ‘detergent’
LATITUDINAL & TEMPORAL VARIATION OF TOTAL O3
? Total O3 in range of 300-400 DU
? Patterns due to stratospheric circulation
? Low total O3 at high southern lat in southern spring
due to ‘ozone hole’
Dobson units
1 DU = 2.69 x 1016 molecules O3 cm-2
Source,Stratospheric Ozone,NASA/GSFC
STRATOSPHERIC O3 CHEMISTRY
THE CHAPMAN MECHANISM FOR STRATOSPHERIC O3
? Cycling between O,O2,and O3
Source,Stratospheric Ozone,NASA/GSFC
MISSING CHEMISTRY IN CHAPMAN MECHANISM
? Global O3 production rate = 5 times destruction rate
Imbalance suggests overest,of prodn,or underest,of loss
? O3 production well constrained by good spectroscopic data
Implies missing chemical sinks for Ox
? Reactions of radicals with O and/or O3
But radicals will also be consumed by reaction
measured
calculated
Source,Stratospheric Ozone,NASA/GSFC
CATALYTIC OX DESTRUCTION IN THE STRATOSPHERE
? Radical chain reactions
X + O3 ? XO + O2
XO + O ? X + O2
Net,O + O3 ? 2O2
? ‘X’ in the stratosphere
H,OH,NO,Cl
? HOx,NOx,and Clx
HOx = H + OH + HO2
NOx = NO + NO2
Clx = Cl + ClO
? Reservoirs tie up active radicals
e.g,ClO + NO2 ? ClONO2
Stratospheric Clx precursors
Source,Stratospheric Ozone,NASA/GSFC
Col
um
n O
3
(DU
)
ANTARCTIC TOTAL OZONE DECREASE
? Depletion of total column O3 starting in mid- to late-70’s
during SH spring
? Gas-phase chemistry predicted smaller decreases & not
over Antarctica
O3
Source,Stratospheric Ozone,NASA/GSFC
Source,Farmann et al.,Nature,v,315,May 1985
ALTITUDE DEPENDENCE OF ANTARCTIC O3 DECREASE
? Strong depletion between 12 and 20 km
? Gas phase chemistry predicted decrease near 40 km
Source,Stratospheric Ozone,NASA/GSFC
TEMPORAL DEPENDENCE OF ANTARCTIC O3 DECREASE
? Depletion begins around Sep 1,& minimum is reached around
Oct 1
Source,NOAA/CMDL
REACTIONS ON POLAR STRATOSPHERIC CLOUDS
? Conversion of inactive Cl to active Cl and removal of NOx
Source,Stratospheric Ozone,NASA/GSFC
ROLE OF METEOROLOGY
Low temps.
PSC formation
?release of active Cl
and removal of NOx
Strong vortex
Isolates air from mid-lats,
?prevents high O3 air influx
Figure shows strong polar vortex (as shown by size of wind
vectors) & low polar temps,(as shown by colors) at various
altitudes in the southern hemisphere stratosphere
Source,Stratospheric Ozone,NASA/GSFC
NORTHERN vs SOUTHERN HEMISPHERE O3 TRENDS
? Vortex not as strong and temps,not as low in NH
Source,Stratospheric Ozone,NASA/GSFC
PROJECTED CHANGES IN STRATOSPHERIC Clx
? Montreal Protocol and subsequent amendments will have
signifcant impacts on projected Clx loading of stratosphere
(p
pb
)
Source,Stratospheric Ozone,NASA/GSFC
WMO 1998 Scientific Assessment of Ozone Depletion
? Ozone depletion in 2050 would be at least 50% at
midlatitudes in the Northern Hemisphere and 70%
at midlatitudes in the Southern Hemisphere,about
10 times larger than today
? Surface UV-B radiation in 2050 would at least double
at midlatitudes in the Northern Hemisphere and quadruple
at midlatitudes in the Southern Hemisphere compared with
an unperturbed atmosphere,This compares to the current
increases of 5% and 8% in the Northern and Southern
Hemispheres,respectively,since 1980
ESTIMATED IMPACTS OF Clx CONTROLS
TROPOSPHERIC O3 CHEMISTRY
Source,EPA
? Tropospheric O3 generally less than 100 ppb away
from urban areas
TROPOSPHERIC O3
Source,Wang et al.,1998
? O3 chemical production in stratosphere followed
by downward transport to the troposphere
O2
O(3P)
Solar radiation,
(< 240 nm)
O3
O2
Solar radiation (<320 nm),M
STRATOSPHERIC SOURCE OF TROPOSPHERIC O3
Strat,chem.
