Background Ozone in Surface Air: Origin, Variability, and Policy Implications Arlene M. Fiore Goddard Institute for Space Studies May 20, 2005

Acknowledgments

Larry Horowitz Chip Levy Jason West Denise Mauzerall Daniel Jacob Mat Evans Duncan Fairlie Brendan Field Qinbin Li Hongyu Liu Bob Yantosca David Streets Suneeta Fernandes Carey Jang

What is the origin of tropospheric ozone? Stratospheric O 3 h

n

NO 2 NO O 3 Stratosphere ~12 km Free Troposphere Hemispheric Pollution OH HO 2 VOC, CH 4 , CO NO x VOC O 3 Direct Intercontinental Transport air pollution (smog) Boundary layer (0-3 km) air pollution (smog) NO x VOC O 3 CONTINENT 1 OCEAN CONTINENT 2

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

m

m (PM 10 ) Particles < 2.5

m

m (PM 2.5

) 0.007

11.1

Carbon monoxide (CO) Lead Any pollutant 0.7

2.7

124 ppbv: 40.2

72.7

EPA [2002] 84 ppbv: 110.3

133.1

0 50 100 Millions of People 150

The Risk Increment Above the Background is Considered when setting the NAAQS for Ozone Environmenta l risk Acceptable added risk Policy-Relevant Background NAAQS Ozone Concentration



Need a quantitative estimate for background ozone



EPA chose a constant value (40 ppbv) in previous review of O 3 standard

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 20 40 60 84 ppbv: threshold for current U.S. O 3 standard 80 100 Used by EPA to assess health risk from O 3 (1996) Range from this work [

Fiore et al., 2003

] Frequent observations previously attributed to natural background [

Lefohn et al., 2001

] O 3 (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 Long-range transport of pollution “Background” air X Human Ocean Fires Land biosphere activity NORTH AMERICA Policy Relevant Background is not directly observable



Must be estimated with models

Approach: Insights from two chemical transport models GEOS-CHEM [ Bey et al ., 2001]

• GEOS-3 GMAO met • 2°x2.5°; 30 s -levels • Synoz upper boundary

MOZART-2 [ Horowitz et al ., 2003]

• NCEP meteorology • T62 (1.9°) ; 28 s -levels • Strat O 3 relaxed to climatology AR4 budgets c/o Jérôme Drevet

Ozone Budgets in IPCC-AR4 Tropospheric Chemistry Models Model

David Stevenson Frank Dentener

PROD LOSS DEP X 19-Model Mean MOZART GEOS-CHEM STRAT CHASER_CTM CHASER_GCM FRSGC GEOS-CHEM GFDL GMICCM GMIDAO gmigis LLNL-IMPACT LMDzINCA LMDzINCAc NCAR STOCHEM_HadAM3 TM4 TM5 UM_CAM MOZECH STOCHEM_HadGEM ULAQ Average Production Loss 5042 5032 5135 4490 5263 5331 5124 4722 5432 4912 4931 4964 5331 4806 4580 3922 6920 5114 5017 5056 4594 4620 4733 3770 5087 5059 4940 4396 5160 4182 4027 4670 4821 4594 4623 3363 6617 3757 3639 4561 P-L 447 412 401 719 176 272 184 326 271 729 903 293 509 211 -43 558 Deposition 948 Strat. Calc.

948 907 1016 963 862 763 856 1014 1232 1227 906 945 720 827 1172 303 1356 1377 495 963 1507 1491 1014 Strat. Model.

501 536 505 296 787 590 579 530 742 502 *** 324 *** 612 435 508 871 614 *** 660 *** 151 *** 114 519 517 506 555 508 939 557 541 537 739 684 383 504 550 688 586

Approach: 2001 CASTNet Observations (EPA, NPS) X Elevated sites (> 1.5 km) Low-lying sites 1. quantify PRB 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

Case Study #1: Voyageurs National Park, Minnesota (May-June 2001)

Lefohn et al.

[2001] suggest a stratospheric source as the likely origin of high-O 3 frequently observed in June events

* + X CASTNet observations Model Background Natural O 3 }

D =

level } Regional pollution

D =

Hemispheric pollution Stratospheric High-O 3 events: dominated by regional pollution; minor stratospheric influence (~2 ppbv) regional pollution hemispheric pollution Model Results from GEOS-CHEM PRB: 15-36 ppbv Natural level: 9-23 ppbv Stratosphere: < 7 ppbv

Case Study #2: Yellowstone National Park, Wyoming (March-May 2001)

Frequent high-O 3 events previously attributed to natural, stratospheric source [

Lefohn et al.,

2001]

* + X CASTNet observations Model PRB Natural O 3 }

D =

level } Regional pollution

D =

Hemispheric pollution Stratospheric regional pollution hemispheric pollution Model Results from GEOS-CHEM Background at high-altitude site (2.5 km) not necessarily representative of background contribution at low-lying sites

* Both models show that PRB ozone is higher at high-altitude site CASTNet observations GEOS-CHEM Model GEOS-CHEM PRB MOZART Model MOZART PRB Elevation: 430 m PRB Range; mean 15-36; 25 14-34; 23 May – June 2001 19 Elevation: 2470 m PRB Range; mean 29-50; 38 21-56; 35 Mar – May 2001

PRB ozone is lower under polluted conditions: typically below 25 ppbv Daily mean afternoon O 3 at 58 low-elevation U.S. CASTNet sites June-July-August * CASTNet sites GEOS-CHEM Model PRB Ozone Ozone (ppbv) Regional Pollution Cumulative Probability



Assuming constant 40 ppbv background underestimates health risks on most polluted days

