Background Ozone in Surface Airover the United States:

Variability, Climate Linkages, and Policy Implications

Arlene M. Fiore

Department of Environmental

Sciences SeminarRutgers University

March 4, 2005

AcknowledgmentsAcknowledgments

Daniel JacobMat EvansDuncan FairlieBrendan Field

David StreetsSuneeta Fernandes

Carey Jang

Qinbin Li Hongyu Liu Bob YantoscaYuxuan Wang

Drew PurvesSteve PacalaJason West

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 2001National 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

Background Estimates are Used When Setting Background Estimates are Used When Setting NAAQSNAAQS

Environmental risk

PollutantconcentrationBackground NAAQS

Acceptableadded

risk

Risk assessments account for risks associated with exposure to the increment of ozone above the background

(i.e., the risk that can potentially be reduced via North American anthropogenic emission controls)

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

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

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

Observations:CASTNetCASTNet Stations Stations

(EPA, Nat’l Park Service)

X Elevated sites (> 1.5 km) Low-lying sites

APPROACH:APPROACH: Use Use 2001 CASTNet data2001 CASTNet data in conjunction with in conjunction with GEOS-CHEMGEOS-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

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

High-O3 events dominated by regional pollution

regional pollution

CASTNet observations Model Base Case (2001) Stratospheric influence Background (no N. Amer. Anthrop emissions; present-day CH4) Natural O3 level (no global anthrop. Emissions; preindustrial CH4)+

* X

Case Studies: Ozone Time Series at Sites used in Case Studies: Ozone Time Series at Sites used in Lefohn et Lefohn et alal. [2001] . [2001]

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

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

Fiore et al., JGR, 2003

Background OBackground O33 even lower under even lower under polluted conditionspolluted conditions

CASTNet sitesModelBackgroundNatural O3 levelStratospheric

+

*

Background on polluted days well below 40 ppbv assumed by EPA health risks underestimated with current (1996) approach

Ozone (ppbv)

Cumulative Probability

RegionalPollution

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

June-July-August

Fiore et al., JGR, 2003

Daily 1-5 p.m. mean surface O3 Mar-Oct 2001

Pro

bab

ility

pp

bv-1

Observed: CASTNet sites

Model at CASTNet

Stratospheric

Natural (no global anthrop. emis. (& CH4 = 700 ppbv))

Background (no anthrop. emis. in N. Amer)

Compiling Results from all (71) CASTNet sites: Compiling Results from all (71) CASTNet sites: Natural vs. Anthropogenic Contributions Natural vs. Anthropogenic Contributions

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

Typical ozone values in U.S. surface air:

Anthropogenic methane enhances “background” above “natural”

Fiore et al., JGR, 2003

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

ANIMALS90

LANDFILLS50

GAS60

COAL40RICE

85

TERMITES25

WETLANDS180

BIOMASSBURNING20

GLOBAL METHANESOURCES (Tg CH4 yr-1)

Anthropogenic Methane Emissions Enhance Anthropogenic Methane Emissions Enhance the the

Tropospheric Ozone BackgroundTropospheric Ozone Background

240

250

260

270

280

290

300

310

320

3301995 basecase50% methane

50% NOx

50% NMVOCs

50%NOx+NMVOCs50% CO

50% all

natural

Sensitivity of global tropospheric ozone inventory in GEOS-CHEM to 50% global reductions in anthropogenic emissions

Anthropogenic emissions of NOx and methane have largestinfluence on tropospheric ozone…. climate? air pollution?

Tg O3

Fiore et al., GRL, 2002

Radiative Forcing of Climate, 1750-Present:Radiative Forcing of Climate, 1750-Present:

Important Contributions from Methane and OzoneImportant Contributions from Methane and Ozone

IPCC [2001]

Level of scientific understanding

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, 2002

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

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

2. Pollution from North America contributes to high-O3 events at remote U.S. sites in spring

3. Hemispheric pollution enhances U.S. background

CONCLUSIONS ...CONCLUSIONS ... and their implications for public policyand their implications for public policy

health risk from O3 underestimated in 1996 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

reducing CH4 decreases both background O3 and greenhouse forcing; enables simultaneous pursuit of air quality & climate goals

global CH4 controls are viable [J. West seminar next Friday!] and complement local-to-regional NOx & NMVOC controls What is the role of uncertainties/changes in “natural” emissions?