destruction
by HOx,NOx,
Clx
Transport to trop.
NO2 NO or O3
OH HO2
CO
O3
O2
solar radiation,O2
Net,CO + 2O2 --> CO2 + O3
? Catalytic role of NOx (NO + NO2) in recycling HO2 to OH
? Coupling between OH and HO2 (HOx) via NO
CO OXIDATION CYCLE – O3 PRODUCTION
CO2
O3
NO or O3
OH HO2
CO
O2
Net,CO + O3 --> CO2 + O2
? Chemical O3 destruction
? Coupling between OH and HO2 (HOx) via O3
CO OXIDATION CYCLE – O3 DESTRUCTION
CO2
O3
2O2
1,O3 + hv ?O2 + O(1D)
2,O(1D) + M ?O + M
3,H2O + O(1D) ? 2OH
4,RH + OH ?RO2 + H2O
5,RO2 + NO ? RO + NO2
6,RO + O2 ? R`CHO + HO2
7,HO2 + NO ? OH + NO2
8,HO2 + HO2 ?H2O2 + O2
9,OH + NO2 + M ?HNO3 + M
SCHEMATIC OF HYDROCARBON CHEMISRY
O2
Net rxns 1-7:
RH + 4O2 ? R’CHO + 2O3 + H2O
Source,Introduction to Atmospheric Chemistry,Jacob,D,J.,1999
can produce more O3
ROLE OF NOX IN O3 CHEMICAL PRODUCTION
? Cycling of HOx (OH + HO2) by NOx vs,radical
termination reactions
? Too little NOx,Radical termination (e.g,HO2 +
HO2) rather than radical cycling (e.g,HO2 + NO)
leading to O3 chemical destruction
? Too much NOx,Radical termination by alternate
route (e.g,OH + NO2) as well as short-term O3
destruction by NO + O3 --> NO2 ==> implications
for O3 peak downwind of strong NOX sources
NOx- AND HYDROCARBON-LIMITED REGIMES
NOx limited Hydrocarbon limited
Complications:
Natural emissions of hydrocarbons are important
Tranport of pollutants into and out of region
Source,Introduction to Atmospheric Chemistry,Jacob,D,J.,1999
? Questions:
– NOx or HC emission controls or combination
– Degree of emission controls
? Uncertainties
– Reliability of emission inventories (e.g,natural
hydrocarbon inventories)
– Reliability of air quality models (e.g,local vs
transported NOx/HC/O3)
ISSUES IN O3 POLLUTION CONTROL
1998 MEASURED SURFACE OZONE CONCENTRATIONS
2nd highest daily max 1-hr (ppb)
<65 65-124 125-164
65-84
205-404
Source,1998 EPA National Trends Report
118
153
169
36
141
155
167
165-204
4th highest daily max 8-hr (ppb)
<65 85-104
105-124 125-374
ESTIMATED GLOBAL EXPOSURE STATISTICS
Population in areas with
max,monthly-mean O3
conc,above a given value
Crops in areas with
growing season mean O3
conc above a given value
? Exposure to O3 pollution,40-60% of population in areas with max,monthly-mean O3 > 50 ppbv
and 10-20% of crops in areas with growing-season mean O3 > 50 ppbv
? Potentially large impact in future years:Year 2100 IPCC scenario from HARVARD model gives
50% of population in areas with max,monthly-mean O3 > 85 ppbv,and 50% of crops in areas
with growing season mean O3 > 70 ppbv
ATMOSPHERIC AEROSOLS AND ACID RAIN
Combustion
generated
? Aerosols and acid rain can effect natural & managed ecosystems
Source,NASA
ATMOSPHERIC
CHEMISTRY
Climate Change
Regional Air Pollution Acid Deposition
Stratospheric Ozone
IMPORTANCE OF ATMOSPHERIC CHEMISTRY
ATMOSPHERE STRUCTURE & BULK COMPOSITION
? Pressure units
