Transcript Background Ozone in Surface Air over the United States: Variability, Climate Linkages, and Policy Implications Arlene M.

Background Ozone in Surface Air
over the United States:
Variability, Climate Linkages,
and Policy Implications
Arlene M. Fiore
Department of Environmental
Sciences Seminar
Rutgers University
March 4, 2005
Acknowledgments
Drew Purves
Steve Pacala
Jason West
Larry Horowitz
Chip Levy
Daniel Jacob
Mat Evans
Duncan Fairlie
Brendan Field
Qinbin Li
Hongyu Liu
Bob Yantosca
Yuxuan Wang
David Streets
Suneeta Fernandes
Carey Jang
What is the origin of tropospheric ozone?
Stratospheric O3
Stratosphere
~12 km
hn
O3
NO2
NO
OH
HO2
Free Troposphere
Hemispheric Pollution
Direct Intercontinental Transport
Boundary layer
(0-3 km)
VOC, CH4, CO
NOx
VOC
air pollution (smog)
O3
air pollution (smog)
CONTINENT 1
OCEAN
NOx
VOC
CONTINENT 2
O3
Number of People Living in U.S. Counties Violating National
Ambient Air Quality Standards (NAAQS) in 2001
EPA [2002]
Nitrogen 0
dioxide
124 ppbv: 40.2
Ozone (O3)
84 ppbv: 110.3
Sulfur dioxide
(SO2)
0.007
Particles < 10 mm (PM10)
11.1
Particles < 2.5 mm (PM2.5)
72.7
Carbon monoxide (CO)
0.7
Lead
2.7
Any pollutant
133.1
0
100
50
Millions of People
150
Background Estimates are Used When Setting NAAQS
Environmenta
l risk
Acceptable
added
risk
Background
NAAQS
Pollutant
concentration
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 O3” estimates in U.S. surface air
Range from prior
global modeling studies
Range considered
by EPA during last
revision of O3 standard
20
40
84 ppbv: threshold for
current U.S. O3 standard
60
80
100
O3 (ppbv)
Used byEPA
EPA
assumed
to assess
health
Frequent observations
background
risk from O3
previously attributed to
natural background
Range from this work
[Lefohn et al.,JGR 106, 9945-9958, 2001]
[Fiore et al., JGR 108, 4787, 2003]
The U.S. EPA considers background levels
when setting the NAAQS
“POLICY RELEVANT BACKGROUND” OZONE:
Ozone concentrations that would exist in the absence of
anthropogenic emissions from North America [EPA CD, 2005]
stratosphere
Outside natural
influences
Lightning
lightning
Long-range transport
of pollution
“Background” air
Ocean
Fires
X
Human
Land
biosphere activity
NORTH AMERICA
Policy Relevant Background is not directly observable
 Must be estimated with models
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
Observations:
CASTNet Stations
(EPA, Nat’l Park Service)
MODEL:
GEOS-CHEM 3D Tropospheric
Chemistry Model
[Bey et al., 2001] (uses assimilated
meteorology; 48 s; 4ºx5º or 2ºx2.5º
horiz. resn., 24 tracers)
X Elevated sites (> 1.5 km)
Low-lying sites
Case Studies: Ozone Time Series at Sites used in Lefohn et al. [2001]
* CASTNet observations
Model Base Case (2001) X Stratospheric influence
Background (no N. Amer. Anthrop emissions; present-day CH4)
+ Natural O3 level (no global anthrop. Emissions; preindustrial CH4)
High-O3 events dominated
by regional pollution
hemispheric
pollution
regional
pollution
Background at highaltitude site (2.5 km)
not representative of
contribution at
low-lying sites
West (>1.5 km)
CASTNet sites
Model
Background
+ Natural O3 level
X Stratospheric
*
Southeast (<1.5 km)
Ozone (ppbv)
Background O3
higher at high-altitude
western sites
Background O3
lower at low-lying
southeastern sites;
decreases with
highest observed O3
Fiore et al., JGR, 2003
Days in March 2001
CASTNet sites
Model
Background
+ Natural O3 level
Stratospheric
*
Background O3 even lower under
polluted conditions
Daily mean afternoon O3 at 58 low-elevation U.S. CASTNet sites
June-July-August
Regional
Pollution
Ozone (ppbv)
Cumulative Probability
Background on polluted days well below 40 ppbv assumed by EPA
 health risks underestimated with current (1996) approach
Fiore et al., JGR, 2003
Compiling Results from all (71) CASTNet sites:
Natural vs. Anthropogenic Contributions
Stratospheric
Probability ppbv-1
Natural (no global anthrop. emis. (& CH4 = 700 ppbv))
Background (no anthrop. emis. in N. Amer)
Observed: CASTNet sites
Model at CASTNet
Daily 1-5 p.m. mean surface O3 Mar-Oct 2001
Typical ozone values in U.S. surface air:
Background 15-35 ppbv; Natural 10-25 ppbv; Stratosphere < 20 ppbv
Anthropogenic methane enhances “background” above “natural”
Fiore et al., JGR, 2003
More than half of global methane emissions
are influenced by human activities
WETLANDS
180
BIOMASS
BURNING ANIMALS
90
20
LANDFILLS
50
GLOBAL METHANE
SOURCES (Tg CH4 yr-1)
TERMITES
25
RICE
85
GAS
60
COAL
40
Anthropogenic Methane Emissions Enhance the
Tropospheric Ozone Background
320
1995 base
case
50% methane
310
50% NOx
330
300
Tg O3
290
280
270
260
250
240
50% NMVOCs
50%
NOx+NMVOCs
50% 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 largest
influence on tropospheric ozone…. climate? air pollution?
Fiore et al., GRL, 2002
Radiative Forcing of Climate, 1750-Present:
Important Contributions from Methane and Ozone
IPCC [2001]
Level of scientific understanding
Radiative Forcing* (W m-2)
Double dividend of Methane Controls:
Decreased greenhouse warming and improved air quality
Number of U.S. summer gridsquare days with O3 > 80 ppbv
50% 50% 50% 2030 2030
anth. anth. anth. A1 B1
VOC CH4 NOx
IPCC
Anthrop. NOx emissions
scenario
(2030 vs. present)
Global
U.S.
1995 50% 50% 50% 2030 2030
(base) anth. anth.anth. A1 B1
VOC CH4 NOx
Methane
emissions
(2030 vs. present)
A1
+80%
-20%
+30%
CH4 links air quality
& climate via
background O3
B1
-5%
-50%
+12%
Fiore et al., GRL, 2002
CONCLUSIONS ... and their implications for public policy
1. Background O3 is typically less than 40 ppbv; even lower under
polluted conditions
health risk from O3 underestimated in 1996 EPA risk assessments
2. Pollution from North America contributes to high-O3 events at
remote U.S. sites in spring
these events do not represent U.S. background conditions and
should not be used to challenge legitimacy of O3 NAAQS
3. Hemispheric pollution enhances U.S. background
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
Isoprene Emissions are generally thought to contribute
to O3 production over the eastern United States
[e.g.Trainer et al., 1987; NRC, 1991]
(VOC)
ISOPRENE
+
+
NOx