First step: BVOC emissions

TEMPPAR

Leaf Area

ISOPRENE + NOx

Isoprene Emissions are generally thought to Isoprene Emissions are generally thought to contribute to Ocontribute to O33 production over the eastern production over the eastern

United States United States [[e.g.Trainer et ale.g.Trainer et al., 1987; ., 1987; NRC,NRC, 1991] 1991]

Vegetation changes Impact on O3?-- Climate-- Land-use

+air pollution (smog)

Harmful to health,vegetation

O3

O3 formation in eastern U.S. is typically more responsive to controls on anthrop. NOx emissions than anthrop. VOCs

(VOC)

GEIA: Global Isoprene Emission Inventory

7.1 Tg C 2.6 Tg C

[1012 atom C cm-2 s-

1]

Isoprene emissions – July 1996

[from Paul Palmer]

Isoprene Emission Inventories uncertain by at least a Isoprene Emission Inventories uncertain by at least a factor of 2factor of 2

BEIS2: Regional Isoprene Emission Inventory

GEIA

GEIA: global inventory

Purves et al., [2004] (based on FIA data; similar to BEIS-2)

(1011 molecules isoprene cm-2 s-1)

ChoiceChoice of isoprene inventory critical for predicting base-case of isoprene inventory critical for predicting base-case OO33

(2001 meteorology; 1x1 nested GEOS-CHEM [(2001 meteorology; 1x1 nested GEOS-CHEM [Wang et al., 2004; Li et al., Wang et al., 2004; Li et al., 2004])2004])

5.6 Tg C

2.8 Tg C

(ppbv)

Difference in July 1-5 p.m. surface O3 (Purves–GEIA)

July Anthrop. NOx emissions

0.43 Tg N

(1011 molec cm-2 s-1)

“isoprene-saturated”

High-NOx

regime

Ju

ly is

op

ren

e e

mis

sio

ns

Isoprene nitratesSink for NOx?

Complicated Chemistry: Isoprene may also decrease surface Complicated Chemistry: Isoprene may also decrease surface OO3 3 in low-NOin low-NOxx, high isoprene settings, high isoprene settings

ISOPRENE + NOx + O3

O3 (slower ) Low-NOx, high-isopreneO3

Thought to occur in pre-industrial [Mickley et al., 2001];and present-day tropical regions [von Kuhlmann et al., 2004]

O3OH RO2

NO (very fast) NO2

High-NOx

ISOPRENE

Isoprene does react directly with O3 in our SE US GEIA simulation:O3+biogenics (10d) comparable to O3+HOx (16d), O3+h-> OH (11d)

Change in July mean 1-5 p.m. surface O3 when isoprene nitrates act as a NOx sink

6-12 ppbv impact!

What is the OWhat is the O33 sensitivity to the uncertain fate of sensitivity to the uncertain fate of organic isoprene nitrates?organic isoprene nitrates?

OH RO2

NO (very fast) NO2

High-NOx

Isoprene nitratesSink for NOx?ISOPRENE

O3

ppbv

MOZART-2

Recent Changes in Biogenic VOC EmissionsRecent Changes in Biogenic VOC Emissions

Isoprene Monoterpenes

[Purves et al., Global Change Biology, 2004]

-20 –10 0 +10 +20 +30Percent Change mid-1980s to mid-1990s

SweetgumInvasion of

Pine plantations

Substantial isoprene increases in southeastern USA largely driven by human land-use decisions

Land-use changes not presently considered in CTMs

Change in July 1-5 p.m. surface O3

Do these recent changes in isoprene emissions influence Do these recent changes in isoprene emissions influence surface Osurface O33??

A Two-Model PerspectiveA Two-Model Perspective

ppbv

-20 –10 0 +10 +20 +30 (%)

Isoprene changes

MOZART-2

O3 increases(1-2 ppbv)

Little change(NOx-sensitive)

GEOS-CHEM

Isoprene nitrates are not a NOx sink in standard MOZART-2;

Results are sensitiveto this assumption!

O3 decreases (same as previous result)

Change in July 1-5 p.m. surface O3 (ppbv) (due to isop emis changes from mid-1980s to mid-1990s)

Chemical uncertainty: MOZART-2 shows similar results to GEOS-Chemical uncertainty: MOZART-2 shows similar results to GEOS-CHEM CHEM

if isoprene nitrates are a NOif isoprene nitrates are a NOxx sink sink

Understanding fate of isop. nitrates essential for predicting sign of response to changes in isoprene emissions

With 12% yield of isoprene nitrates

GEOS-CHEM: GEIA

GEOS-CHEM: Purves

MOZART-2: GEIA

ppbv

• Better constrained isoprene emissions are needed to quantify:1. isoprene contribution to Eastern U.S. surface O3 2. how O3 responds to both anthrop. and biogenic emission changes

Utility of satellite CH2O columns? New inventories (MEGAN, BEIS-3) more accurate? Insights from aircraft campaigns?

• Recent isoprene increases may have reduced surface O3 in the SE

Does this regime actually exist? Fate of organic nitrates produced during isoprene oxidation?

Conclusions and Remaining ChallengesConclusions and Remaining Challenges

Fiore et al., JGR, 2005