Presentation on theme: "Linking regional air pollution with global chemistry and climate:"— Presentation transcript:
1 Linking regional air pollution with global chemistry and climate: The role of background ozoneArlene M. FioreAdviser: Daniel J. JacobApril 22, 2002
2 Tropospheric ozone links air pollution & climate change (1) primary constituent of smog in surface air [NRC, 1991] (2) 3rd most important greenhouse gas [IPCC, 2001]O3greenhouse gasFree TropospherehnNO2NOHemispheric PollutionOHHO2Direct Intercontinental TransportBoundary layer(0-2.5 km)VOC, CH4, COair pollution (smog)NOxNMVOCsO3NOxNMVOCsO3air pollution (smog)CONTINENT 1CONTINENT 2OCEAN
3 Ozone abatement strategies evolve as our understanding of the O3 problem improves O3 smog recognizedas an URBAN problem: Los Angeles,Haagen-Smit identifieschemical mechanismSmog consideredREGIONAL problem;role of biogenicVOCs discoveredA GLOBAL perspective:role of intercontinentaltransport, background1980sPresent1950sAbatement Strategy:NMVOCs+ NOx+ CH4??
4 Historical and recent evidence suggest that human activities are increasing the hemispheric O3 background8-h daily maximum ozone probabilitydistribution at rural U.S. sites[Lin et al., 2000]Ozone trend at European mountain sites, [Marenco et al., 1994]Cumulative ProbabilityOzone (ppbv)~ 5-fold increase overpast century~ 3 ppbv increase overpast 20 years
5 Need to quantify U.S. background O3 in surface air for current review of National Ambient Air Quality Standard for O3 (NAAQS)What background concentrations should be used to assess the human health risk associated with exposure to O3?Is the present (or more stringent future) NAAQS too close to background concentrations?“REGULATORY BACKGROUND” DEFINITION:Ozone concentrations that would exist in the absence of anthrop. emissions from North America [EPA, 2003]25-45 ppbv [EPA, 1996]
6 An approach for estimating O3 background from observations Summer p.m. observations atHarvard Forest [Munger et al., 1996, 1998]U.S. Ozone Standardbackgroundpresently used in EPA risk assessmentsFrequently observed surface O3 concentrations attributedto natural background byLefohn et al. Ozone (ppbv)Range of background that wasconsidered for revised O3 standardIntercept30 ppbbackground(clean air)Range of Observed Background(Pollution coordinate)NOy-NOx (ppbv)(Index of Aged Pollution)
7 QUESTION: What role does background O3 play in linking regional air quality with global chemistry & climate?TOOL: GEOS-CHEM 3D Tropospheric Chemistry Model [Bey et al., 2001](uses assim. met.; s; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers)RESEARCH OBJECTIVES:Assess whether GEOS-CHEM is a suitable tool for quantifying the O3 background over the U.S.-- Standard statistical metrics + EOF analysis2. Quantify background contribution to average vs. polluted days3. Identify origin of background and variability of its sources4. Diagnose source of high-O3 events at remote U.S. sites in spring5. Determine combined air quality and climatic implications of various O3 control strategies-- Sensitivity simulations & tagged tracers in GEOS-CHEM
8 Summer 1995 afternoon (1-5 p.m.) ozone in surface air over the United States AIRSObservationsGEOS-CHEM 2ºx2.5ºr = 0.66, bias=5 ppbv(for 4ºx5º resolutionr = 0.84, bias=2 ppbv)Broad overview here of model evaluation – will present more detailed comparisons later in the talk with science resultsExplain SE overestimate
10 Same fundamental, synoptic-scale processes modulate observed O3 variability at scale of global model horizontal resolutionOBS (AIRS)GEOS-CHEM 2°x2.5°r2 = 0.68Slope = 1.0East-westEOFr2 = 0.74Slope = 1.2r2 = 0.54Slope = 0.8Midwest-NortheastEOFr2 = 0.27Slope = 1.0r2 = 0.78Slope = 1.0SoutheastEOFr2 = 0.90Slope = 1.0
11 Mean Afternoon Surface Ozone Background (ppbv) in GEOS-CHEM model, Summer 1995 Note: although not using EPA background definition here, consistent results – will switch to later.Background is tagged as ozone produced outside theN. American boundary layer (surface-700 hPa)What is the contribution of the background to pollution episodes?
