1. Background Ozone in Surface Air over 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

2. Acknowledgments Daniel Jacob Brendan Field Hongyu Liu Bob Yantosca Qinbin Li Duncan Fairlie David Streets Suneeta Fernandes Carey Jang Jason West

3. NO x VOC NO x VOC CONTINENT 2 OCEAN O3O3 Boundary layer (0-3 km) Free Troposphere CONTINENT 1 What is the origin of tropospheric ozone? OH HO 2 VOC, CH 4, CO NO NO 2 h O3O3 O3O3 Hemispheric Pollution Direct Intercontinental Transport air pollution (smog) Stratospheric O 3 Stratosphere ~12 km

4. 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 (O 3 ) Particles < 10  m (PM 10 ) Particles < 2.5  m (PM 2.5 ) Sulfur dioxide (SO 2 ) Any pollutant Millions of People 0 0.007 11.1 0.7 72.7 2.7 133.1 0 0 50 100 150

5. The Risk Increment Above the Background is Considered when setting the NAAQS for Ozone Environmenta l risk Ozone Concentration BackgroundNAAQS Acceptable added risk  Need a quantitative estimate for background ozone  EPA chose a constant value (40 ppbv) in previous review of O 3 standard

6. Range of “background O 3 ” estimates in U.S. surface air 20406080100 Used by EPA to assess health risk from O 3 O 3 (ppbv) Range considered by EPA during last revision of O 3 standard 84 ppbv: threshold for current U.S. O 3 standard Frequent observations previously attributed to natural background [Lefohn et al.,JGR 106, 9945-9958, 2001] Range from prior global modeling studies Range from this work [Fiore et al., JGR 108, 4787, 2003] EPA assumed background Range from this work [Fiore et al., JGR 108, 4787, 2003] EPA assumed background

7. FiresLand biosphere Human activity Ocean NORTH AMERICA NORTH AMERICA Lightning “Background” air X Outside natural influences Long-range transport of pollution “REGULATORY BACKGROUND OZONE” DEFINITION: Ozone concentrations that would exist in the absence of anthropogenic emissions from North America [EPA, 2003] stratosphere lightning

8. Intercontinental Transport of Pollution at Northern Midlatitudes Anthropogenic NO x emissions, Transport: 2-4 weeks Reduced Visibility from Transpacific Transport of Asian Dust O3O3 Clear DayApril 16, 2001 Glen Canyon, Arizona Contributes to Hemispheric Background O 3

9. Regional Pollution = Standard – Background Hemispheric Pollution = Background – Natural O 3 level EPA-Defined Background is Not Directly Observable  Must be Estimated with Models compare with observationsStandard simulation …..2x2.5 GEOS-CHEM, compare with observations 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 Simulations for Ozone Source Attribution: Mar-Oct 2001 GEOS-CHEM 3DTropospheric Chemistry Model TOOL: GEOS-CHEM 3D Tropospheric Chemistry Model [Bey et al., 2001] (uses assimilated meteorology; 48  ; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers)

10. 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 O 3 and its various sources 2. diagnose origin of springtime high-O 3 events at remote U.S. sites, previously attributed to natural, stratospheric influence

11. Case Study: Voyageurs National Park, Minnesota (May-June 2001) CASTNet observations Model Background Natural O 3 level Stratospheric + * Hemispheric pollution Regional pollution }  } Background: 15-36 ppbv Natural level: 9-23 ppbv Stratosphere: < 7 ppbv High-O 3 events: dominated by regional pollution; minor stratospheric influence (~2 ppbv) Lefohn et al. [2001] suggest a stratospheric source as the likely origin of high-O 3 events frequently observed in June regional pollution hemispheric pollution X

12. Case Study: Yellowstone NP, Wyoming (March-May 2001) CASTNet observations Model Background Natural O 3 level Stratospheric + * Hemispheric pollution Regional pollution }  } Background at high-altitude site (2.5 km) not necessarily representative of background contribution at low-lying sites Frequent high-O 3 events previously attributed to natural, stratospheric source [Lefohn et al., 2001] X regional pollution hemispheric pollution

