Background Ozone in Surface Air over the United States: Variability, Climate Linkages, and Policy Implications

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1 Background Ozone in Surface Air over the United States: Variability, Climate Linkages, and Policy Implications Arlene M. Fiore Department of Environmental Sciences Seminar Rutgers University March 4, 2005

2 Acknowledgments Larry Horowitz Chip Levy Drew Purves Steve Pacala Jason West Daniel Jacob Mat Evans Duncan Fairlie Brendan Field Qinbin Li Hongyu Liu Bob Yantosca Yuxuan Wang Carey Jang David Streets Suneeta Fernandes

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

4 Number of People Living in U.S. Counties Violating National Ambient Air Quality Standards (NAAQS) in 2001 Nitrogen dioxide Ozone (O 3 ) Sulfur dioxide (SO 2 ) Particles < 10 µm (PM 10 ) Particles < 2.5 µm (PM 2.5 ) Carbon monoxide (CO) Lead Any pollutant ppbv: ppbv: EPA [2002] Millions of People

5 Background Estimates are Used When Setting NAAQS Environmenta l risk Acceptable added risk Background NAAQS Pollutant concentration Risk assessments account for risks associated with exposure to the increment of ozone above the background (i.e., the risk that can potentially be reduced via North American anthropogenic emission controls)

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

7 POLICY RELEVANT BACKGROUND OZONE: Ozone concentrations that would exist in the absence of anthropogenic emissions from North America [EPA CD, 2005] Outside natural influences Long-range transport of pollution stratosphere lightning Lightning Background air Ocean Fires Land biosphere X Human activity NORTH AMERICA Policy Relevant Background is not directly observable Must be estimated with models

8 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 Observations: CASTNet Stations (EPA, Nat l Park Service) MODEL: GEOS-CHEM 3D Tropospheric Chemistry Model [Bey et al., 2001] (uses assimilated meteorology; 48 σ; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers) X Elevated sites (> 1.5 km) Low-lying sites

9 Case Studies: Ozone Time Series at Sites used in Lefohn et al. [2001] * CASTNet observations Model Base Case (2001) X Stratospheric influence Background (no N. Amer. Anthrop emissions; present-day CH 4 ) + Natural O 3 level (no global anthrop. Emissions; preindustrial CH 4 ) High-O 3 events dominated by regional pollution regional pollution Background at highaltitude site (2.5 km) not representative of contribution at low-lying sites hemispheric pollution

10 * + X CASTNet sites Model Background Natural O 3 level Stratospheric West (>1.5 km) Southeast (<1.5 km) Background O 3 higher at high-altitude western sites Ozone (ppbv) Background O 3 lower at low-lying southeastern sites; decreases with highest observed O 3 Fiore et al., JGR, 2003 Days in March 2001

11 * + CASTNet sites Model Background Natural O 3 level Stratospheric Background O 3 even lower under polluted conditions Daily mean afternoon O 3 at 58 low-elevation U.S. CASTNet sites June-July-August Ozone (ppbv) Regional Pollution Cumulative Probability Background on polluted days well below 40 ppbv assumed by EPA health risks underestimated with current (1996) approach Fiore et al., JGR, 2003

12 Compiling Results from all (71) CASTNet sites: Natural vs. Anthropogenic Contributions Stratospheric Natural (no global anthrop. emis. (& CH 4 = 700 ppbv)) Probability ppbv -1 Background (no anthrop. emis. in N. Amer) Observed: CASTNet sites Model at CASTNet Daily 1-5 p.m. mean surface O 3 Mar-Oct 2001 Typical ozone values in U.S. surface air: Background ppbv; Natural ppbv; Stratosphere < 20 ppbv Anthropogenic methane enhances background above natural Fiore et al., JGR, 2003

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

14 Anthropogenic Methane Emissions Enhance the Tropospheric Ozone Background Tg O base case 50% methane 50% NOx 50% NMVOCs 50% NOx+NMVOCs 50% CO 50% all natural Sensitivity of global tropospheric ozone inventory in GEOS-CHEM to 50% global reductions in anthropogenic emissions Anthropogenic emissions of NO x and methane have largest influence on tropospheric ozone. climate? air pollution? Fiore et al., GRL, 2002

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

16 Double dividend of Methane Controls: Decreased greenhouse warming and improved air quality Radiative Forcing* (W m -2 ) 50% anth. VOC 50% anth. CH 4 50% anth. NO x 2030 A B1 Number of U.S. summer gridsquare days with O 3 > 80 ppbv % (base) anth. VOC 50% 50% 2030 anth. anth. A1 CH 4 NO x 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% CH 4 links air quality & climate via background O 3 Fiore et al., GRL, 2002