O3
air pollution (smog)
TEMP
Harmful to health,
vegetation
PAR
Leaf Area
O3 formation in eastern U.S. is typically
more responsive to controls on anthrop.
NOx emissions than anthrop. VOCs
Vegetation changes  Impact on O3?
-- Climate
-- Land-use
Isoprene Emission Inventories uncertain by at least a factor of 2
Isoprene emissions – July 1996
GEIA: Global Isoprene
Emission Inventory
7.1 Tg C
BEIS2: Regional Isoprene
Emission Inventory
2.6 Tg C
[from Paul Palmer]
[1012 atom C cm-2 s-1]
Choice of isoprene inventory critical for predicting base-case O3
(2001 meteorology; 1x1 nested GEOS-CHEM [Wang et al., 2004; Li et al., 2004])
July isoprene emissions
GEIA: global inventory
5.6 Tg C
July Anthrop. NOx emissions
0.43 Tg N
(1011 molec cm-2 s-1)
Purves et al., [2004] (based on
FIA data; similar to BEIS-2)
Difference in July 1-5 p.m.
surface O3 (Purves–GEIA)
High-NOx
regime
2.8 Tg C
GEIA
“isoprenesaturated”
(ppbv)
(1011 molecules isoprene cm-2 s-1)
Complicated Chemistry: Isoprene may also decrease surface O3 in
low-NOx, high isoprene settings
ISOPRENE
+
+
NOx

O3
High-NOx
(very fast)
OH
RO2
NO
NO2
O3
Isoprene nitrates
Sink for NOx?
ISOPRENE
O3
(slower )
O3
Low-NOx, high-isoprene
Thought to occur in pre-industrial [Mickley et al., 2001];
and present-day tropical regions [von Kuhlmann et al., 2004]
Isoprene does react directly with O3 in our SE US GEIA simulation:
O3+biogenics (10d) comparable to O3+HOx (16d), O3+hn -> OH (11d)
What is the O3 sensitivity to the uncertain fate of
organic isoprene nitrates?
High-NOx
(very fast)
OH
RO2
ISOPRENE
NO
NO2
O3
Isoprene nitrates
Sink for NOx?
MOZART-2
Change in July mean 1-5 p.m.
surface O3 when isoprene
nitrates act as a NOx sink
 6-12 ppbv impact!
ppbv
Recent Changes in Biogenic VOC Emissions
[Purves et al., Global Change Biology, 2004]
 Substantial isoprene increases in southeastern USA
largely driven by human land-use decisions
 Land-use changes not presently considered in CTMs
Isoprene
Sweetgum
Invasion of
Pine plantations
Monoterpenes
-20 –10 0 +10 +20 +30
Percent Change mid-1980s to mid-1990s
Do these recent changes in isoprene emissions influence surface O3?
A Two-Model Perspective
Change in July 1-5 p.m. surface O3
GEOS-CHEM
MOZART-2
Little change
(NOx-sensitive)
O3 decreases
(same as previous result)
ppbv
O3 increases
(1-2 ppbv)
Isoprene nitrates are
not a NOx sink in
standard MOZART-2;
Isoprene changes
-20 –10 0 +10 +20 +30 (%)
Results are sensitive
to this assumption!
Chemical uncertainty: MOZART-2 shows similar results to GEOS-CHEM
if isoprene nitrates are a NOx sink
Change in July 1-5 p.m. surface O3 (ppbv)
(due to isop emis changes from mid-1980s to mid-1990s)
With 12% yield of isoprene nitrates
GEOS-CHEM: GEIA
GEOS-CHEM: Purves
MOZART-2: GEIA
ppbv
Understanding fate of isop. nitrates essential for predicting
sign of response to changes in isoprene emissions
Conclusions and Remaining Challenges
•
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?
Fiore et al., JGR, 2005