Background ozone in surface air over the United States
Arlene M. FioreDaniel J. Jacob
US EPA Workshop on Developing Criteria for the Chemistry and Physics of Atmospheric Ozone
College Park, Maryland, March 17, 2003
Discussion points
• Methods to characterize regional O3 spatial and temporal variability– EOF Analysis for eastern United States
• Background ozone over the United States– average vs. polluted conditions– seasonal & regional variability– during events of elevated O3
– origin (stratosphere; natural; hemispheric pollution)
• Linkages between O3 and aerosols– surface O3 response to heterogenous & radiative effects of aerosols
• Linkages between air quality and climate – influence of CH4 on background O3
Conventional model evaluation: Correlation of simulated vs. observed time series
MAQSIP regional model36 km2
Correlation coefficient (r)
Daily afternoon (1-5 p.m. local time) mean surface O3 Summer 1995, eastern U.S.
GEOS-CHEM global model2°x2.5°
Fiore et al., in press, JGR
EOF ANALYSIS: Characterize spatiotemporal variability of surface O3
(daily 1-5 p.m. mean concentrations in summer 1995 over eastern U.S.) OBS (AIRS) MAQSIP (36 km2)
Fiore et al., in press, JGR
r2 = 0.60Slope = 0.9
r2 = 0.57Slope = 0.8
r2 = 0.68Slope = 0.7
EOF 1: East-west
EOF 2: Midwest-Northeast
EOF 3: Southeast
r2 = 0.86Slope = 1.0
r2 = 0.76Slope = 1.0
r2 = 0.80Slope = 1.0
Same fundamental, synoptic-scale processes modulate observed O3
variability at scale of global model horizontal resolution
EOF 1: East-west
EOF 2: Midwest-Northeast
EOF 3: Southeast
OBS (AIRS) GEOS-CHEM 2°x2.5°
Fiore et al., in press, JGR
r2 = 0.74Slope = 1.2
r2 = 0.27Slope = 1.0
r2 = 0.90Slope = 1.0
r2 = 0.68Slope = 1.0
r2 = 0.54Slope = 0.8
r2 = 0.78Slope = 1.0
Mean Afternoon Surface Ozone Background (ppbv) in GEOS-CHEM model, Summer 1995
Background is tagged as ozone produced outside the N. American boundary layer (surface-700 hPa)
What is the contribution of the background to pollution episodes?
Ozone Background is depleted during regional pollution episodes(due to deposition and chemical loss under stagnant conditions)
Daily mean afternoon O3 vs. (NOy-NOx) At Harvard Forest, MA
Index of Aged Pollution
Background(clean conditions)
Background in model(pollution episode)
Total Surface Ozone in Model
Ozo
ne
(pp
bv)
Fiore et al., JGR, August, 2002.
Background O3: produced outside the N. Americanboundary layer (surface-700 hPa)
Observations
U.S. Ozone Standard
Frequency Distribution of Afternoon Background Ozone Concentrations in U.S. Surface Air Summer 1995 (GEOS-CHEM model)
summer ensemble vs. pollution episodes
Convection upwindoccasionally results inhigh background duringpollution episodes
Background Ozone Concentration (ppbv)
Pro
bab
ilit
y
Fiore et al., JGR, August, 2002.
RANGE OF ASIAN/EUROPEAN POLLUTION SURFACE OZONE ENHANCEMENTS OVER THE U.S. IN SUMMER
as determined from a simulation without these emissions
Max Asian/European pollution enhancements(up to 14 ppbv) occur at intermediate ozone levels (50-70 ppbv)
MAJOR CONCERNIF OZONE STANDARDWERE TO DECREASE!
tropical air
Subsidence of Asian pollution+ local production
stagnation
Fiore et al., JGR, August, 2002.
Two Questions Central to Background O3 Discussion:
How can we further address these questions?
Analyze 2001 CASTNet O3 data (representative year)
Apply GEOS-CHEM to interpret observations
1. What background concentrations should be used
to assess risk?
2. Is the present NAAQS for O3 too close to
background concentrations?
Observed concentrations above 50-80 ppbv in spring have been attributed to natural causes [Lefohn, 1997; Lefohn et al., 2001]
Suggests current 25-45 ppbv background definition may be inadequate
Implies that NAAQS for O3 may be unattainable via domestic emissions reductions
Model captures percentages of occurrences ≥ 50 ppbv in 2001at CASTNet sites except for SE
% o
ccu
rren
ce
s ≥
50
pp
bv
All hourly obsHourly obs 1-5 p.m.Mean obs 1-5 p.m.Mean model 1-5 p.m.
