Background ozone in surface air over the United States Arlene M. Fiore Daniel 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 O 3 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 O 3 –origin (stratosphere; natural; hemispheric pollution) Linkages between O 3 and aerosols –surface O 3 response to heterogenous & radiative effects of aerosols Linkages between air quality and climate –influence of CH 4 on background O 3

Conventional model evaluation: Correlation of simulated vs. observed time series MAQSIP regional model 36 km 2 Correlation coefficient (r) Daily afternoon (1-5 p.m. local time) mean surface O 3 Summer 1995, eastern U.S. GEOS-CHEM global model 2°x2.5 ° Fiore et al., in press, JGR

EOF ANALYSIS: Characterize spatiotemporal variability of surface O 3 (daily 1-5 p.m. mean concentrations in summer 1995 over eastern U.S.) OBS (AIRS) MAQSIP (36 km 2 ) Fiore et al., in press, JGR r 2 = 0.60 Slope = 0.9 r 2 = 0.57 Slope = 0.8 r 2 = 0.68 Slope = 0.7 EOF 1: East-west EOF 2: Midwest- Northeast EOF 3: Southeast r 2 = 0.86 Slope = 1.0 r 2 = 0.76 Slope = 1.0 r 2 = 0.80 Slope = 1.0

Same fundamental, synoptic-scale processes modulate observed O 3 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 r 2 = 0.74 Slope = 1.2 r 2 = 0.27 Slope = 1.0 r 2 = 0.90 Slope = 1.0 r 2 = 0.68 Slope = 1.0 r 2 = 0.54 Slope = 0.8 r 2 = 0.78 Slope = 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 O 3 vs. (NO y -NO x ) At Harvard Forest, MA Index of Aged Pollution Background (clean conditions) Background in model (pollution episode) Total Surface Ozone in Model Ozone (ppbv) Fiore et al., JGR, August, Background O 3 : produced outside the N. American boundary 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 upwind occasionally results in high background during pollution episodes Background Ozone Concentration (ppbv) Probability 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 CONCERN IF OZONE STANDARD WERE TO DECREASE! tropical air Subsidence of Asian pollution + local production stagnation Fiore et al., JGR, August, 2002.

Two Questions Central to Background O 3 Discussion: How can we further address these questions? Analyze 2001 CASTNet O 3 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 O 3 too close to background concentrations? Observed concentrations above ppbv in spring have been attributed to natural causes [Lefohn, 1997; Lefohn et al., 2001]  Suggests current ppbv background definition may be inadequate  Implies that NAAQS for O 3 may be unattainable via domestic emissions reductions

Model captures percentages of occurrences ≥ 50 ppbv in 2001 at CASTNet sites except for SE % occurrences ≥ 50 ppbv All hourly obs Hourly obs 1-5 p.m. Mean obs 1-5 p.m. Mean model 1-5 p.m. NWNE SESW

Sensitivity Simulations for source attribution Standard simulation …..2x2.5 GEOS-CHEM, 48 sigma levels 2001 Background ………………no anthrop. NO x, CO, NMVOC emissions from N. America Natural O 3 level ………….no anthrop. NO x, CO, NMVOC emissions globally; CH 4 = 700 ppbv Stratospheric …………….tagged O 3 tracer simulation Regional Pollution = Standard – Background Hemispheric Pollution = Background – Natural O 3 level Note: Background in the following results is as defined by EPA How does background O 3 vary with season and region?

Seasonal cycle in mean afternoon (1-5 p.m.) O 3 in surface air CASTNet sites Model at CASTNet Model entire region Background Natural O 3 level Stratospheric + * Regional Pollution (from N. Amer. emissions) { { { { Hemispheric Pollution enhancement

Ozone Time Series at selected CASTNet stations in 2001 CASTNet sites Model Background Natural O 3 level Stratospheric + * Hemispheric pollution Regional pollution }  }

APR-MAY 2000 High-O 3 “Haywood County” event in North Carolina (model box centered at 85W34N) Regional pollution contributes significantly to high-O 3 events in NC; Model does not indicate substantial stratospheric influence CASTNet sites Model Background Natural O 3 level Stratospheric Continental lower troposphere + * Hemispheric pollution Regional pollution }  } APR-MAY 2001

CASTNet sites Model Background Natural O 3 level Stratospheric + * Ozone (ppbv) Days in March 2001 Southeast West Background increases with highest observed O 3 at western sites in March Background decreases with highest observed O 3 at SE sites in March

Cumulative probability distributions for daily mean afternoon O 3, April-June “background sites” (all in western U.S.) 34 “polluted sites” (mostly in eastern U.S.) CASTNet sites Model Background Natural O 3 level Stratospheric + * Background decreases under polluted conditions! Some enhancement from N. Amer and hemis. pollution for highest values

Cumulative probability distributions for daily mean afternoon O 3, July-August “background sites” from previous slide (western U.S.) 34 “polluted sites” (mostly in eastern U.S.) CASTNet sites Model Background Natural O 3 level Stratospheric + * Background is even lower during high-O 3 events in summer Sites are influenced by pollution in summer months Background is lower

GEOS-CHEM: August  O 3 (ppbv) Martin et al., JGR, February, Ozone-aerosol linkage: (simulation with aerosols) – (simulation without aerosols) PM  O 3 over U.S.

Air Quality-Climate Linkage: Impacts of future changes in global anthropogenic emissions (GEOS-CHEM) 50% anth. NO x 2030 A1 50% anth. CH 4 50% anth. VOC 2030 B (base) 50% anth. VOC 50% anth. CH 4 50% anth. NO x 2030 A B1 IPCC scenario Anthrop. NO x 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 O 3 > 80 ppbv Radiative Forcing* (W m -2 ) CH 4 links air quality & climate via background O 3 Fiore et al.,GRL, Oct., 2002.

Rising emissions from developing countries lengthen the O 3 pollution season in the United States 2030 A Base Case Fiore et al.,GRL, Oct., 2002.

CH 4 NO x NMVOCs NO x NMVOCs O3O3 Chemical loss Deposition CONTINENT 2 OCEAN O3O3 O3O3 NO x emissions local impact; little effect on climate Boundary layer (0-2.5 km) Free Troposphere CH 4 emissions global impact: Lower background O 3 Negative radiative forcing Intercontinental transport, hemispheric O 3 background increases in 2030 A1 simulation CONTINENT 1 Double dividend of methane emissions reductions: lower global O 3 background and improve air quality everywhere