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

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

What is the origin of tropospheric ozone? Stratospheric O3 Stratosphere O3 ~12 km hn Free Troposphere NO2 NO Hemispheric Pollution OH HO2 Direct Intercontinental Transport Boundary layer (0-3 km) VOC, CH4, CO air pollution (smog) NOx VOC O3 NOx VOC O3 air pollution (smog) CONTINENT 1 CONTINENT 2 OCEAN

Number of People Living in U. S Number of People Living in U.S. Counties Violating National Ambient Air Quality Standards (NAAQS) in 2001 EPA [2002] Nitrogen dioxide 124 ppbv: 40.2 Ozone (O3) 84 ppbv: 110.3 Sulfur dioxide (SO2) 0.007 Particles < 10 mm (PM10) 11.1 Particles < 2.5 mm (PM2.5) 72.7 Carbon monoxide (CO) 0.7 Lead 2.7 Any pollutant 133.1 50 100 150 Millions of People

Background Estimates are Used When Setting NAAQS Environmental risk Acceptable added risk Pollutant concentration Background NAAQS 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)

global modeling studies Range of “background O3” estimates in U.S. surface air Range from prior global modeling studies Range considered by EPA during last revision of O3 standard 84 ppbv: threshold for current U.S. O3 standard 20 40 60 80 100 Used by EPA to assess health risk from O3 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 Frequent observations previously attributed to natural background [Lefohn et al.,JGR 106, 9945-9958, 2001] O3 (ppbv) The U.S. EPA considers background levels when setting the NAAQS

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

Observations: CASTNet Stations (EPA, Nat’l Park Service) APPROACH: Use 2001 CASTNet data in conjunction with GEOS-CHEM to 1. quantify background O3 and its various sources 2. diagnose origin of springtime high-O3 events at remote U.S. sites, previously attributed to natural, stratospheric influence MODEL: GEOS-CHEM 3D Tropospheric Chemistry Model [Bey et al., 2001] (uses assimilated meteorology; 48 s; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers) Observations: CASTNet Stations (EPA, Nat’l Park Service) X Elevated sites (> 1.5 km) Low-lying sites

Case Studies: Ozone Time Series at Sites used in Lefohn et al. [2001] * CASTNet observations Model Base Case (2001) Stratospheric influence Background (no N. Amer. Anthrop emissions; present-day CH4) Natural O3 level (no global anthrop. Emissions; preindustrial CH4) X + High-O3 events dominated by regional pollution regional pollution hemispheric pollution Background at high- altitude site (2.5 km) not representative of contribution at low-lying sites

* West (>1.5 km) Southeast (<1.5 km) CASTNet sites Model Background Natural O3 level Stratospheric + X Background O3 higher at high-altitude western sites Ozone (ppbv) Background O3 lower at low-lying southeastern sites; decreases with highest observed O3 Fiore et al., JGR, 2003 Days in March 2001

Background O3 even lower under polluted conditions CASTNet sites Model Background Natural O3 level Stratospheric + * Background O3 even lower under polluted conditions Daily mean afternoon O3 at 58 low-elevation U.S. CASTNet sites June-July-August Regional Pollution Ozone (ppbv) 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

Daily 1-5 p.m. mean surface O3 Mar-Oct 2001 Compiling Results from all (71) CASTNet sites: Natural vs. Anthropogenic Contributions Stratospheric Natural (no global anthrop. emis. (& CH4 = 700 ppbv)) Background (no anthrop. emis. in N. Amer) Observed: CASTNet sites Probability ppbv-1 Model at CASTNet Daily 1-5 p.m. mean surface O3 Mar-Oct 2001 Typical ozone values in U.S. surface air: Background 15-35 ppbv; Natural 10-25 ppbv; Stratosphere < 20 ppbv Anthropogenic methane enhances “background” above “natural” Fiore et al., JGR, 2003

More than half of global methane emissions are influenced by human activities BIOMASS BURNING 20 ANIMALS 90 WETLANDS 180 LANDFILLS 50 GLOBAL METHANE SOURCES (Tg CH4 yr-1) GAS 60 TERMITES 25 COAL 40 RICE 85

Anthropogenic Methane Emissions Enhance the Tropospheric Ozone Background Tg O3 Sensitivity of global tropospheric ozone inventory in GEOS-CHEM to 50% global reductions in anthropogenic emissions Anthropogenic emissions of NOx and methane have largest influence on tropospheric ozone…. climate? air pollution? Fiore et al., GRL, 2002

