Download PPT

TROPOSPHERIC OZONE AS A CLIMATE GAS AND AIR
POLLUTANT: THE CASE FOR CONTROLLING METHANE
Daniel J. Jacob
with Loretta J. Mickley, Arlene M. Fiore, Yaping Xiao
MILLENIAL TEMPERATURE TREND
[Mann et al., 1999; adopted by IPCC 2001]
RADIATIVE FORCING AS INDEX OF CLIMATE FORCING
Climate models (GCMs) indicate DTsurface = l DF where l (climate sensitivity
parameter) ranges from 0.3 to 1.4 K m2 W-1 depending on the GCM
RADIATIVE FORCING OF CLIMATE, 1750-PRESENT
IPCC [2001]
“Kyoto also failed to address two major pollutants that have an impact on
warming: black soot and tropospheric ozone. Both are proven health
hazards. Reducing both would not only address climate change, but also
dramatically improve people's health.” (George W. Bush, June 11 2001 Rose
Garden speech)
TROPOSPHERIC vs. STRATOSPHERIC OZONE
NOx = NO + NO2: nitrogen oxide radicals
VOC (volatile organic compounds) = light hydrocarbons
and substituted organic compounds
TERRESTRIAL RADIATION OBSERVED FROM SPACE
Scene over Niger Valley, northern Africa
Ozone absorption
feature at 9.6 mm;
pressure-broadened
in troposphere
Ozone also has “shortwave forcing” by absorption of solar UV radiation
GLOBAL BUDGET OF TROPOSPHERIC OZONE
Tropospheric ozone is the primary source of OH, the main atmospheric oxidant
Global sources and sinks, Tg O3 yr-1 (GEOS-CHEM model)
O2
hn
O3
STRATOSPHERE
8-18 km
Chem prod in
troposphere
4900
Chem loss in
troposphere
4200
Transport from
stratosphere
500
Deposition
1200
TROPOSPHERE
hn
O3
Deposition
NO2
NO
OH
HO2
hn, H2O
Pacman of the
atmosphere! CO, CH4, VOC
Main sink for CH4
H2O2
NOx EMISSIONS (Tg N yr-1) TO THE TROPOSPHERE
NOx is the limiting precursor for tropospheric ozone formation
LIGHTNING
5.8
STRATOSPHERE
0.2
SOILS
5.1
BIOMASS
BURNING
BIOFUEL
5.2
AIRCRAFT
2.2
0.5
FOSSIL FUEL
23.1
GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE
Lifetime is < 1 wk in surface air, several wks in free troposphere
Climatology of observed
ozone at 400 hPa in July
from
ozonesondes
and
MOZAIC aircraft (circles)
and corresponding GEOSCHEM model results for
1997 (contours).
GEOS-CHEM tropospheric
ozone columns for July 1997
Li et al. [2001]
CURRENT GENERATION OF OZONE MODELS (IPCC)
UNDERESTIMATES OZONE RISE IN 20th CENTURY
Preindustrial
ozone models
}
Observations at mountain
sites in Europe
[Marenco et al., 1994]
RADIATIVE FORCING BY TROPOSPHERIC OZONE
COULD THUS BE MUCH LARGER THAN IPCC VALUE
Global simulation of late 19th century
ozone observations with the GISS GCM
Standard model:
DF = 0.44 W m-2
“Adjusted” model
(lightning and soil NOx decreased,
biogenic hydrocarbons increased):
DF = 0.80 W m-2
[Mickley et al., 2001]
RADIATIVE FORCING FROM TROPOSPHERIC OZONE
Annual mean values: note heterogeneity
Mickley et al.
[1999]
DF = 0.46 W m-2
How good is radiative forcing as an indicator of climate change,
when forcing is so heterogeneous?
GISS GCM ANALYSIS OF CLIMATIC RESPONSE TO
TROPOSPHERIC OZONE CHANGE OVER 20th CENTURY
GCM equilibrium simulation for present-day climate
with present vs. preindustrial tropospheric ozone;
sea surface temperatures allowed to respond
equilibrium
climate
present-day ozone
Preindustrial ozone
DF = 0.46 W m-2
DT = 0.3oC
L.J. Mickley, Harvard
INHOMOGENEITY OF CLIMATE RESPONSE
TO OZONE CHANGE OVER 20th CENTURY
• Greater warming in northern
hemisphere (due to more
ozone and albedo feedback in
Arctic)
•Strong cooling in
stratosphere:
Stratospheric
ozone
Tropospheric
ozone
9.6 mm
L.J. Mickley, Harvard
Surface
CLIMATE RESPONSE EXPERIMENTS WITH IDENTICAL
GLOBAL RADIATIVE FORCINGS (0.46 W m-2) FROM:
1.