17 CONCLUSIONS... and their implications for public policy 1. Background O 3 is typically less than 40 ppbv; even lower under polluted conditions health risk from O 3 underestimated in 1996 EPA risk assessments 2. Pollution from North America contributes to high-o 3 events at remote U.S. sites in spring these events do not represent U.S. background conditions and should not be used to challenge legitimacy of O 3 NAAQS 3. Hemispheric pollution enhances U.S. background international agreements to reduce hemispheric background should improve air quality & facilitate compliance w/ more stringent standards reducing CH 4 decreases both background O 3 and greenhouse forcing; enables simultaneous pursuit of air quality & climate goals global CH 4 controls are viable [J. West seminar next Friday!] and complement local-to-regional NO x & NMVOC controls What is the role of uncertainties/changes in natural emissions? First step: BVOC emissions

18 Isoprene Emissions are generally thought to contribute to O 3 production over the eastern United States [e.g.trainer et al., 1987; NRC, 1991] TEMP (VOC) ISOPRENE + + NO x PAR O 3 air pollution (smog) Harmful to health, vegetation Leaf Area O 3 formation in eastern U.S. is typically more responsive to controls on anthrop. NO x emissions than anthrop. VOCs Vegetation changes Impact on O 3? -- Climate -- Land-use

19 Isoprene Emission Inventories uncertain by at least a factor of 2 Isoprene emissions July 1996 GEIA: Global Isoprene Emission Inventory BEIS2: Regional Isoprene Emission Inventory 7.1 Tg C 2.6 Tg C [from Paul Palmer] [10 12 atom C cm -2 s -1 ]

20 Choice of isoprene inventory critical for predicting base-case O 3 (2001 meteorology; 1x1 nested GEOS-CHEM [Wang et al., 2004; Li et al., 2004]) GEIA: global inventory July Anthrop. NO x emissions July isoprene emissions 5.6 Tg C Purves et al., [2004] (based on FIA data; similar to BEIS-2) (10 11 molec cm -2 s -1 ) 0.43 Tg N Difference in July 1-5 p.m. surface O 3 (Purves GEIA) High-NO x regime GEIA 2.8 Tg C isoprenesaturated (10 11 molecules isoprene cm -2 s -1 ) (ppbv)

21 Complicated Chemistry: Isoprene may also decrease surface O 3 in low-no x, high isoprene settings ISOPRENE + + NO x O 3 High-NO x (very fast) NO RO NO O OH ISOPRENE Isoprene nitrates Sink for NO x? O 3 (slower ) O 3 Low-NO x, high-isoprene Thought to occur in pre-industrial [Mickley et al., 2001]; and present-day tropical regions [von Kuhlmann et al., 2004] Isoprene does react directly with O 3 in our SE US GEIA simulation: O 3 +biogenics (10d) comparable to O 3 +HO x (16d), O 3 +hν -> OH (11d)

22 What is the O 3 sensitivity to the uncertain fate of organic isoprene nitrates? High-NO x (very fast) NO RO NO OH 2 2 O 3 ISOPRENE Change in July mean 1-5 p.m. surface O 3 when isoprene nitrates act as a NO x sink Isoprene nitrates Sink for NO x? MOZART ppbv impact! ppbv

23 Recent Changes in Biogenic VOC Emissions Isoprene [Purves et al., Global Change Biology, 2004] Substantial isoprene increases in southeastern USA largely driven by human land-use decisions Land-use changes not presently considered in CTMs Monoterpenes Sweetgum Invasion of Pine plantations Percent Change mid-1980s to mid-1990s

24 Do these recent changes in isoprene emissions influence surface O 3? A Two-Model Perspective Change in July 1-5 p.m. surface O 3 GEOS-CHEM MOZART-2 Little change (NO x -sensitive) O 3 decreases (same as previous result) ppbv O 3 increases (1-2 ppbv) Isoprene nitrates are not a NO x sink in standard MOZART-2; Isoprene changes (%) Results are sensitive to this assumption!

25 Chemical uncertainty: MOZART-2 shows similar results to GEOS-CHEM if isoprene nitrates are a NO x sink Change in July 1-5 p.m. surface O 3 (ppbv) (due to isop emis changes from mid-1980s to mid-1990s) With 12% yield of isoprene nitrates GEOS-CHEM: GEIA GEOS-CHEM: Purves MOZART-2: GEIA ppbv Understanding fate of isop. nitrates essential for predicting sign of response to changes in isoprene emissions

26 Conclusions and Remaining Challenges Better constrained isoprene emissions are needed to quantify: 1. isoprene contribution to Eastern U.S. surface O 3 2. how O 3 responds to both anthrop. and biogenic emission changes Utility of satellite CH 2 O columns? New inventories (MEGAN, BEIS-3) more accurate? Insights from aircraft campaigns? Recent isoprene increases may have reduced surface O 3 in the SE Does this regime actually exist? Fate of organic nitrates produced during isoprene oxidation? Fiore et al., JGR, 2005