background ozone in surface air over the united states: variability, climate linkages, and policy...
TRANSCRIPT
Background Ozone in Surface Airover the United States:
Variability, Climate Linkages, and Policy Implications
Arlene M. Fiore
Department of Atmospheric and Oceanic Sciences Seminar
University of Wisconsin-Madison
December 6, 2004
AcknowledgmentsAcknowledgments
Daniel JacobBrendan FieldHongyu Liu Bob YantoscaQinbin Li Duncan Fairlie
David StreetsSuneeta Fernandes
Carey JangJason West
NOx
VOC
NOx
VOC
CONTINENT 2 OCEAN
O3
Boundary layer
(0-3 km)
Free Troposphere
CONTINENT 1
What is the origin of tropospheric ozone?
OH HO2
VOC, CH4, CO
NONO2
hO3
O3
Hemispheric Pollution
Direct Intercontinental Transport
air pollution (smog)
air pollution (smog)
Stratospheric O3 Stratosphere
~12 km
Number of People Living in U.S. Counties Violating National Ambient Air Quality Standards (NAAQS) in 2001
EPA [2002]
124 ppbv: 40.2
84 ppbv: 110.3
Carbon monoxide (CO)
Lead
Nitrogen dioxide
Ozone (O3)
Particles < 10 m (PM10)
Particles < 2.5 m (PM2.5)
Sulfur dioxide(SO2)
Any pollutant
Millions of People
0
0.007
11.1
0.7
72.7
2.7
133.1
0
0
50 100 150
The Risk Increment Above the Background is Considered
when setting the NAAQS for OzoneEnvironmenta
l risk
OzoneConcentrationBackground NAAQS
Acceptableadded
risk
Need a quantitative estimate for background ozone EPA chose a constant value (40 ppbv) in previous
review of O3 standard
Range of “background O3” estimates in U.S. surface air
20 40 60 80 100
Used by EPA to assess healthrisk from O3
O3 (ppbv)
Range considered by EPA during last revision of O3 standard
84 ppbv: threshold forcurrent U.S. O3 standard
Frequent observations previously attributed to natural background[Lefohn et al.,JGR 106, 9945-9958, 2001]
Range from priorglobal modeling studies
Range from this work[Fiore et al., JGR 108, 4787, 2003]
EPAassumedbackground
Range from this work[Fiore et al., JGR 108, 4787, 2003]
EPAassumedbackground
Fires Landbiosphere
Humanactivity
Ocean
NORTH AMERICANORTH AMERICA
Lightning
“Background” air
X
Outside naturalinfluences
Long-range transportof pollution
““REGULATORY BACKGROUND OZONE” DEFINITION:REGULATORY BACKGROUND OZONE” DEFINITION:
Ozone concentrations that would exist in the absence of Ozone concentrations that would exist in the absence of anthropogenic emissions from North America [anthropogenic emissions from North America [EPAEPA, 2003], 2003]
stratosphere
lightning
Intercontinental Transport of Pollution at Northern Midlatitudes
AnthropogenicNOx emissions,
Transport: 2-4 weeks
Reduced Visibility from Transpacific Transport of Asian Dust
O3
Clear Day April 16, 2001
Glen Canyon, Arizona
Contributes to Hemispheric Background O3
Regional Pollution = Standard – Background
Hemispheric Pollution = Background – Natural O3 level
EPA-Defined Background is Not Directly Observable Must be Estimated with Models
• Standard simulation…..2x2.5 GEOS-CHEM, compare compare with observations with observations
• 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
Simulations for Ozone Source Attribution: Mar-Oct 2001
TOOL: GEOS-CHEM 3DGEOS-CHEM 3D Tropospheric Chemistry Tropospheric Chemistry ModelModel [Bey et al., 2001] (uses assimilated meteorology; 48 ; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers)
2001 CASTNet Stations (EPA, Nat’l Park Service)
X Elevated sites (> 1.5 km)
Low-lying sites
APPROACH: Use 2001 CASTNet data in conjunction with GEOS-CHEM to
1. quantify background O3 and its various sources
2. diagnose origin of springtime high-O3 events at remote U.S. sites, previously attributed to natural,
stratospheric influence
Case Study: Case Study: Voyageurs National Park, MinnesotaVoyageurs National Park, Minnesota(May-June 2001)(May-June 2001)
CASTNet observationsModelBackgroundNatural O3 levelStratospheric
+
*
Hemisphericpollution
Regionalpollution}
}
Background: 15-36 ppbvNatural level: 9-23 ppbvStratosphere: < 7 ppbv
High-O3 events: dominated by regional pollution; minor stratospheric influence (~2 ppbv)
Lefohn et al. [2001] suggest a stratosphericsource as the likely origin of high-O3 eventsfrequently observed in June
regional pollution
hemispheric pollution
X
Case Study: Case Study: Yellowstone NP, WyomingYellowstone NP, Wyoming(March-May 2001) (March-May 2001)
CASTNet observationsModelBackgroundNatural O3 levelStratospheric
+
*
Hemisphericpollution
Regionalpollution}
}
Background at high-altitude site (2.5 km) not necessarily representative of background contribution at low-lying sites
Frequent high-O3 events previously attributed to natural, stratospheric source [Lefohn et al., 2001]
X
regional pollution
hemispheric pollution
CASTNet sitesModelBackgroundNatural O3 levelStratospheric
+
*
Ozo
ne
(pp
bv)
Days in March 2001
Southeast (<1.5 km)West (>1.5 km)
Background O3 higher at high-altitude western sites
Background O3
lower at low-lyingsoutheastern sites; decreases with highest observed O3
X
Background O3 even lower under polluted conditions
CASTNet sitesModelBackgroundNatural O3 levelStratospheric
+
*
Background on polluted days well below 40 ppbv assumed by EPA health risks underestimated with current approach
Ozone (ppbv)
Cumulative Probability
RegionalPollution
Daily mean afternoon O3 at 58 U.S. CASTNet sites
June-July-August
Monthly mean afternoon (1-5 p.m.) surface O3
CASTNet sitesModel at CASTNetModel entire regionBackgroundNatural O3 levelStratospheric
+
*
{
{
{
{
Hemispheric pollutionenhancement(5-12 ppbv)
Mean background: 20-35 ppbvMean natural level: 13-27 ppbvMean stratosphere: 2-7 ppbv
Regional pollutionfrom N. Am. emis.
