Background ozone in surface air over the United States

1. 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

2. 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

3. Conventional model evaluation: Correlation of simulated vs. observed time series Daily afternoon (1-5 p.m. local time) mean surface O3 Summer 1995, eastern U.S. MAQSIP regional model 36 km2 GEOS-CHEM global model 2°x2.5° Correlation coefficient (r) Fiore et al., in press, JGR

4. 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) r2 = 0.60 Slope = 0.9 EOF 1: East-west r2 = 0.86 Slope = 1.0 EOF 2: Midwest- Northeast r2 = 0.57 Slope = 0.8 r2 = 0.76 Slope = 1.0 r2 = 0.68 Slope = 0.7 EOF 3: Southeast r2 = 0.80 Slope = 1.0 Fiore et al., in press, JGR

5. Same fundamental, synoptic-scale processes modulate observed O3 variability at scale of global model horizontal resolution OBS (AIRS) GEOS-CHEM 2°x2.5° r2 = 0.68 Slope = 1.0 EOF 1: East-west r2 = 0.74 Slope = 1.2 r2 = 0.54 Slope = 0.8 EOF 2: Midwest- Northeast r2 = 0.27 Slope = 1.0 r2 = 0.78 Slope = 1.0 EOF 3: Southeast r2 = 0.90 Slope = 1.0 Fiore et al., in press, JGR

6. 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?

7. 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 Background O3: produced outside the N. American boundary layer (surface-700 hPa) Observations U.S. Ozone Standard Total Surface Ozone in Model Ozone (ppbv) Background in model (pollution episode) Background (clean conditions) Index of Aged Pollution Fiore et al., JGR, August, 2002.

8. 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 Probability Background Ozone Concentration (ppbv) Fiore et al., JGR, August, 2002.

9. RANGE OF ASIAN/EUROPEAN POLLUTION SURFACE OZONE ENHANCEMENTS OVER THE U.S. IN SUMMERas determined from a simulation without these emissions Subsidence of Asian pollution + local production Max Asian/European pollution enhancements (up to 14 ppbv) occur at intermediate ozone levels (50-70 ppbv) stagnation tropical air MAJOR CONCERN IF OZONE STANDARD WERE TO DECREASE! Fiore et al., JGR, August, 2002.

10. Two Questions Central to Background O3 Discussion: • What background concentrations should be used to assess risk? • 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 How can we further address these questions? Analyze 2001 CASTNet O3 data (representative year) Apply GEOS-CHEM to interpret observations

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

12. Sensitivity Simulations for source attribution Note: Background in the following results is as defined by EPA • 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 How does background O3 vary with season and region?

13. * CASTNet sites Model at CASTNet Model entire region Background Natural O3 level Stratospheric + Seasonal cycle in mean afternoon (1-5 p.m.) O3 in surface air Regional Pollution (from N. Amer. emissions) { { Hemispheric Pollution enhancement { {

14. * CASTNet sites Model Background Natural O3 level Stratospheric + } Regional pollution D = } D = Hemispheric pollution Ozone Time Series at selected CASTNet stations in 2001

15. * CASTNet sites Model Background Natural O3 level Stratospheric Continental lower troposphere + } Regional pollution D = High-O3 “Haywood County”event in North Carolina(model box centered at 85W34N) } D = Hemispheric pollution APR-MAY 2000 APR-MAY 2001 Regional pollution contributes significantly to high-O3 events in NC; Model does not indicate substantial stratospheric influence

16. * CASTNet sites Model Background Natural O3 level Stratospheric + West Southeast Background decreases with highest observed O3 at SE sites in March Ozone (ppbv) Background increases with highest observed O3 at western sites in March Days in March 2001

17. * CASTNet sites Model Background Natural O3 level Stratospheric + Cumulative probability distributions for daily mean afternoon O3, April-June 2001 Some enhancement from N. Amer and hemis. pollution for highest values 11 “background sites” (all in western U.S.) 34 “polluted sites” (mostly in eastern U.S.) Background decreases under polluted conditions!

18. * CASTNet sites Model Background Natural O3 level Stratospheric + Cumulative probability distributions for daily mean afternoon O3, July-August 2001 Sites are influenced by pollution in summer months Background is lower 11 “background sites” from previous slide (western U.S.) Background is even lower during high-O3 events in summer 34 “polluted sites” (mostly in eastern U.S.)

19. PM  O3 over U.S. Ozone-aerosol linkage: GEOS-CHEM: August • O3 (ppbv) (simulation with aerosols) – (simulation without aerosols) Martin et al., JGR, February, 2003.

20. Air Quality-Climate Linkage: Impacts of future changes in global anthropogenic emissions (GEOS-CHEM) Number of U.S. summer grid-square days with O3 > 80 ppbv Radiative Forcing* (W m-2) 50% anth. CH4 50% anth. NOx 50% anth. VOC 2030 A1 2030 B1 1995 (base) 50% anth. VOC 50% anth. CH4 50% anth. NOx 2030 A1 2030 B1 CH4 links air quality & climate via background O3 Fiore et al.,GRL, Oct., 2002.

21. Rising emissions from developing countries lengthen the O3 pollution season in the United States 1995 Base Case 2030 A1 Fiore et al.,GRL, Oct., 2002.

22. NOx NMVOCs Double dividend of methane emissions reductions: lower global O3 background and improve air quality everywhere Free Troposphere CH4 emissions global impact: Lower background O3 Negative radiative forcing CH4 Intercontinental transport, hemispheric O3 background increases in 2030 A1 simulation O3 Boundary layer (0-2.5 km) NOx emissions local impact; little effect on climate Chemical loss Deposition NOx NMVOCs O3 O3 CONTINENT 1 CONTINENT 2 OCEAN