12 Daily mean afternoon O3 vs. (NOy-NOx) At Harvard Forest, MA 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, MABackground O3:produced outsidethe N. Americanboundary layer(surface-700 hPa)ObservationsU.S. OzoneStandardTotal Surface Ozonein ModelOzone (ppbv)Background in model(pollution episode)Background(clean conditions)Index of Aged Pollution
13 Frequency Distribution of Afternoon Background Ozone Concentrations in U.S. Surface Air Summer 1995 (GEOS-CHEM model) summer ensemble vs. pollution eventsConvection upwindoccasionally results inhigh background duringpollution eventsProbabilityBackground Ozone Concentration (ppbv)
14 Range of Asian/European Pollution Surface Ozone Enhancements Over the U.S. in summer as determined from a simulation without these emissions (4°x5°)Subsidence of Asian pollution+ local productionMax Asian/European pollution enhancements(up to 14 ppbv) occur at intermediate ozone levels (50-70 ppbv)stagnationtropicalairMAJOR CONCERNIF OZONE STANDARDWERE TO DECREASE!
15 Sensitivity Simulations for source attribution Mar-Oct 2001 Note: Background in the following results is as defined by EPAStandard simulation…..2x2.5 GEOS-CHEM, 48 sigma levels2001Background………………no anthrop. NOx, CO, NMVOC emissions from N. AmericaNatural O3 level………….no anthrop. NOx, CO, NMVOC emissions globally; CH4 = 700 ppbvStratospheric…………….tagged O3 tracer simulationRegional Pollution = Standard – BackgroundHemispheric Pollution = Background – Natural O3 levelUse 2001 CASTNet data in conjunction with GEOS-CHEM toinvestigate how background O3 varies with season and region
17 High-O3 “Haywood County” event in North Carolina Time series at CASTNet sites where high-O3 events in spring were previously attributed to stratospheric origin on basis of back-trajectoriesYellowstone NP, WyomingMarch-May 2001CASTNet sitesModelBackgroundNatural O3 levelStratosphericContinental lowertroposphere+*}D =Regionalpollution}D =HemisphericpollutionHigh-O3 “Haywood County” event in North CarolinaAPR-MAY 2000APR-MAY 2001Regional pollution largely controls high-O3 events in spring;Model explains these events without a large stratospheric influence
18 * CASTNet sites Model Background Natural O3 level Stratospheric + West SoutheastBackgroundincreaseswith highestobserved O3 atwestern sitesin MarchOzone (ppbv)Backgrounddecreaseswith highestobserved O3at SE sitesin MarchDays in March 2001
19 Background O3 for risk assessment = f (season, altitude, total O3 concentration) CASTNet sitesModelBackgroundNatural O3 levelStratospheric+*Enhancement from N. Amer & hemis. pollution forhigh O3 concentrationsLower background;larger pollution influencein summer (& fall)12 elevatedsites;all in west58 surfacesitesRegionalPollutionCumulative Probability (daily mean afternoon O3)Lower background at surface sites;Maximum contribution at the centerof the O3 distributionUsing average background forpollution episodes underestimatesrisk to human health!
20 Number of U.S. summer grid-square days with O3 > 80 ppbv Air Quality-Climate Linkage: Impacts of future changes in global anthropogenic emissions (GEOS-CHEM; 4°x5°)Number of U.S. summer grid-square days with O3 > 80 ppbvRadiative Forcing* (W m-2)50%anth.VOC50%anth.CH450%anth.NOx2030A12030B11995(base)50%anth.VOC50%anth.CH450%anth.NOx2030A12030B1IPCC scenarioAnthrop. NOx emissions(2030 vs. present)Global U.S.Methane emissionsA1+80%-20%+30%B1-5%-50%+12%CH4 links air quality& climate viabackground O3
21 Rising emissions from developing countries lengthen the O3 pollution season in the United States Degradation of U.S.air quality from risein global emissionsdespite domesticreductions1995 Base Case2030 A1
22 CONCLUSIONS and their implications for policy Global models can adequately simulate the synoptic conditions important for resolving background and high-O3 conditions2. Background O3 varies with season, site elevation, and total surface O3 concentrations-- highest at high-altitude western U.S. sites in spring-- lower at surface sites and in summer-- depleted during polluted conditionshealth risk from O3 underestimated in present EPA risk assessments3. High-O3 events at remote U.S. sites in spring previously attributed to a natural stratospheric source are explained largely by regional pollutionthese events should not be used to challenge legitimacy of O3 NAAQS4. Hemispheric pollution enhances U.S. background; may rise in future international agreements to reduce hemispheric background would improve U.S. air quality & facilitate compliance w/ more stringent standards5. Methane emission controls decrease hemispheric background & greenhouse warming co-benefits for air quality & climate change mitigation objectives