Background Ozone in Surface Air:Background Ozone in Surface Air:

Origin, Variability, and Policy Origin, Variability, and Policy Implications Implications

Arlene M. Fiore

Goddard Institute for Space

StudiesMay 20, 2005

Acknowledgments

Daniel JacobMat EvansDuncan FairlieBrendan Field

David StreetsSuneeta Fernandes

Carey Jang

Qinbin Li Hongyu Liu Bob Yantosca

Jason WestDenise Mauzerall

Larry HorowitzChip Levy

NOx

VOC

NOx

VOC

CONTINENT 2 OCEAN

O3

Boundary layer

(0-3 km)

Free Troposphere

CONTINENT 1

What is the origin of tropospheric ozone? What is the origin of tropospheric ozone?

OH HO2

VOC, CH4, CO

NONO2

hO3

O3

Hemispheric Pollution

Direct Intercontinental Transport

air pollution (smog)

air pollution (smog)

Stratospheric O3 Stratosphere

~12 km

Number of People Living in U.S. Counties Violating Number of People Living in U.S. Counties Violating National Ambient Air Quality Standards (NAAQS) in National Ambient Air Quality Standards (NAAQS) in

20012001EPA [2002]

124 ppbv: 40.2

84 ppbv: 110.3

Carbon monoxide (CO)

Lead

Nitrogen dioxide

Ozone (O3)

Particles < 10 m (PM10)

Particles < 2.5 m (PM2.5)

Sulfur dioxide(SO2)

Any pollutant

Millions of People

0

0.007

11.1

0.7

72.7

2.7

133.1

0

0

50 100 150

The Risk Increment Above the Background is The Risk Increment Above the Background is Considered Considered

when setting the NAAQS for Ozonewhen setting the NAAQS for OzoneEnvironmenta

l risk

OzoneConcentrationPolicy-Relevant

BackgroundNAAQS

Acceptableadded

risk

Need a quantitative estimate for background ozone EPA chose a constant value (40 ppbv) in previous

review of O3 standard

Range of “background ORange of “background O33” estimates in U.S. ” estimates in U.S. surface airsurface air

20 40 60 80 100

O3 (ppbv)

Range considered by EPA during last revision of O3 standard

84 ppbv: threshold forcurrent U.S. O3 standard

Frequent observations previously attributed to natural background[Lefohn et al., 2001]

Range from priorglobal modeling studies

Range from this work [Fiore et al., 2003]

The U.S. EPA considers background levels when setting the NAAQS

Used by EPA to assess healthrisk from O3 (1996)

Fires Landbiosphere

Humanactivity

Ocean

NORTH AMERICANORTH AMERICA

Lightning

“Background” air

X

Outside naturalinfluences

Long-range transportof pollution

““POLICY RELEVANT BACKGROUND” OZONEPOLICY RELEVANT BACKGROUND” OZONE::

Ozone concentrations that would exist in the absence of Ozone concentrations that would exist in the absence of anthropogenic emissions from North America [anthropogenic emissions from North America [EPA CDEPA CD, 2005], 2005]

stratosphere

lightning

Policy Relevant Background is not directly observable Must be estimated with models

GEOS-CHEM[Bey et al., 2001]

MOZART-2[Horowitz et al., 2003]