NW NE
SESW
Sensitivity Simulations for source attribution
• Standard simulation…..2x2.5 GEOS-CHEM, 48 sigma levels
2001
• Background………………no anthrop. NOx, CO, NMVOC emissions from N. America
• Natural O3 level………….no anthrop. NOx, CO, NMVOC
emissions globally; CH4 = 700 ppbv
• Stratospheric…………….tagged O3 tracer simulation
Regional Pollution = Standard – Background
Hemispheric Pollution = Background – Natural O3 level
Note: Background in the following results is as defined by EPA
How does background O3 vary with season and region?
Seasonal cycle in mean afternoon (1-5 p.m.) O3 in surface air
CASTNet sitesModel at CASTNetModel entire regionBackgroundNatural O3 levelStratospheric
+
*
RegionalPollution(from N. Amer. emissions)
{
{
{
{
HemisphericPollutionenhancement
Ozone Time Series at selected CASTNet stations in 2001
CASTNet sitesModelBackgroundNatural O3 levelStratospheric
+
*
Hemisphericpollution
Regionalpollution}
}
APR-MAY 2000
High-O3 “Haywood County”event in North Carolina(model box centered at 85W34N)
Regional pollution contributes significantly to high-O3 events in NC;
Model does not indicate substantial stratospheric influence
CASTNet sitesModelBackgroundNatural O3 levelStratosphericContinental lower troposphere
+
*
Hemisphericpollution
Regionalpollution}
}
APR-MAY 2001
CASTNet sitesModelBackgroundNatural O3 levelStratospheric
+
*
Ozo
ne
(pp
bv)
Days in March 2001
SoutheastWest
Background increaseswith highest observed O3 at western sites in March
Background decreaseswith highest observed O3 at SE sitesin March
Cumulative probability distributions for daily mean afternoon O3, April-June 2001
11 “background sites”(all in western U.S.)
34 “polluted sites”(mostly in eastern U.S.)
CASTNet sitesModelBackgroundNatural O3 levelStratospheric
+
*
Backgrounddecreases underpolluted conditions!
Some enhancementfrom N. Amerand hemis.pollutionfor highest values
Cumulative probability distributions for daily mean afternoon O3, July-August 2001
11 “background sites”from previous slide(western U.S.)
34 “polluted sites”(mostly in eastern U.S.)
CASTNet sitesModelBackgroundNatural O3 levelStratospheric
+
*
Backgroundis even lowerduring high-O3
events in summer
Sites areinfluenced bypollution in summer months
Background islower
GEOS-CHEM: August
O3 (ppbv)
Martin et al., JGR, February, 2003.
Ozone-aerosol linkage:
(simulation with aerosols) – (simulation without aerosols)
PM O3 over U.S.
Air Quality-Climate Linkage: Impacts of future changes in global anthropogenic emissions (GEOS-CHEM)
50% anth. NOx
2030 A1
50% anth. CH4
50% anth.VOC
2030 B1
1995(base)
50% anth.VOC
50% anth.CH4
50% anth.NOx
2030 A1
2030 B1
IPCC scenario
Anthrop. NOx emissions
(2030 vs. present)
Global U.S.
Methane emissions
(2030 vs. present)
A1 +80% -20% +30%
B1 -5% -50% +12%
Number of U.S. summer grid-square days with O3 > 80 ppbv
Rad
iati
ve F
orc
ing
* (W
m-2)
CH4 links air quality & climate via
background O3
Fiore et al.,GRL, Oct., 2002.
Rising emissions from developing countries lengthen the O3 pollution season in the United States
2030 A1
1995 Base Case
Fiore et al.,GRL, Oct., 2002.
CH4
NOx
NMVOCs
NOx
NMVOCsO3
Chemical lossDeposition
CONTINENT 2OCEAN
O3
O3
NOx emissions local impact;
little effect on climate
Boundary layer(0-2.5 km)
Free Troposphere CH4 emissions global impact:
Lower background O3
Negative radiative forcingIntercontinental transport,
hemispheric O3 backgroundincreases in
2030 A1 simulation
CONTINENT 1
Double dividend of methane emissions reductions: lower global O3 background and improve air quality everywhere