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

Double dividend of Methane Controls: Decreased greenhouse warming and improved air quality Number of U.S. summer grid-square days with O3 > 80 ppbv Radiative Forcing* (W m-2) 50% anth. VOC 50% anth. CH4 50% anth. NOx 2030 A1 2030 B1 1995 (base) 50% anth. VOC 50% anth. CH4 50% anth. NOx 2030 A1 2030 B1 IPCC scenario Anthrop. NOx emissions (2030 vs. present) Global U.S. Methane emissions A1 +80% -20% +30% B1 -5% -50% +12% CH4 links air quality & climate via background O3 Fiore et al., GRL, 2002

Hemispheric pollution enhances U.S. background CONCLUSIONS ... and their implications for public policy Background O3 is typically less than 40 ppbv; even lower under polluted conditions Pollution from North America contributes to high-O3 events at remote U.S. sites in spring Hemispheric pollution enhances U.S. background health risk from O3 underestimated in 1996 EPA risk assessments these events do not represent U.S. background conditions and should not be used to challenge legitimacy of O3 NAAQS international agreements to reduce hemispheric background should improve air quality & facilitate compliance w/ more stringent standards reducing CH4 decreases both background O3 and greenhouse forcing; enables simultaneous pursuit of air quality & climate goals global CH4 controls are viable [J. West seminar next Friday!] and complement local-to-regional NOx & NMVOC controls What is the role of uncertainties/changes in “natural” emissions? First step: BVOC emissions

O3 + + NOx  TEMP PAR Leaf Area Vegetation changes  Impact on O3? Isoprene Emissions are generally thought to contribute to O3 production over the eastern United States [e.g.Trainer et al., 1987; NRC, 1991] (VOC) O3 + + NOx  ISOPRENE air pollution (smog) Harmful to health, vegetation TEMP PAR Leaf Area O3 formation in eastern U.S. is typically more responsive to controls on anthrop. NOx emissions than anthrop. VOCs Vegetation changes  Impact on O3? -- Climate -- Land-use

Isoprene emissions – July 1996 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] [1012 atom C cm-2 s-1]

Choice of isoprene inventory critical for predicting base-case O3 (2001 meteorology; 1x1 nested GEOS-CHEM [Wang et al., 2004; Li et al., 2004]) GEIA: global inventory July Anthrop. NOx emissions 0.43 Tg N (1011 molec cm-2 s-1) 5.6 Tg C July isoprene emissions Purves et al., [2004] (based on FIA data; similar to BEIS-2) (ppbv) Difference in July 1-5 p.m. surface O3 (Purves–GEIA) High-NOx regime 2.8 Tg C “isoprene- saturated” GEIA (1011 molecules isoprene cm-2 s-1)

Complicated Chemistry: Isoprene may also decrease surface O3 in low-NOx, high isoprene settings + O3 O3 OH RO2 NO (very fast) NO2 High-NOx ISOPRENE Isoprene nitrates Sink for NOx? O3 (slower ) Low-NOx, 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 O3 in our SE US GEIA simulation: O3+biogenics (10d) comparable to O3+HOx (16d), O3+hn -> OH (11d)

O3 What is the O3 sensitivity to the uncertain fate of organic isoprene nitrates? OH RO2 NO (very fast) NO2 High-NOx Isoprene nitrates Sink for NOx? ISOPRENE O3 ppbv MOZART-2 Change in July mean 1-5 p.m. surface O3 when isoprene nitrates act as a NOx sink  6-12 ppbv impact!

Recent Changes in Biogenic VOC Emissions [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 Isoprene Monoterpenes Sweetgum Invasion of Pine plantations -20 –10 0 +10 +20 +30 Percent Change mid-1980s to mid-1990s

Change in July 1-5 p.m. surface O3 Do these recent changes in isoprene emissions influence surface O3? A Two-Model Perspective Change in July 1-5 p.m. surface O3 GEOS-CHEM MOZART-2 Little change (NOx-sensitive) O3 increases (1-2 ppbv) ppbv O3 decreases (same as previous result) Isoprene nitrates are not a NOx sink in standard MOZART-2; Results are sensitive to this assumption! -20 –10 0 +10 +20 +30 (%) Isoprene changes

GEOS-CHEM: GEIA GEOS-CHEM: Purves MOZART-2: GEIA ppbv Chemical uncertainty: MOZART-2 shows similar results to GEOS-CHEM if isoprene nitrates are a NOx sink Change in July 1-5 p.m. surface O3 (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

Conclusions and Remaining Challenges Better constrained isoprene emissions are needed to quantify: isoprene contribution to Eastern U.S. surface O3 how O3 responds to both anthrop. and biogenic emission changes Utility of satellite CH2O columns? New inventories (MEGAN, BEIS-3) more accurate? Insights from aircraft campaigns? Recent isoprene increases may have reduced surface O3 in the SE Does this regime actually exist? Fate of organic nitrates produced during isoprene oxidation? Fiore et al., JGR, 2005