Background Ozone in Surface Airover the United States:

Variability, Climate Linkages, and Policy Implications

Arlene M. Fiore

Department of Atmospheric and Oceanic Sciences Seminar

University of Wisconsin-Madison

December 6, 2004

AcknowledgmentsAcknowledgments

Daniel JacobBrendan FieldHongyu Liu Bob YantoscaQinbin Li Duncan Fairlie

David StreetsSuneeta Fernandes

Carey JangJason West

NOx

VOC

NOx

VOC

CONTINENT 2 OCEAN

O3

Boundary layer

(0-3 km)

Free Troposphere

CONTINENT 1

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 National Ambient Air Quality Standards (NAAQS) in 2001

EPA [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 Considered

when setting the NAAQS for OzoneEnvironmenta

l risk

OzoneConcentrationBackground NAAQS

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 O3” estimates in U.S. surface air

20 40 60 80 100

Used by EPA to assess healthrisk from O3

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.,JGR 106, 9945-9958, 2001]

Range from priorglobal modeling studies

Range from this work[Fiore et al., JGR 108, 4787, 2003]

EPAassumedbackground

Range from this work[Fiore et al., JGR 108, 4787, 2003]

EPAassumedbackground

Fires Landbiosphere

Humanactivity

Ocean

NORTH AMERICANORTH AMERICA

Lightning

“Background” air

X

Outside naturalinfluences

Long-range transportof pollution

““REGULATORY BACKGROUND OZONE” DEFINITION:REGULATORY BACKGROUND OZONE” DEFINITION:

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 [EPAEPA, 2003], 2003]

stratosphere

lightning

Intercontinental Transport of Pollution at Northern Midlatitudes

AnthropogenicNOx emissions,

Transport: 2-4 weeks

Reduced Visibility from Transpacific Transport of Asian Dust

O3

Clear Day April 16, 2001

Glen Canyon, Arizona

Contributes to Hemispheric Background O3

Regional Pollution = Standard – Background

Hemispheric Pollution = Background – Natural O3 level

EPA-Defined Background is Not Directly Observable Must be Estimated with Models

• Standard simulation…..2x2.5 GEOS-CHEM, compare compare with observations with observations

• Background………………no anthrop. NOx, CO, NMVOC emissions from N. America

• Natural O3 level………….no anthrop. NOx, CO, NMVOC

emissions globally; CH4 = 700 ppbv

• Stratospheric…………….tagged O3 tracer simulation

Simulations for Ozone Source Attribution: Mar-Oct 2001

TOOL: GEOS-CHEM 3DGEOS-CHEM 3D Tropospheric Chemistry Tropospheric Chemistry ModelModel [Bey et al., 2001] (uses assimilated meteorology; 48 ; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers)

2001 CASTNet Stations (EPA, Nat’l Park Service)

X Elevated sites (> 1.5 km)

Low-lying sites

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

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

CASTNet observationsModelBackgroundNatural O3 levelStratospheric

+

*

Hemisphericpollution

Regionalpollution}

}

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

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

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

regional pollution

hemispheric pollution

X

Case Study: Case Study: Yellowstone NP, WyomingYellowstone NP, Wyoming(March-May 2001) (March-May 2001)

CASTNet observationsModelBackgroundNatural O3 levelStratospheric

+

*

Hemisphericpollution

Regionalpollution}

}

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

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

X

regional pollution

hemispheric pollution

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Ozo

ne

(pp

bv)

Days in March 2001

Southeast (<1.5 km)West (>1.5 km)

Background O3 higher at high-altitude western sites

Background O3

lower at low-lyingsoutheastern sites; decreases with highest observed O3

X

Background O3 even lower under polluted conditions

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Background on polluted days well below 40 ppbv assumed by EPA health risks underestimated with current approach

Ozone (ppbv)

Cumulative Probability

RegionalPollution

Daily mean afternoon O3 at 58 U.S. CASTNet sites

June-July-August

Monthly mean afternoon (1-5 p.m.) surface O3

CASTNet sitesModel at CASTNetModel entire regionBackgroundNatural O3 levelStratospheric

+

*

{

{

{

{

Hemispheric pollutionenhancement(5-12 ppbv)

Mean background: 20-35 ppbvMean natural level: 13-27 ppbvMean stratosphere: 2-7 ppbv

Regional pollutionfrom N. Am. emis.