(8-30 ppbv)
Background O3 varies with season
Daily afternoon (1-5 p.m.) surface ODaily afternoon (1-5 p.m.) surface O3 3
March-October 2001 March-October 2001
Background 15-35 ppbv; Natural 10-25 ppbv; Stratosphere < 20 ppbv
Pro
bab
ility
pp
bv-1
Typical ozone values in U.S. surface air:
Compiling results from all CASTNet sites…
CASTNet sites
Model at CASTNet
Stratospheric
NaturalBackground
1. Background O3 is typically less than 40 ppbv; even lower under polluted conditions
2. High-O3 events at remote U.S. sites in spring can be explained largely by pollution from North America
3. Hemispheric pollution enhances U.S. background
CONCLUSIONS ...and their implications for public policy
health risk from O3 underestimated in present EPA risk assessments
these events do not represent U.S. background conditions and should not be used to challenge legitimacy of O3 NAAQS
international agreements to reduce hemispheric background should improve air quality & facilitate compliance w/ more stringent standards
Climate Linkage
NOx
NMVOCs
NOx
NMVOCs
CONTINENT 2 OCEAN
O3
Boundary layer
(0-3 km)
Free Troposphere
CONTINENT 1
Air quality-Climate Linkage:CH4, O3 are important greenhouse gases
CH4 contributes to background O3 in surface air
OH HO2
VOC, CH4, CO
NONO2
hO3
O3
Global Background O3
(Hemispheric Pollution)
Direct Intercontinental Transport
greenhouse gas
air pollution (smog)
air pollution (smog)
More than half of global methane emissions are influenced by human activities
ANIMALS90
LANDFILLS50
GAS60
COAL40RICE
85
TERMITES25
WETLANDS180
BIOMASSBURNING20
GLOBAL METHANESOURCES (Tg CH4 yr-1)
Radiative Forcing of Climate, 1750-Present:
Important Contributions from Methane and Ozone
IPCC [2001]
Level of scientific understanding
Impacts of future changes in global anthropogenic emissions on climate
NOx OH
CH4
50% anthr. NOx
2030 A1
50% anthr. CH4
50% anthr.
NMVOC
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%
Change in radiative forcing (W m-2) relative to 1995 base case
Fiore et al., GRL, 2002
Decrease in July 1995 Mean Surface O3 concentrationsfrom 50% reductions in global anthropogenic CH4 emissions
Methane emission controls reduce surface O3 everywhere
Methane emission reductions shift entire summer afternoon surface O3 frequency distribution over U.S.
Pro
bab
ilit
y p
pb
v-1
Ozone Concentration (ppbv)
Impacts of future changes in global anthrop. emis. on U.S. air quality:
1995(base)
50% anthr.VOC
50% anthr.CH4
50% anthr.NOx
2030 A1
Increase in U.S. pollution events despite domesticdecline in anthrop.emissions
Number of summer grid-square days with O3 > 80 ppbv
2030 B1
IPCC 2030
scenario
Anthrop. NOx emissions
Global U.S.
Methane emissions
A1 +80% -20% +30%
B1 -5% -50% +12%
Rising emissions from developing countries lengthen the O3 pollution season in the United
States
2030 A1
1995 Base CaseDegradation of U.S. air quality from rise in global emissions despite domestic reductions
1. Background O3 is typically less than 40 ppbv; even lower under polluted conditions
2. High-O3 events at remote U.S. sites in spring can be explained largely by pollution from North America
3. Hemispheric pollution enhances U.S. background
CONCLUSIONS,Public policy implications, and Next steps
health risk from O3 underestimated in present EPA risk assessments
these events do not represent U.S. background conditions and should not be used to challenge legitimacy of O3 NAAQS
international agreements to reduce hemispheric background should improve air quality & facilitate compliance w/ more stringent standards
compare additional model estimates of background (MOZART-2) for use in risk assessments
feasibility of methane control [West and Fiore, submitted; AGU poster]
better characterize relationship between methane sources and surface ozone response
extend source attribution for background