Compiling daily afternoon (1-5 p.m. mean) surface ozone from all CASTNet sites for March-October 2001: PRB ozone is typically 20-35 ppbv Natural 18 ±5 ppbv GEOS-CHEM PRB 26 ±7 ppbv GEOS-CHEM PRB 29 ±9 ppbv MOZART-2 CASTNet sites GEOS-CHEM Model at CASTNet



Hemispheric Pollution enhances U.S. PRB by 8 ±4 ppbv



Prior work suggests 6 ppbv from anthrop. CH 4 on average

Radiative Forcing of Climate from Preindustrial to Present: Important Contributions from Methane and Ozone

Hansen, Scientific American

, 2004

Double dividend of Methane Controls: Decreased greenhouse warming and improved air quality CLIMATE: Radiative Forcing (W m -2 ) AIR QUALITY: Number of U.S. summer grid-square days with O 3 > 80 ppbv



50% anth.

VOC 50% anth.

CH 4 50% anth.

NO x 1995 (base) 50% anth.

VOC 50% anth.

CH 4 50% anth.

NO x Methane links air quality and climate via background ozone

Fiore et al., GRL, 2002

Response of Global Surface Ozone to 50% decrease in global methane emissions (actually changing uniform concentration from 1700 to 1000 ppbv)

• Ozone decreases by 1-6 ppb • ~3 ppb over land in US summer ** ~60% of reduction in 10 yr; ~80% in 20 yr.

How Much Methane Can Be Reduced?

Top bar: IEA (2003), for 5 industrial sectors. Lower bar: EPA (2003), for 4 industrial and 1 agricultural sector.

Comparison: Clean Air Interstate Rule (proposed) reduces 0.86 ppb over the eastern US, at $0.88 billion yr -1 , through NO X control.

West & Fiore, ES&T, 2005

Tropospheric ozone response to anthropogenic methane emission changes is fairly linear X

MOZART-2 (this work) TM3 [

Dentener et al

.,

ACPD

, 2005] GISS [

Shindell et al., GRL

, 2005 GEOS-CHEM [

Fiore et al., GRL

, 2002] IPCC TAR [

Prather et al

., 2001] Shindell et al., 2005 report that tropospheric ozone responds linearly to 10, 25, 50, 75, 100% decreases in anthropogenic CH 4

Methane Emissions in EDGAR inventory early 1990s; Tg CH 4 yr -1 Energy, landfills, wastewater Ruminants Rice Biomass burning Ocean Biogenic 95 93 60 86 10 204 TOTAL 547



Are ozone decreases independent of CH 4 source location?

Cut Global Anthropogenic by 40%: (1) All in Asia (2) Everywhere in the globe

July 2000 surface O 3 change mainly independent of CH 4 source location, slightly larger response in source region (Transient simulations with EDGAR 1990 emissions, beginning 1990) Zero anthrop. CH 4 emissions from Asia Global 40% decrease in anthrop. CH 4 emis.

Zero Asia – global 40%

U.S. Surface Afternoon Ozone Response in Summer also independent of methane emission location MEAN DIFFERENCE NO ASIAN CH 4 MAX DIFFERENCE (Max daily mean afternoon JJA) GLOBAL 40% DECREASE IN ANTHROP. CH 4



Stronger Sensitivity in NO x -saturated regions (Los Angeles)

Local CH 4 oxidation contributes up to ~30% of the total decrease in surface O 3 from lowering CH 4 concentrations in a NO x -saturated region Change in MOZART-2 surface afternoon (1-5 p.m.) O 3 concentration after 1 day, when a 300 ppbv decrease in CH 4 concentrations is imposed below 800 hPa in Los Angeles 7/23/00 -1.00

-0.75

-0.50 -0.25

ppbv O 3

CLIMATE IMPACTS : Change in July 2000 Trop. O 3 (to 200 hPa) Columns 40% decrease in global anthrop.

CH 4 emissions Zero CH 4 emissions from Asia (= 40% decrease in global anthrop.) Dobson Units No Asia – (40% global decrease) DU Tropospheric O 3 column response is independent of CH 4 emission location except for small (~10%) local changes

AIR POLLUTION IMPACTS : 2030 Avoided Premature Mortalities (A2 scenario - 65 Tg CH 4 emissions) Change in 8-hr summer surface ozone from a 20% decrease in global anthrop. methane emissions Assume 25 ppb threshold, CH 4 reductions starting in 2000 Avoided Mortalities in 2030 Reducing anthrop. CH 4 emis. by 20% (starting in 2000): -- decreases global surface ozone (~1 ppbv in populated areas) -- prevents ~34,000 premature deaths in 2030

West et al., 2005

Conclusions

1. Policy-Relevant Background - 25±5 ppbv in surface air over the United States: at low-elevation sites in summer -- varies with season, altitude; anti-correlated with high domestic O 3 -- 8±4 ppbv from hemispheric pollution (~5-7 ppbv from anthr. CH



Should these be science conclusions from paper incorporated 40 ppbv previously used by EPA underestimates health risks 4 )



international negotiations to reduce hemispheric background would facilitate compliance with more stringent standards 2. 20% reductions in anthrop. methane emissions possible now: -- reduce surface ozone globally by ~1 ppbv in populated regions -- lower global radiative forcing by ~0.13 W m -2 -- avoid 34,000 premature mortalities in 2030 3. Climate and air quality benefits from controls on anthropogenic methane emissions are largely independent of source location -- small enhancements (~10%) in source region -- enhanced surface ozone response (up to ~30% total response) to CH 4 changes in NO x -saturated regions