2.
3.
Tropospheric ozone
Uniform tropospheric ozone (18 ppv)
Carbon dioxide (25 ppmv)
• CO2 is a more effective
warming agent at surface
• In lower stratosphere,
CO2 causes warming
while tropospheric ozone
causes cooling
L.J. Mickley, Harvard
LOWER STRATOSPHERIC COOLING FROM TROPOSPHERIC
OZONE IS STRONGEST IN ARCTIC WINTER
particularly sensitive region for recovery of ozone layer!
GCM temperature change in lower stratosphere in DJF (oC)
from increasing tropospheric ozone over 20th century
L.J. Mickley, Harvard
WHY IS CO2 MORE EFFECTIVE THAN OZONE
FOR SURFACE WARMING AT SAME RADIATIVE FORCING?
Correlation of forcing with 500 hPa humidity in tropics (25N-25S)
Ozone
CO2
DFCO2 – DFO3
Overlap of CO2 and H2O bands causes CO2
forcing to shift poleward where ice feedback
enhances warming
L.J. Mickley, Harvard
GCM SURFACE WARMING PATTERNS (oC) FROM INCREASING
TROPOSPHERIC OZONE OVER 20th CENTURY – JJA SURFACE
Tropospheric ozone
Equivalent uniform CO2
Difference
(white = insignificant or high altitude)
Largest warmings downwind of ozone
source regions
L.J. Mickley, Harvard
SURFACE OZONE IS THE #1 AIR POLLUTANT IN U.S.
Mean # summer days (1980-1998) exceeding U.S. ozone air quality standard
(84 ppbv, 8-hour average)
EPA/AIRS data
[Lin et al., 2001]
ANTHROPOGENIC ENHANCEMENT OF TROPOSPHERIC
OZONE BACKGROUND IS A SIZABLE INCREMENT
TOWARDS VIOLATION OF U.S. AIR QUALITY STANDARDS
(even more so for European standards!)
Europe
(8-h avg.)
Europe
(seasonal)
0
preindustrial
20
40
present
background
U.S.
(8-h avg.)
60
80
U.S.
(1-h avg.)
100
120 ppbv
SUMMER 1995 MEAN AFTERNOON OZONE IN SURFACE AIR
Fiore et al. [2002]
AIRS observations
GEOS-CHEM model
(r2 = 0.4, bias=3 ppbv)
“Background ozone” produced outside the North American boundary layer
contributes 15-35 ppbv to mean surface air concentrations in the model
Combined effects of future anthropogenic emission trends
on U.S. ozone air quality and on global climate
Ozone pollution
IPCC
scenario
Fossil fuel NOx emissions
(2020 vs. present)
Global
U.S.
Methane concentration
(2020 vs. present)
A1
+80%
-30%
+35%
B1
+10%
-60%
+20%
Fiore et al.
[2002]
HISTORICAL METHANE TRENDS AND IPCC PROJECTIONS
Recent methane trend
Historical methane trend
IPCC
projections
All IPCC scenarios project
increases in CH4 emissions
over next 50 years – but can we
try to decrease CH4 instead?
PRESENT-DAY EMISSIONS OF METHANE:
ASIA IS A MAJOR SOURCE REGION
Y. Xiao,
Harvard
QUANTIFYING ASIAN SOURCES OF METHANE USING
AIRCRAFT OBSERVATIONS OF ASIAN OUTFLOW
NASA/TRACE-P mission,
Feb-Apr 2001
TESTING a priori ASIAN METHANE SOURCE ESTIMATES
WITH TRACE-P CORRELATIONS FOR CH4-C2H6-CO
Y. Xiao, Harvard
GEOS-CHEM vs. observed CH4:
a priori Asian CH4 source too high by 25%
GEOS-CHEM vs. observed C2H6/CH4:
Coal mining CH4source likely too high