13. CASTNet sites Model Background Natural O 3 level Stratospheric + * Ozone (ppbv) Days in March 2001 Southeast (<1.5 km) West (>1.5 km) Background O 3 higher at high-altitude western sites Background O 3 lower at low-lying southeastern sites; decreases with highest observed O 3 X

14. Background O 3 even lower under polluted conditions CASTNet sites Model Background Natural O 3 level Stratospheric + * Background on polluted days well below 40 ppbv assumed by EPA  health risks underestimated with current approach Ozone (ppbv) Cumulative Probability Regional Pollution Daily mean afternoon O 3 at 58 U.S. CASTNet sites June-July-August

15. Monthly mean afternoon (1-5 p.m.) surface O 3 CASTNet sites Model at CASTNet Model entire region Background Natural O 3 level Stratospheric + * { { { { Hemispheric pollution enhancement (5-12 ppbv) Mean background: 20-35 ppbv Mean natural level: 13-27 ppbv Mean stratosphere: 2-7 ppbv Regional pollution from N. Am. emis. (8-30 ppbv) Background O 3 varies with season

16. Daily afternoon (1-5 p.m.) surface O 3 March-October 2001 Background 15-35 ppbv; Natural 10-25 ppbv; Stratosphere < 20 ppbv Probability ppbv -1 Typical ozone values in U.S. surface air: Compiling results from all CASTNet sites… CASTNet sites Model at CASTNet Stratospheric Natural Background

17. 1.Background O 3 is typically less than 40 ppbv; even lower under polluted conditions 2.High-O 3 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 O 3 underestimated in present EPA risk assessments  these events do not represent U.S. background conditions and should not be used to challenge legitimacy of O 3 NAAQS  international agreements to reduce hemispheric background should improve air quality & facilitate compliance w/ more stringent standards  Climate Linkage

18. NO x NMVOCs NO x NMVOCs CONTINENT 2 OCEAN O3O3 Boundary layer (0-3 km) Free Troposphere CONTINENT 1 Air quality-Climate Linkage: CH 4, O 3 are important greenhouse gases CH 4 contributes to background O 3 in surface air OH HO 2 VOC, CH 4, CO NO NO 2 h O3O3 O3O3 Global Background O 3 (Hemispheric Pollution) Direct Intercontinental Transport greenhouse gas air pollution (smog)

19. More than half of global methane emissions are influenced by human activities ANIMALS 90 LANDFILLS 50 GAS 60 COAL 40 RICE 85 TERMITES 25 WETLANDS 180 BIOMASS BURNING 20 GLOBAL METHANE SOURCES (Tg CH 4 yr -1 )

20. Radiative Forcing of Climate, 1750-Present: Important Contributions from Methane and Ozone IPCC [2001] Level of scientific understanding

21. Impacts of future changes in global anthropogenic emissions on climate NO x  OH  CH 4 50% anthr. NO x 2030 A1 50% anthr. CH 4 50% anthr. NMVOC 2030 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% Change in radiative forcing (W m -2 ) relative to 1995 base case Fiore et al., GRL, 2002

22. Decrease in July 1995 Mean Surface O 3 concentrations from 50% reductions in global anthropogenic CH 4 emissions Methane emission controls reduce surface O 3 everywhere

23. Methane emission reductions shift entire summer afternoon surface O 3 frequency distribution over U.S. Probability ppbv -1 Ozone Concentration (ppbv )

24. Impacts of future changes in global anthrop. emis. on U.S. air quality: 1995 (base) 50% anthr. VOC 50% anthr. CH 4 50% anthr. NO x 2030 A1 Increase in U.S. pollution events despite domestic decline in anthrop. emissions Number of summer grid-square days with O 3 > 80 ppbv 2030 B1 IPCC 2030 scenario Anthrop. NO x emissions Global U.S. Methane emissions A1+80%-20%+30% B1-5%-50%+12%

25. Rising emissions from developing countries lengthen the O 3 pollution season in the United States 2030 A1 1995 Base Case Degradation of U.S. air quality from rise in global emissions despite domestic reductions

26. 1.Background O 3 is typically less than 40 ppbv; even lower under polluted conditions 2.High-O 3 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 O 3 underestimated in present EPA risk assessments  these events do not represent U.S. background conditions and should not be used to challenge legitimacy of O 3 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