Background Ozone in Surface Air over the United States: Variability, Climate Linkages,
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Transcript of Background Ozone in Surface Air over the United States: Variability, Climate Linkages,
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