1 atm = 1.013 x 106 g cm-1 s-2
1 mbar = 103 g cm-1 s-2
1 hPa = 103 g cm-1 s-2
? Ideal gas law
p V = ? Rg T
p = (? Rg T)/Ma
Source,Environmental Science,Cunningham,P,W,and B,W,Saigo,2001
ATMOSPHERIC TRANSPORT TIME SCALES
Source,Introduction to Atmospheric Chemistry,Jacob,D,J.,1999
TYPICAL ATMOSPHERIC COMPOSITION
N
2
0,78
O2 0,21
Ar 0,093
H
2
O <0,000 1 - 0,04
CO 2 3 70 ppm
CH
4
1,7 ppm
O
3
1 0 ppb – 10 ppm
CO 5 0 ppb – 300 ppb
NO
x
5 ppb – 1 ppt
OH < 1 pp t
(mole fraction) ? Typical units
Number concentration,molecules cm-3
Mixing ratio,ppm,ppb,ppt
? Molecular weight of air
Ma = 28.96 g mole-1
? Trace chemistry
Reactants are present in trace amounts
OXIDIZING NATURE OF THE ATMOSPHERE
? Earth’s atmosphere is oxidizing due to presence of O2
SO2 --> SO42-; CH4 --> CO; CO --> CO2; NO2 --> HNO3
? Radicals are oxidizing agents in the atmosphere
OH is the cleansing agent of the atmosphere
? Key to understanding atmospheric oxidant chemistry
Understand radical cycling ? but radical cycle is
intimately connected to oxidant chemistry of other
trace compounds including O3
? Start with O3
O3 is important from chemical,climate,and health
perspectives
ATMOSPHERIC O3 – A BRIEF HISTORY
? 1840,Ozone discovered in 1840 by C,F,Sch?nbein
– thought it was made up of oxygen and hydrogen
? 1848,Systematic measurement attempts – curiousity,growing
interest in env.,health effects,‘economy of nature’
? 1861,Odling suggested that ozone was O3
? 1930,Chemical mechanism for O3 layer postulated
? 1952,O3 identified as component of chemical smog
Christian Frederich Sch?nbein
OZONE AND HEALTH
? 90% of O3 is in the stratosphere; O3 layer with max ~ 9 ppm
? Absorption of ? = 200-320 nm (UV-B and UV-C) by strat,O3
Source,Stratospheric Ozone,NASA/GSFC
OZONE AND STRATOSPHERIC TEMPERATURE
? Local heating of the stratosphere due to UV absorption by O3
? Tropospheric O3 is also an important greenhouse gas
Source,Stratospheric Ozone,NASA/GSFCSource,Environmental Science,Cunningham,P,W,and B,W,Saigo,2001
O3
O(1D)
O(3P)
2OH
Solar radiation,
wavelength 290-320 nm
H2OO2,N2
O2
OZONE AND ATMOSPHERIC CHEMISTRY
? O3 is the primary source of tropospheric OH
? OH is atmospheric ‘detergent’
LATITUDINAL & TEMPORAL VARIATION OF TOTAL O3
? Total O3 in range of 300-400 DU
? Patterns due to stratospheric circulation
? Low total O3 at high southern lat in southern spring
due to ‘ozone hole’
Dobson units
1 DU = 2.69 x 1016 molecules O3 cm-2
Source,Stratospheric Ozone,NASA/GSFC
STRATOSPHERIC O3 CHEMISTRY
THE CHAPMAN MECHANISM FOR STRATOSPHERIC O3
? Cycling between O,O2,and O3
Source,Stratospheric Ozone,NASA/GSFC
MISSING CHEMISTRY IN CHAPMAN MECHANISM
? Global O3 production rate = 5 times destruction rate
Imbalance suggests overest,of prodn,or underest,of loss
? O3 production well constrained by good spectroscopic data
Implies missing chemical sinks for Ox
? Reactions of radicals with O and/or O3
