Background ozone in surface air over the United States

Arlene M. FioreDaniel J. Jacob

US EPA Workshop on Developing Criteria for the Chemistry and Physics of Atmospheric Ozone

College Park, Maryland, March 17, 2003

Discussion points

• Methods to characterize regional O3 spatial and temporal variability– EOF Analysis for eastern United States

• Background ozone over the United States– average vs. polluted conditions– seasonal & regional variability– during events of elevated O3

– origin (stratosphere; natural; hemispheric pollution)

• Linkages between O3 and aerosols– surface O3 response to heterogenous & radiative effects of aerosols

• Linkages between air quality and climate – influence of CH4 on background O3

Conventional model evaluation: Correlation of simulated vs. observed time series

MAQSIP regional model36 km2

Correlation coefficient (r)

Daily afternoon (1-5 p.m. local time) mean surface O3 Summer 1995, eastern U.S.

GEOS-CHEM global model2°x2.5°

Fiore et al., in press, JGR

EOF ANALYSIS: Characterize spatiotemporal variability of surface O3

(daily 1-5 p.m. mean concentrations in summer 1995 over eastern U.S.) OBS (AIRS) MAQSIP (36 km2)

Fiore et al., in press, JGR

r2 = 0.60Slope = 0.9

r2 = 0.57Slope = 0.8

r2 = 0.68Slope = 0.7

EOF 1: East-west

EOF 2: Midwest-Northeast

EOF 3: Southeast

r2 = 0.86Slope = 1.0

r2 = 0.76Slope = 1.0

r2 = 0.80Slope = 1.0

Same fundamental, synoptic-scale processes modulate observed O3

variability at scale of global model horizontal resolution

EOF 1: East-west

EOF 2: Midwest-Northeast

EOF 3: Southeast

OBS (AIRS) GEOS-CHEM 2°x2.5°

Fiore et al., in press, JGR

r2 = 0.74Slope = 1.2

r2 = 0.27Slope = 1.0

r2 = 0.90Slope = 1.0

r2 = 0.68Slope = 1.0

r2 = 0.54Slope = 0.8

r2 = 0.78Slope = 1.0

Mean Afternoon Surface Ozone Background (ppbv) in GEOS-CHEM model, Summer 1995

Background is tagged as ozone produced outside the N. American boundary layer (surface-700 hPa)

What is the contribution of the background to pollution episodes?

Ozone Background is depleted during regional pollution episodes(due to deposition and chemical loss under stagnant conditions)

Daily mean afternoon O3 vs. (NOy-NOx) At Harvard Forest, MA

Index of Aged Pollution

Background(clean conditions)

Background in model(pollution episode)

Total Surface Ozone in Model

Ozo

ne

(pp

bv)

Fiore et al., JGR, August, 2002.

Background O3: produced outside the N. Americanboundary layer (surface-700 hPa)

Observations

U.S. Ozone Standard

Frequency Distribution of Afternoon Background Ozone Concentrations in U.S. Surface Air Summer 1995 (GEOS-CHEM model)

summer ensemble vs. pollution episodes

Convection upwindoccasionally results inhigh background duringpollution episodes

Background Ozone Concentration (ppbv)

Pro

bab

ilit

y

Fiore et al., JGR, August, 2002.

RANGE OF ASIAN/EUROPEAN POLLUTION SURFACE OZONE ENHANCEMENTS OVER THE U.S. IN SUMMER

as determined from a simulation without these emissions

Max Asian/European pollution enhancements(up to 14 ppbv) occur at intermediate ozone levels (50-70 ppbv)

MAJOR CONCERNIF OZONE STANDARDWERE TO DECREASE!

tropical air

Subsidence of Asian pollution+ local production

stagnation

Fiore et al., JGR, August, 2002.

Two Questions Central to Background O3 Discussion:

How can we further address these questions?

Analyze 2001 CASTNet O3 data (representative year)

Apply GEOS-CHEM to interpret observations

1. What background concentrations should be used

to assess risk?

