Variability in surface ozone background over the United States: Implications for air quality policy Arlene Fiore 1, Daniel J. Jacob, Hongyu Liu 2, Robert M. Yantosca, T. Duncan Fairlie 2, Qinbin Li Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 1 Now at Princeton/GFDL, Princeton, NJ 2 Now at National Institute of Aerospace, Hampton, VA When setting the National Ambient Air Quality Standard (NAAQS) for ozone (O 3 ) in surface air over the United States, the U.S. EPA accounts for a background O 3 level above which risk to human health is assessed. Introduction “REGULATORY BACKGROUND” DEFINITION: Ozone concentrations that would exist in the absence of anthrop. emissions from North America [EPA, 2003] ppbv [EPA, 1996] i.e., background includes: Natural O 3 from the stratosphere or produced from natural precursor emissions anywhere Anthropogenic O 3 produced from precursor emissions outside North America Two questions under debate in the current review of the O 3 NAAQS: 1. Is the present (or more stringent future) NAAQS too close to background concentrations? 2. What background concentrations should be used to assess the health risk associated with exposure to O 3 ? CASTNet sites Model at CASTNet Model entire region Background Natural O 3 level Stratospheric + * Mean background: 36 ppbv Case Studies: Background contribution to high-O 3 events in spring Haywood County, North Carolina (83W, 35N) Ozone (ppbv) Health and Environmental Risk 1040observed risk from natural O 3 levels risk from background O 3 total risk from observed O 3 Excess risk associated with observed O 3 concentration above background EPA presently uses a 40 ppbv background Range of Observed Background Intercept 30 ppb background (clean air) (Pollution coordinate) U.S. Ozone Standard Degree of Regional Pollution Ozone (ppbv) (NOy-NOx (ppbv)) Summer p.m. observations at Harvard Forest [Munger et al., 1996, 1998 ] Range of background that was considered for revised O 3 standard Frequently observed surface O 3 concentrations attributed to natural background by Lefohn et al. [2001] background presently used in EPA risk assessments One approach for estimating O 3 background from observations Approach Sensitivity Simulations in GEOS-CHEM for source attribution; Mar-Oct 2001 Regional Pollution = Standard – Background Hemispheric Pollution = Background – Natural O 3 level SimulationDescription Standard2x2.5 GEOS-CHEM, 48 sigma levels, 2001 Backgroundno anthrop. NO x, CO, NMVOC emis. from N. America Natural O 3 levelno anthrop. NO x, CO, NMVOC emis. globally; CH 4 = 700 ppbv Stratospherictagged O 3 tracer simulation Apply GEOS-CHEM 3D Tropospheric Chemistry Model [Bey et al., 2001] (uses assim. met.; s; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers) in conjunction with O 3 observations from the Clean Air Status and Trends Network (CASTNet): to investigate how background O 3 and its sources vary with season and region to diagnose the source of high-O 3 events at remote sites in spring Monthly mean afternoon (1-5 p.m.) surface O 3 { { { Hemispheric pollution enhancement (5-12 ppbv) Mean background: ppbv Mean natural level: ppbv Mean stratosphere: 2-7 ppbv Regional pollution from N. Amer. emis. (8-30 ppbv) Mar-Oct 2001 U.S. daily mean afternoon surface O 3 Background < 50 ppbv; typically ppbv Natural level < 40 ppbv; typically ppbv Stratosphere < 20 ppbv; typically < 10 ppbv { CASTNet sites Model Background Natural O 3 level Stratospheric + * Hemispheric pollution Regional pollution }  } 1. Background O 3 is generally ppbv, with some incidences of ppbv in the west in spring at high- elevation sites (> 2km). The background declines from spring to summer and further decreases during O3 pollution episodes. It is best defined as a function of season, site altitude, and total O 3 concentration 2. Previous interpretations of a stratospheric source for high-O 3 events at western U.S. sites in spring underestimated the role of regional and hemispheric pollution. Health risk is underestimated with 40 ppbv background presently used in EPA risk assessments, esp. under polluted conditions These events do not represent U.S. background conditions and do not compromise the current O 3 standard. Conclusions Acknowledgments References 3. Natural O 3 levels are typically ppbv and always < 40 ppbv. International