But radicals will also be consumed by reaction
measured
calculated
Source,Stratospheric Ozone,NASA/GSFC
CATALYTIC OX DESTRUCTION IN THE STRATOSPHERE
? Radical chain reactions
X + O3 ? XO + O2
XO + O ? X + O2
Net,O + O3 ? 2O2
? ‘X’ in the stratosphere
H,OH,NO,Cl
? HOx,NOx,and Clx
HOx = H + OH + HO2
NOx = NO + NO2
Clx = Cl + ClO
? Reservoirs tie up active radicals
e.g,ClO + NO2 ? ClONO2
Stratospheric Clx precursors
Source,Stratospheric Ozone,NASA/GSFC
Col
um
n O
3
(DU
)
ANTARCTIC TOTAL OZONE DECREASE
? Depletion of total column O3 starting in mid- to late-70’s
during SH spring
? Gas-phase chemistry predicted smaller decreases & not
over Antarctica
O3
Source,Stratospheric Ozone,NASA/GSFC
Source,Farmann et al.,Nature,v,315,May 1985
ALTITUDE DEPENDENCE OF ANTARCTIC O3 DECREASE
? Strong depletion between 12 and 20 km
? Gas phase chemistry predicted decrease near 40 km
Source,Stratospheric Ozone,NASA/GSFC
TEMPORAL DEPENDENCE OF ANTARCTIC O3 DECREASE
? Depletion begins around Sep 1,& minimum is reached around
Oct 1
Source,NOAA/CMDL
REACTIONS ON POLAR STRATOSPHERIC CLOUDS
? Conversion of inactive Cl to active Cl and removal of NOx
Source,Stratospheric Ozone,NASA/GSFC
ROLE OF METEOROLOGY
Low temps.
PSC formation
?release of active Cl
and removal of NOx
Strong vortex
Isolates air from mid-lats,
?prevents high O3 air influx
Figure shows strong polar vortex (as shown by size of wind
vectors) & low polar temps,(as shown by colors) at various
altitudes in the southern hemisphere stratosphere
Source,Stratospheric Ozone,NASA/GSFC
NORTHERN vs SOUTHERN HEMISPHERE O3 TRENDS
? Vortex not as strong and temps,not as low in NH
Source,Stratospheric Ozone,NASA/GSFC
PROJECTED CHANGES IN STRATOSPHERIC Clx
? Montreal Protocol and subsequent amendments will have
signifcant impacts on projected Clx loading of stratosphere
(p
pb
)
Source,Stratospheric Ozone,NASA/GSFC
WMO 1998 Scientific Assessment of Ozone Depletion
? Ozone depletion in 2050 would be at least 50% at
midlatitudes in the Northern Hemisphere and 70%
at midlatitudes in the Southern Hemisphere,about
10 times larger than today
? Surface UV-B radiation in 2050 would at least double
at midlatitudes in the Northern Hemisphere and quadruple
at midlatitudes in the Southern Hemisphere compared with
an unperturbed atmosphere,This compares to the current
increases of 5% and 8% in the Northern and Southern
Hemispheres,respectively,since 1980
ESTIMATED IMPACTS OF Clx CONTROLS
TROPOSPHERIC O3 CHEMISTRY
Source,EPA
? Tropospheric O3 generally less than 100 ppb away
from urban areas
TROPOSPHERIC O3
Source,Wang et al.,1998
? O3 chemical production in stratosphere followed
by downward transport to the troposphere
O2
O(3P)
Solar radiation,
(< 240 nm)
O3
O2
Solar radiation (<320 nm),M
STRATOSPHERIC SOURCE OF TROPOSPHERIC O3
Strat,chem.
destruction
by HOx,NOx,
Clx
Transport to trop.