(8-30 ppbv)

Background O3 varies with season

Daily afternoon (1-5 p.m.) surface ODaily afternoon (1-5 p.m.) surface O3 3

March-October 2001 March-October 2001

Background 15-35 ppbv; Natural 10-25 ppbv; Stratosphere < 20 ppbv

Pro

bab

ility

pp

bv-1

Typical ozone values in U.S. surface air:

Compiling results from all CASTNet sites…

CASTNet sites

Model at CASTNet

Stratospheric

NaturalBackground

1. Background O3 is typically less than 40 ppbv; even lower under polluted conditions

2. High-O3 events at remote U.S. sites in spring can be explained largely by pollution from North America

3. Hemispheric pollution enhances U.S. background

CONCLUSIONS ...and their implications for public policy

health risk from O3 underestimated in present 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

Climate Linkage

NOx

NMVOCs

NOx

NMVOCs

CONTINENT 2 OCEAN

O3

Boundary layer

(0-3 km)

Free Troposphere

CONTINENT 1

Air quality-Climate Linkage:CH4, O3 are important greenhouse gases

CH4 contributes to background O3 in surface air

OH HO2

VOC, CH4, CO

NONO2

hO3

O3

Global Background O3

(Hemispheric Pollution)

Direct Intercontinental Transport

greenhouse gas

air pollution (smog)

air pollution (smog)

More than half of global methane emissions are influenced by human activities

ANIMALS90

LANDFILLS50

GAS60

COAL40RICE

85

TERMITES25

WETLANDS180

BIOMASSBURNING20

GLOBAL METHANESOURCES (Tg CH4 yr-1)

Radiative Forcing of Climate, 1750-Present:

Important Contributions from Methane and Ozone

IPCC [2001]

Level of scientific understanding

Impacts of future changes in global anthropogenic emissions on climate

NOx OH

CH4

50% anthr. NOx

2030 A1

50% anthr. CH4

50% anthr.

NMVOC

2030 B1

IPCC scenario

Anthrop. NOx emissions

(2030 vs. present)

Global U.S.

Methane emissions

(2030 vs. present)

A1 +80% -20% +30%

B1 -5% -50% +12%

Change in radiative forcing (W m-2) relative to 1995 base case

Fiore et al., GRL, 2002

Decrease in July 1995 Mean Surface O3 concentrationsfrom 50% reductions in global anthropogenic CH4 emissions

Methane emission controls reduce surface O3 everywhere

Methane emission reductions shift entire summer afternoon surface O3 frequency distribution over U.S.

Pro

bab

ilit

y p

pb

v-1

Ozone Concentration (ppbv)

Impacts of future changes in global anthrop. emis. on U.S. air quality:

1995(base)

50% anthr.VOC

50% anthr.CH4

50% anthr.NOx

2030 A1

Increase in U.S. pollution events despite domesticdecline in anthrop.emissions

Number of summer grid-square days with O3 > 80 ppbv

2030 B1

IPCC 2030

scenario

Anthrop. NOx emissions

Global U.S.

Methane emissions

A1 +80% -20% +30%

B1 -5% -50% +12%

Rising emissions from developing countries lengthen the O3 pollution season in the United

States

2030 A1

1995 Base CaseDegradation of U.S. air quality from rise in global emissions despite domestic reductions

1. Background O3 is typically less than 40 ppbv; even lower under polluted conditions

2. High-O3 events at remote U.S. sites in spring can be explained largely by pollution from North America

3. Hemispheric pollution enhances U.S. background

CONCLUSIONS,Public policy implications, and Next steps

health risk from O3 underestimated in present 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

compare additional model estimates of background (MOZART-2) for use in risk assessments

feasibility of methane control [West and Fiore, submitted; AGU poster]

better characterize relationship between methane sources and surface ozone response

extend source attribution for background