Approach: Insights from two chemical transport models

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

• NCEP meteorology• T62 (1.9°) ; 28 -levels

• Strat O3 relaxed to climatology

PROD LOSS DEP STRAT

Ozo

ne

(Tg

yr-1

) X 19-Model Mean MOZART GEOS-CHEM

Ozone Budgets in IPCC-AR4 Tropospheric Chemistry ModelsOzone Budgets in IPCC-AR4 Tropospheric Chemistry ModelsModel Production Loss P-L Deposition Strat. Calc. Strat. Model.CHASER_CTM 5042 4594 447 948 501 517CHASER_GCM 5032 4620 412 948 536 506FRSGC 5135 4733 401 907 505 555GEOS-CHEM 4490 3770 719 1016 296 508GFDL 5263 5087 176 963 787 939GMICCM 5331 5059 272 862 590 557GMIDAO 5124 4940 184 763 579 541gmigis 4722 4396 326 856 530 537LLNL-IMPACT 5432 5160 271 1014 742 739LMDzINCA 4912 4182 729 1232 502 ***LMDzINCAc 4931 4027 903 1227 324 ***NCAR 4964 4670 293 906 612 684STOCHEM_HadAM3 5331 4821 509 945 435 383TM4 4806 4594 211 720 508 504TM5 4580 4623 -43 827 871 ***UM_CAM 3922 3363 558 1172 614 550MOZECH 6920 6617 303 963 660 ***STOCHEM_HadGEM 5114 3757 1356 1507 151 ***ULAQ 5017 3639 1377 1491 114 688Average 5056 4561 495 1014 519 586

AR4 budgets c/oJérôme DrevetDavid StevensonFrank Dentener

X Elevated sites (> 1.5 km)

Low-lying sites

1. quantify PRB O3 and its various sources

2. diagnose origin of springtime high-O3 events at remote U.S. sites, previously attributed to natural,

stratospheric influence

ApproachApproach: 2001 CASTNet Observations (EPA, NPS): 2001 CASTNet Observations (EPA, NPS)

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

CASTNet observationsModelBackgroundNatural O3 levelStratospheric

+

*

Hemisphericpollution

Regionalpollution}

}

PRB: 15-36 ppbvNatural level: 9-23 ppbvStratosphere: < 7 ppbv

High-O3 events: dominated by regional pollution; minor stratospheric influence (~2 ppbv)

regional pollution

hemispheric pollution

X

Lefohn et al. [2001] suggest a stratosphericsource as the likely origin of high-O3 eventsfrequently observed in June

Model Results from GEOS-CHEM

CASTNet observationsModelPRBNatural O3 levelStratospheric

+

*

Hemisphericpollution

Regionalpollution}

}

Background at high-altitude site (2.5 km) not necessarily representative of background contribution at low-lying sites

X

regional pollution

hemispheric pollution

Frequent high-O3 events previously attributed to natural, stratospheric source [Lefohn et al., 2001]

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

Model Results from GEOS-CHEM

Both models show that PRB ozone is higher at high-altitude Both models show that PRB ozone is higher at high-altitude sitesite

19

May – June 2001

* CASTNet observations

Mar – May 2001

GEOS-CHEM ModelGEOS-CHEM PRB

MOZART ModelMOZART PRB

15-36; 25 14-34; 23

29-50; 38 21-56; 35

PRB Range; mean

PRB Range; mean

Elevation: 430 m

Elevation: 2470 m

PRB ozone is lower under polluted conditions:PRB ozone is lower under polluted conditions:typically below 25 ppbvtypically below 25 ppbv

RegionalPollution

Ozone (ppbv)

Cumulative Probability

Daily mean afternoon O3 at 58 low-elevation U.S. CASTNet sites

June-July-August

CASTNet sitesGEOS-CHEM ModelPRB Ozone

*

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

Compiling daily afternoon (1-5 p.m. mean) surface ozone from all Compiling daily afternoon (1-5 p.m. mean) surface ozone from all CASTNet sites for March-October 2001: CASTNet sites for March-October 2001:

PRB ozone is typically 20-35 ppbvPRB ozone is typically 20-35 ppbv P

rob

abili

ty p

pb

v-1

CASTNet sites

GEOS-CHEM Model at CASTNet

Natural 18±5 ppbvGEOS-CHEM

PRB 26±7 ppbvGEOS-CHEM

PRB 29±9 ppbv MOZART-2

Hemispheric Pollution enhances U.S. PRB by 8±4 ppbvPrior work suggests 6 ppbv from anthrop. CH4 on average

Radiative Forcing of Climate from Preindustrial to Present:Radiative Forcing of Climate from Preindustrial to Present:

Important Contributions from Methane and OzoneImportant Contributions from Methane and Ozone

Hansen, Scientific American, 2004

50% anth. NOx

50% anth. CH4

50% anth.VOC

1995(base)

50% anth.VOC

50% anth.CH4

50% anth.NOx

AIR QUALITY: Number of U.S. summer grid-square days with

O3 > 80 ppbvCLIMATE: Radiative Forcing (W m-2)

Fiore et al., GRL, 2002

Double dividend of Methane Controls:Double dividend of Methane Controls: Decreased greenhouse warming and improved air qualityDecreased greenhouse warming and improved air quality

Methane links air quality and climate via background ozone

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 NOX control.

