1. 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

2. 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
Carey Jang
David Streets
Suneeta Fernandes

3. What is the origin of tropospheric ozone?
Stratospheric O3
Stratosphere O3
~12 km hn
Free Troposphere
NO2 NO
Hemispheric Pollution OH
HO2
Direct Intercontinental Transport
Boundary layer
(0-3 km)
VOC, CH4, CO
air pollution (smog)
NOx
VOC O3
NOx
VOC O3
air pollution (smog)
CONTINENT 1
CONTINENT 2
OCEAN

4. Number of People Living in U. S
Number of People Living in U.S. Counties Violating National Ambient Air Quality Standards (NAAQS) in 2001
EPA [2002]
Nitrogen
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 50
100
150
Millions of People

5. Background Estimates are Used When Setting NAAQS
Environmental risk
Acceptable
added
risk
Pollutant
concentration
Background
NAAQS
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)

6. global modeling studies
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
84 ppbv: threshold for
current U.S. O3 standard 20 40 60 80
100
Used by EPA
to assess health
risk from O3
Range from this work
[Fiore et al., JGR 108, 4787, 2003]
EPA
assumed
background
Range from this work
[Fiore et al., JGR 108, 4787, 2003]
EPA
assumed
background
Frequent observations previously attributed to natural background
[Lefohn et al.,JGR 106, , 2001]
O3 (ppbv)
The U.S. EPA considers background levels
when setting the NAAQS

7. “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 X
Fires
Land
biosphere
Human
activity
Ocean
NORTH AMERICA
Policy Relevant Background is not directly observable  Must be estimated with models

8. Observations: CASTNet Stations (EPA, Nat’l Park Service)
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
MODEL:
GEOS-CHEM 3D Tropospheric Chemistry Model
[Bey et al., 2001] (uses assimilated meteorology; 48 s; 4ºx5º or 2ºx2.5º horiz. resn., 24 tracers)
Observations: CASTNet Stations (EPA, Nat’l Park Service)
X Elevated sites (> 1.5 km)
Low-lying sites

9. Case Studies: Ozone Time Series at Sites used in Lefohn et al. [2001]*
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 +
High-O3 events dominated
by regional pollution
regional
pollution
hemispheric
pollution
Background at high-
altitude site (2.5 km)
not representative of
contribution at
low-lying sites

10. * West (>1.5 km) Southeast (<1.5 km) CASTNet sites Model
Background
Natural O3 level
Stratospheric + X
Background O3
higher at high-altitude western sites
Ozone (ppbv)
Background O3
lower at low-lying
southeastern sites;
decreases with
highest observed O3
Fiore et al., JGR, 2003
Days in March 2001

11. Background O3 even lower under polluted conditions
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

12. Daily 1-5 p.m. mean surface O3 Mar-Oct 2001
Compiling Results from all (71) CASTNet sites:
Natural vs. Anthropogenic Contributions
Stratospheric
Natural (no global anthrop. emis. (& CH4 = 700 ppbv))
Background (no anthrop. emis. in N. Amer)
Observed: CASTNet sites
Probability ppbv-1
Model at CASTNet
Daily 1-5 p.m. mean surface O3 Mar-Oct 2001
Typical ozone values in U.S. surface air:
Background ppbv; Natural ppbv; Stratosphere < 20 ppbv
Anthropogenic methane enhances “background” above “natural”
Fiore et al., JGR, 2003

13. More than half of global methane emissions
are influenced by human activities
BIOMASS
BURNING 20
ANIMALS 90
WETLANDS
180
LANDFILLS 50
GLOBAL METHANE
SOURCES (Tg CH4 yr-1)
GAS 60
TERMITES 25
COAL 40
RICE 85

14. Anthropogenic Methane Emissions Enhance the Tropospheric Ozone Background
Tg O3
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

15. Radiative Forcing of Climate, 1750-Present: Important Contributions from Methane and Ozone
IPCC [2001]
Level of scientific understanding

16. Double dividend of Methane Controls:
Decreased greenhouse warming and improved air quality
Number of U.S. summer grid-square days with O3 > 80 ppbv
Radiative Forcing* (W m-2)
50%
anth.
VOC
50%
anth.
CH4
50%
anth.
NOx
2030 A1
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 A1
+80%
-20%
+30% B1
-5%
-50%
+12%
CH4 links air quality
& climate via
background O3
Fiore et al., GRL, 2002

17. Hemispheric pollution enhances U.S. background
CONCLUSIONS ...
and their implications for public policy
Background O3 is typically less than 40 ppbv; even lower under polluted conditions
Pollution from North America contributes to high-O3 events at remote U.S. sites in spring
Hemispheric pollution enhances U.S. background
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

18. O3 + + NOx  TEMP PAR Leaf Area Vegetation changes  Impact on O3?
Isoprene Emissions are generally thought to contribute to O3 production over the eastern United States [e.g.Trainer et al., 1987; NRC, 1991]
(VOC) O3 +
+ NOx 
ISOPRENE
air pollution (smog)
Harmful to health,
vegetation
TEMP
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

19. Isoprene emissions – July 1996
Isoprene Emission Inventories uncertain by at least a factor of 2
Isoprene emissions – July 1996
GEIA: Global Isoprene Emission Inventory
BEIS2: Regional Isoprene Emission Inventory
7.1 Tg C
2.6 Tg C
[from Paul Palmer]
[1012 atom C cm-2 s-1]

20. Choice of isoprene inventory critical for predicting base-case O3
(2001 meteorology; 1x1 nested GEOS-CHEM [Wang et al., 2004; Li et al., 2004])
GEIA: global inventory
July Anthrop. NOx emissions
0.43 Tg N
(1011 molec cm-2 s-1)
5.6 Tg C
July isoprene emissions
Purves et al., [2004] (based on FIA data; similar to BEIS-2)
(ppbv)
Difference in July 1-5 p.m.
surface O3 (Purves–GEIA)
High-NOx
regime
2.8 Tg C
“isoprene-
saturated”
GEIA
(1011 molecules isoprene cm-2 s-1)

21. Complicated Chemistry: Isoprene may also decrease surface O3 in low-NOx, high isoprene settings+ O3 O3 OH
RO2 NO
(very fast)
NO2
High-NOx
ISOPRENE
Isoprene nitrates
Sink for NOx? O3
(slower )
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)

22. O3 What is the O3 sensitivity to the uncertain fate of
organic isoprene nitrates? OH
RO2 NO
(very fast)
NO2
High-NOx
Isoprene nitrates
Sink for NOx?
ISOPRENE O3
ppbv
MOZART-2
Change in July mean 1-5 p.m.
surface O3 when isoprene
nitrates act as a NOx sink
 6-12 ppbv impact!

23. 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
Monoterpenes
Sweetgum
Invasion of
Pine plantations
-20 –
Percent Change mid-1980s to mid-1990s

24. Change in July 1-5 p.m. surface O3
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 increases
(1-2 ppbv)
ppbv
O3 decreases
(same as previous result)
Isoprene nitrates are
not a NOx sink in
standard MOZART-2;
Results are sensitive
to this assumption!
-20 – (%)
Isoprene changes

25. GEOS-CHEM: GEIA GEOS-CHEM: Purves MOZART-2: GEIA ppbv
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

26. Conclusions and Remaining Challenges
Better constrained isoprene emissions are needed to quantify:
isoprene contribution to Eastern U.S. surface O3
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