controls to reduce the hemispheric pollution background would facilitate compliance with an AOT40-type standard (cumulative exposure to O 3 above 40 ppbv) in the United States. We thank David McKee and Joe Pinto at U.S. EPA for helpful conversations. This work was supported by The EPA Office of Air Quality Planning and Standards (OAQPS) under the Intercontinental Transport and Climatic Effects of Air Pollutants (ICAP) program. Bey, I., D.J. Jacob, R.M. Yantosca, J.A. Logan, B. Field, A.M. Fiore, Q. Li, H. Liu, L.J. Mickley, and M. Schultz, Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res., 106, 23,073-23,096, Lefohn, A.S., S.J. Oltmans, T. Dann, and H.B. Singh, Present-day variability of background ozone in the lower troposphere, J. Geophys. Res., 106, , Munger, J.W., S. C. Wofsy, P. S. Bakwin, S.-M. Fan, M. L. Goulden, B. C. Daube, A H. Goldstein, K. E. Moore, and D. R. Fitzjarrald, Atmospheric Deposition of Reactive Nitrogen Oxides and Ozone in a Temperate Deciduous Forest and a Sub-arctic Woodland. 1. Measurements and mechanisms, J. Geophys. Res. 101, 12,639-12,657, Munger, J.W., S-M. Fan, P.S. Bakwin, M.L. Goulden, A.H. Goldstein, A.S. Colman, and S.C. Wofsy, Regional budgets for nitrogen oxides from continental sources: Variations of rates for oxidation and deposition with season and distance from source regions, J. Geophys. Res. 103, , Spatial and Seasonal Variation in mean background concentrations X High-O 3 event May 1, 2000 Regional pollution largely controls high-O 3 events in spring; Model explains these events without a large stratospheric influence. O 3 produced in continental lower trop Isentropic back-trajectory calculated from the GEOS meteorological fields with the NASA Langley 3-D trajectory model at T = 300K for air arriving in North Carolina on May 1, Natural vs. Anthropogenic Contributions to High-O 3 occurrences The May 1, 2000 high-O 3 event has been attributed to stratospheric origin on basis of back-trajectory analysis [see associates.com/ncreport.htm]. The GEOS-CHEM met fields do predict strong subsidence over the region, consistent with the previous trajectories, yet the model indicates that the event is largely controlled by regional pollution. associates.com/ncreport.htm Background: ppbv Natural level: 9-23 ppbv Stratosphere: < 7 ppbv High-O 3 events: dominated by regional pollution (44 and 50 ppbv on June 26 & 29) with < 30 ppbv background and minor stratospheric influence (~2 ppbv) Background O 3 for risk assessment: f (season, altitude, total O 3 ) Mean background: 27 ppbv Mean background: 30 ppbv Mean background: 23 ppbv Cumulative Probability (%) Ozone (ppbv) 12 elevated sites (>1.5 km) (X in figure to left) 58 surface sites (<1.5 km) ( in figure to left) X X Elevated (> 1.5 km) Surface (<1.5 km) CASTNet Stations measuring surface O 3 in 2001 Using average background for pollution episodes underestimates risk to human health! Lower background at surface sites; Maximum contribution at the center of the O 3 distribution Enhancement from North American (& hemispheric) pollution for high-O 3 events in all seasons Regional Pollution Lower background; larger pollution influence in summer & fall Large range in quantitative estimates of background concentrations in U.S. surface air Yellowstone NP, Wyoming (110W, 45N) Voyageurs NP, Minnesota (93W, 48N) WestSoutheast March 2001 Results Model captures overall distribution of observed concentrations Hemispheric pollution (difference of background & natural) is generally 4-12 ppbv, rarely exceeds 20 ppbv. 40 ppbv background too high  underestimates health risk Why are simulated background concentrations lower than estimates based on obs at remote/rural sites? Observations may contain regional pollution. (Note: hemispheric pollution is included in current EPA background definition.) Why are the intepretations based on similar back-trajectories so different? The back-trajectories diagnose subsidence from the free trop. Free-trop O 3 contains a substantial anthropogenic enhancement. Subsiding conditions are associated with strong inversions and clear skies that promote O 3 production in the boundary layer. Background, natural & stratospheric O 3 contributions to surface O 3 are higher at elevated western sites than eastern sites. Peak O 3 associated with lower background concentrations