2. Is the present NAAQS for O3 too close to

background concentrations?

Observed concentrations above 50-80 ppbv in spring have been attributed to natural causes [Lefohn, 1997; Lefohn et al., 2001]

Suggests current 25-45 ppbv background definition may be inadequate

Implies that NAAQS for O3 may be unattainable via domestic emissions reductions

Model captures percentages of occurrences ≥ 50 ppbv in 2001at CASTNet sites except for SE

% o

ccu

rren

ce

s ≥

50

pp

bv

All hourly obsHourly obs 1-5 p.m.Mean obs 1-5 p.m.Mean model 1-5 p.m.

NW NE

SESW

Sensitivity Simulations for source attribution

• Standard simulation…..2x2.5 GEOS-CHEM, 48 sigma levels

2001

• 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

Regional Pollution = Standard – Background

Hemispheric Pollution = Background – Natural O3 level

Note: Background in the following results is as defined by EPA

How does background O3 vary with season and region?

Seasonal cycle in mean afternoon (1-5 p.m.) O3 in surface air

CASTNet sitesModel at CASTNetModel entire regionBackgroundNatural O3 levelStratospheric

+

*

RegionalPollution(from N. Amer. emissions)

{

{

{

{

HemisphericPollutionenhancement

Ozone Time Series at selected CASTNet stations in 2001

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Hemisphericpollution

Regionalpollution}

}

APR-MAY 2000

High-O3 “Haywood County”event in North Carolina(model box centered at 85W34N)

Regional pollution contributes significantly to high-O3 events in NC;

Model does not indicate substantial stratospheric influence

CASTNet sitesModelBackgroundNatural O3 levelStratosphericContinental lower troposphere

+

*

Hemisphericpollution

Regionalpollution}

}

APR-MAY 2001

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Ozo

ne

(pp

bv)

Days in March 2001

SoutheastWest

Background increaseswith highest observed O3 at western sites in March

Background decreaseswith highest observed O3 at SE sitesin March

Cumulative probability distributions for daily mean afternoon O3, April-June 2001

11 “background sites”(all in western U.S.)

34 “polluted sites”(mostly in eastern U.S.)

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Backgrounddecreases underpolluted conditions!

Some enhancementfrom N. Amerand hemis.pollutionfor highest values

Cumulative probability distributions for daily mean afternoon O3, July-August 2001

11 “background sites”from previous slide(western U.S.)

34 “polluted sites”(mostly in eastern U.S.)

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Backgroundis even lowerduring high-O3

events in summer

Sites areinfluenced bypollution in summer months

Background islower

GEOS-CHEM: August

O3 (ppbv)

Martin et al., JGR, February, 2003.

Ozone-aerosol linkage:

(simulation with aerosols) – (simulation without aerosols)

PM O3 over U.S.

Air Quality-Climate Linkage: Impacts of future changes in global anthropogenic emissions (GEOS-CHEM)

50% anth. NOx

2030 A1

50% anth. CH4

50% anth.VOC

2030 B1

1995(base)

50% anth.VOC

50% anth.CH4

50% anth.NOx

2030 A1

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%

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

Rad

iati

ve F

orc

ing

* (W

m-2)

CH4 links air quality & climate via

background O3

Fiore et al.,GRL, Oct., 2002.

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

2030 A1

1995 Base Case

Fiore et al.,GRL, Oct., 2002.

CH4

NOx

NMVOCs

NOx

NMVOCsO3

Chemical lossDeposition

CONTINENT 2OCEAN

O3

O3

NOx emissions local impact;

little effect on climate

Boundary layer(0-2.5 km)

Free Troposphere CH4 emissions global impact:

Lower background O3

Negative radiative forcingIntercontinental transport,

hemispheric O3 backgroundincreases in

2030 A1 simulation

CONTINENT 1

Double dividend of methane emissions reductions: lower global O3 background and improve air quality everywhere