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QUESTIONS 1. Is hexane more or less reactive with OH than propane? 2. Is pentene or isoprene more reactive with OH? LARGE SUPPLY OF BIOGENIC VOCs – unrecognized until the 1990s Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime! recognized in Revised Clean Air Act of 1999 Anthropogenic VOCs Isoprene (biogenic VOC) Jacob et al., [1993] Isoprene (C5H8) and monoterpenes (C10H16) are oxidized by OH, O3 and NO3 (generally analogous to alkene rxn) secondary organic aerosol (SOA) LATEST INVENTORIES OF BIOGENIC vs. ANTHROPOGENIC VOCs …notice difference in scale! Millet et al. [2007] GLOBAL DISTRIBUTION OF ISOPRENE EMISSIONS E = f (T, h) MEGAN biogenic emission model (Guenther et al., 2006) CONSTRAINT ON VOC EMISSIONS FROM SPACE OBSERVATIONS OF FORMALDEHYDE GOME satellite observations (July 1996) 2.5x1016 molecules cm-2 2 1.5 1 0.5 detection limit South Atlantic Anomaly (disregard) High values are associated with biogenic emissions (eastern US), anthropogenic emissions (China), fires (Africa, Siberia) 0 -0.5 SIMULATED SENSITIVITY OF SURFACE OZONE TO EMISSION CONTROLS U.S is NOx-limited! Jacob et al., [1993b] U.S. GROWTH MEASURES (2008) U.S. EMISSIONS OF OZONE PRECURSORS and trends over past 20 years • Vehicles • Fires Fuel combustion • vehicles • power plants Flat/down Down 40% Anthropogenic VOCs •Fuel combustion & transport • Solvents Down 30% Isoprene (biogenic VOC) Vegetation Flat OZONE TRENDS IN U.S. http://www.epa.gov/airtrends/ National trend Fort Collins trend TREND IN 4th-HIGHEST 8-HOUR OZONE, 2004-2006 vs 1990-1992 DEPENDENCE OF OZONE PRODUCTION ON NOx AND HYDROCARBONS NET: RH + 4O2 R’CHO + 2O3 + H2O O3 HOxfamily RO2 RH NO 5 RO 4 O3 PHOx 6 O2 7 OH NO NO2, M 9 HNO3 2k4 PHOx [ RH ] P (O3 ) k9 [ NO2 ][ M ] “NOx- saturated” or “hydrocarbon-limited” regime HO2 8 O3 H2O2 PHOx 1/ 2 P (O3 ) 2k7 ( ) [ NO] k8 “NOx-limited” regime ALTHOUGH THE O3 PRODUCTION RATE IS ~ [NOx], THE TOTAL O3 PRODUCED IS HYDROCARBON-DEPENDENT AND [O3] = f(ENOx) IS STRONGLY NONLINEAR P(O3) L(NOx) HO2,RO2,O3 OH, O3 NO hv NO2 Emission HNO3 Deposition Define ozone production efficiency (OPE) as the total number of O3 molecules produced per unit NOx emitted. Assuming NOx steady state, efficient HOx cycling, and loss of NO2 by reaction with OH: P (O3 ) 2k7 [ HO2 ][ NO] 2k4 [ RH ] OPE = L( NOx ) k9 [ NO2 ][OH ] k9 [ NO2 ] OPE m as NOx k e strong nonlinearity 1999-2004 NOx EMISSION REDUCTIONS AND SIMULATED EFFECTS ON SURFACE OZONE Hudman et al. [2008] 50% decrease in power plant emissions 20% decrease in total U.S. emissions TRENDS IN 4th-HIGHEST 8-HOUR OZONE AT NATIONAL PARKS, 1992-2001 [EPA, 2003] OBSERVED TREND IN OZONE BACKGROUND OVER CALIFORNIA IN SPRING SUGGESTS 10-15 ppbv INCREASE OVER PAST 20 YEARS Jaffe et al. [2003] Trend: 0.5-0.8 ppbv yr-1 Background: concentration that would be present in absence of local anthropogenic emissions RISING OZONE BACKGROUND IN EUROPE Hohenpeissenberg/ Payerne polluted 3-5 km background Naja et al. [2003] Changes in anthrop. NOx emissions Mace Head, 1987-2004 [Simmonds et al., 2004] HEMISPHERIC OZONE POLLUTION: IMPLICATIONS OF ENHANCED OZONE BACKGROUND FOR MEETING AIR QUALITY STANDARDS (AQS) Europe AQS (8-h avg.) Europe AQS (seasonal) U.S. AQS (8-h avg.) U.S. AQS (1-h avg.) Was here until 2008! 0 Preindustrial ozone background 20 40 60 Present-day ozone background at northern midlatitudes 80 100 120 ppbv GLOBAL OZONE BACKGROUND: METHANE AND NOx ARE THE LIMITING PRECURSORS Sensitivity of global tropospheric ozone inventory (Tg) to 50% global reductions in anthropogenic precursor emissions GEOS-Chem model [Fiore et al., 2002] 1995 base case 330 320 50% methane 310 300 50% NOx 290 280 270 260 250 50% NMVOCs 50% NOx+NMVOCs 50% CO 240 50% all Anthropogenic methane enhances surface ozone natural by 4-6 ppbv worldwide PROJECTIONS OF GLOBAL NOx EMISSIONS Anthropogenic NOx emissions [IPCC, 2001] 2000 “Optimistic” IPCC scenario: OECD, U.S. m20%, Asia k 50% 2020 109 atoms N cm-2 s-1 EFFECT OF INCREASING SIBERIAN FOREST FIRES ON SUMMER SURFACE OZONE IN PACIFIC NORTHWEST Observations GEOS-Chem ozone enhancements Siberian fires Ozone Mean summer 2003 enhancement of 5-9 ppbv (9-17 ppbv in events) Jaffe et al. [2004] EFFECT OF CLIMATE CHANGE ON OZONE AIR QUALITY Probability of max 8-h O3 > 84 ppbv vs. daily max. T Ozone exceedances of 90 ppbv, summer 2003 Lin et al. [Atm. Env. 2001] Correlation of high ozone with temperature is driven by (1) stagnation, (2) biogenic hydrocarbon emissions, (3) chemistry EFFECT OF CLIMATE CHANGE ON REGIONAL STAGNATION GISS GCM simulations for 2050 vs. present-day climate using pollution tracers with constant emissions weather map illustrating cyclonic ventilation of the eastern U.S. 2045-2052 summer 1995-2002 Pollution episodes double in duration in 2050 due to decreasing frequency of cyclones ventilating the eastern U.S; expected result of greenhouse warming. Mickley et al. [2004]