West & Fiore, ES&T, 2005

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

Shindell et al., 2005 report that tropospheric ozone responds linearly to 10, 25, 50, 75, 100% decreases in anthropogenic CH4

X

MOZART-2 (this work)TM3 [Dentener et al., ACPD, 2005]GISS [Shindell et al., GRL, 2005GEOS-CHEM [Fiore et al., GRL, 2002]IPCC TAR [Prather et al., 2001]

Methane Emissions in EDGAR inventory early 1990s; Tg CH4 yr-1

Energy, landfills, wastewater

95

Ruminants 93

Rice 60

Biomass burning 86

Ocean 10

Biogenic 204

TOTAL 547

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

Are ozone decreases independent of CH4 source location?

An

thro

po

ge

nic

Na

tura

l

Zero anthrop. CH4 emissions from Asia

Global 40% decreasein anthrop. CH4 emis. Zero Asia – global 40%

July 2000 surface O3 change mainly independent of CH4 source location, slightly larger response in source region

(Transient simulations with EDGAR 1990 emissions, beginning 1990)

OZ

ON

EM

ET

HA

NE

U.S. Surface Afternoon Ozone Response in Summer also independent of methane emission location

MEAN DIFFERENCE MAX DIFFERENCE(Max daily mean afternoon JJA)

NO ASIAN CH4

GLOBAL 40% DECREASE IN ANTHROP. CH4

Stronger Sensitivity in NOx-saturated regions (Los Angeles)

Local CH4 oxidation contributes up to ~30% of the total decrease in surface O3 from lowering CH4

concentrationsin a NOx-saturated region

Change in MOZART-2 surface afternoon (1-5 p.m.) O3 concentration after 1 day, when a 300 ppbv decrease in CH4 concentrations

is imposed below 800 hPa in Los Angeles

-1.00 -0.75 -0.50 -0.25

7/23/00

ppbv O3

CLIMATE IMPACTS: Change in July 2000 Trop. O3 Columns (to 200 hPa)

40% decrease in global anthrop.CH4 emissions

Zero CH4 emissions from Asia(= 40% decrease in global anthrop.)

No Asia – (40% global decrease)

Tropospheric O3 column response is independent of CH4 emission location except for small (~10%) local changes

Dobson Units

DU

Assume 25 ppb threshold,

CH4 reductions starting in 2000

AIR POLLUTION IMPACTS: 2030 Avoided Premature Mortalities (A2 scenario - 65 Tg CH4 emissions)

West et al., 2005

Change in 8-hr summer surface ozone from a 20% decrease in global anthrop. methane emissions

Avoided Mortalities in 2030

Reducing anthrop. CH4 emis. by 20% (starting in 2000):

-- decreases global surface ozone (~1 ppbv in populated areas)

-- prevents ~34,000 premature deaths in 2030

• Should these be science conclusions from paper incorporated into Criteria document

• And/or a summary of the talk focused on model evaluation?

1. Policy-Relevant Background in surface air over the United States:-- 25±5 ppbv at low-elevation sites in summer-- varies with season, altitude; anti-correlated with high domestic O3

-- 8±4 ppbv from hemispheric pollution (~5-7 ppbv from anthr. CH4) 40 ppbv previously used by EPA underestimates health risksinternational negotiations to reduce hemispheric background would facilitate compliance with more stringent standards

ConclusionsConclusions

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 CH4 changes in NOx-saturated regions