NO2 NO or O3
OH HO2
CO
O3
O2
solar radiation,O2
Net,CO + 2O2 --> CO2 + O3
? Catalytic role of NOx (NO + NO2) in recycling HO2 to OH
? Coupling between OH and HO2 (HOx) via NO
CO OXIDATION CYCLE – O3 PRODUCTION
CO2
O3
NO or O3
OH HO2
CO
O2
Net,CO + O3 --> CO2 + O2
? Chemical O3 destruction
? Coupling between OH and HO2 (HOx) via O3
CO OXIDATION CYCLE – O3 DESTRUCTION
CO2
O3
2O2
1,O3 + hv ?O2 + O(1D)
2,O(1D) + M ?O + M
3,H2O + O(1D) ? 2OH
4,RH + OH ?RO2 + H2O
5,RO2 + NO ? RO + NO2
6,RO + O2 ? R`CHO + HO2
7,HO2 + NO ? OH + NO2
8,HO2 + HO2 ?H2O2 + O2
9,OH + NO2 + M ?HNO3 + M
SCHEMATIC OF HYDROCARBON CHEMISRY
O2
Net rxns 1-7:
RH + 4O2 ? R’CHO + 2O3 + H2O
Source,Introduction to Atmospheric Chemistry,Jacob,D,J.,1999
can produce more O3
ROLE OF NOX IN O3 CHEMICAL PRODUCTION
? Cycling of HOx (OH + HO2) by NOx vs,radical
termination reactions
? Too little NOx,Radical termination (e.g,HO2 +
HO2) rather than radical cycling (e.g,HO2 + NO)
leading to O3 chemical destruction
? Too much NOx,Radical termination by alternate
route (e.g,OH + NO2) as well as short-term O3
destruction by NO + O3 --> NO2 ==> implications
for O3 peak downwind of strong NOX sources
NOx- AND HYDROCARBON-LIMITED REGIMES
NOx limited Hydrocarbon limited
Complications:
Natural emissions of hydrocarbons are important
Tranport of pollutants into and out of region
Source,Introduction to Atmospheric Chemistry,Jacob,D,J.,1999
? Questions:
– NOx or HC emission controls or combination
– Degree of emission controls
? Uncertainties
– Reliability of emission inventories (e.g,natural
hydrocarbon inventories)
– Reliability of air quality models (e.g,local vs
transported NOx/HC/O3)
ISSUES IN O3 POLLUTION CONTROL
1998 MEASURED SURFACE OZONE CONCENTRATIONS
2nd highest daily max 1-hr (ppb)
<65 65-124 125-164
65-84
205-404
Source,1998 EPA National Trends Report
118
153
169
36
141
155
167
165-204
4th highest daily max 8-hr (ppb)
<65 85-104
105-124 125-374
ESTIMATED GLOBAL EXPOSURE STATISTICS
Population in areas with
max,monthly-mean O3
conc,above a given value
Crops in areas with
growing season mean O3
conc above a given value
? Exposure to O3 pollution,40-60% of population in areas with max,monthly-mean O3 > 50 ppbv
and 10-20% of crops in areas with growing-season mean O3 > 50 ppbv
? Potentially large impact in future years:Year 2100 IPCC scenario from HARVARD model gives
50% of population in areas with max,monthly-mean O3 > 85 ppbv,and 50% of crops in areas
with growing season mean O3 > 70 ppbv
ATMOSPHERIC AEROSOLS AND ACID RAIN
Combustion
generated
? Aerosols and acid rain can effect natural & managed ecosystems
