Background Ozone in Surface Air

1
Background Ozone in Surface Air over the United
States Variability, Climate Linkages, and
Policy Implications
Arlene M. Fiore Department of Atmospheric and
Oceanic Sciences Seminar University of
Wisconsin-Madison December 6, 2004
2
Acknowledgments
Daniel Jacob Brendan Field Hongyu Liu Bob
Yantosca Qinbin Li Duncan Fairlie
David Streets Suneeta Fernandes
Carey Jang Jason West
3
What is the origin of tropospheric ozone?
Stratospheric O3
Stratosphere

O3
12 km
hn
Free Troposphere
NO
NO2
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 2
CONTINENT 1
OCEAN
4
Number of People Living in U.S. Counties
Violating National Ambient Air Quality Standards
(NAAQS) in 2001
EPA 2002
Nitrogen dioxide
0
0
124 ppbv 40.2
Ozone (O3)
84 ppbv 110.3
Sulfur dioxide (SO2)
0.007
Particles lt 10 mm (PM10)
11.1
Particles lt 2.5 mm (PM2.5)
72.7
Carbon monoxide (CO)
0.7
Lead
2.7
Any pollutant
133.1
0
100
50
150
Millions of People
5
The Risk Increment Above the Background is
Considered when setting the NAAQS for Ozone
Environmental risk
Acceptable added risk
Ozone Concentration
Background
NAAQS

• Need a quantitative estimate for background ozone
• EPA chose a constant value (40 ppbv) in previous
  review of O3 standard

6
Range of background O3 estimates in U.S.
surface air
Range considered by EPA during last revision of
O3 standard
84 ppbv threshold for current U.S. O3 standard
20
40
60
80
100
O3 (ppbv)
7
REGULATORY BACKGROUND OZONE DEFINITION Ozone
concentrations that would exist in the absence of
anthropogenic emissions from North America EPA,
2003
stratosphere
Outside natural influences
Lightning
lightning
Long-range transport of pollution
Background air
X
Human activity
Fires
Land biosphere
Ocean
NORTH AMERICA
8
Intercontinental Transport of Pollution at
Northern Midlatitudes
Reduced Visibility from Transpacific Transport of
Asian Dust
Glen Canyon, Arizona
O3
Contributes to Hemispheric Background O3
9
EPA-Defined Background is Not Directly Observable
? Must be Estimated with Models
TOOL GEOS-CHEM 3D Tropospheric Chemistry Model
Bey et al., 2001 (uses assimilated
meteorology 48 s 4ºx5º or 2ºx2.5º horiz.
resn., 24 tracers)
Simulations for Ozone Source Attribution Mar-Oct
2001

• Standard simulation..2x2.5 GEOS-CHEM, compare
  with observations
• Backgroundno 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
10
2001 CASTNet Stations (EPA, Natl 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
X Elevated sites (gt 1.5 km) Low-lying sites
11
Case Study Voyageurs National Park,
Minnesota(May-June 2001)

CASTNet observations Model Background Natural O3
level Stratospheric

Regional pollution
D

D
Hemispheric pollution

X
Lefohn et al. 2001 suggest a stratospheric sourc
e as the likely origin of high-O3
events frequently observed in June
Background 15-36 ppbv Natural level 9-23
ppbv Stratosphere lt 7 ppbv
12
Case Study Yellowstone NP, Wyoming(March-May
2001)

CASTNet observations Model Background Natural O3
level Stratospheric

Regional pollution
D

D
Hemispheric pollution

X
Frequent high-O3 events previously attributed to
natural, stratospheric source Lefohn et al.,
2001
Background at high-altitude site (2.5 km) not
necessarily representative of background
contribution at low-lying sites
13
West (gt1.5 km)
Southeast (lt1.5 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
Days in March 2001
14
Background O3 even lower under polluted
conditions
Daily mean afternoon O3 at 58 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 approach
15
Monthly mean afternoon (1-5 p.m.) surface O3
Background O3 varies with season


Regional pollution from N. Am. emis. (8-30 ppbv)
Hemispheric pollution enhancement (5-12 ppbv)


Mean background 20-35 ppbv Mean natural level
13-27 ppbv Mean stratosphere 2-7 ppbv
16
Daily afternoon (1-5 p.m.) surface O3
March-October 2001
Compiling results from all CASTNet sites
Stratospheric
Natural
Background
CASTNet sites
Probability ppbv-1
Model at CASTNet
Typical ozone values in U.S. surface air
Background 15-35 ppbv Natural 10-25 ppbv
Stratosphere lt 20 ppbv
17
and their implications for public policy
CONCLUSIONS ...

• Background O3 is typically less than 40 ppbv
  even lower under polluted conditions
• High-O3 events at remote U.S. sites in spring can
  be explained largely by pollution from North
  America
• Hemispheric pollution enhances U.S. background

?health risk from O3 underestimated in present
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

? Climate Linkage
18
Air quality-Climate LinkageCH4, O3 are
important greenhouse gasesCH4 contributes to
background O3 in surface air
O3
greenhouse gas
Free Troposphere
hn
Global Background O3 (Hemispheric Pollution)
NO
NO2
OH
HO2
Boundary layer (0-3 km)
Direct Intercontinental Transport
VOC, CH4, CO
air pollution (smog)
NOx NMVOCs
NOx NMVOCs
O3
O3
air pollution (smog)
CONTINENT 2
CONTINENT 1
OCEAN
19
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
20
Radiative Forcing of Climate, 1750-PresentImpor
tant Contributions from Methane and Ozone
IPCC 2001
Level of scientific understanding
21
Impacts of future changes in global anthropogenic
emissions on climate
Change in radiative forcing (W m-2) relative to
1995 base case
50 anthr. CH4
50 anthr. NOx
2030 A1
50 anthr. NMVOC
2030 B1
IPCC scenario Anthrop. NOx emissions (2030 vs. present) Global U.S. Anthrop. NOx emissions (2030 vs. present) Global U.S. Methane emissions (2030 vs. present)
A1 80 -20 30
B1 -5 -50 12
Fiore et al., GRL, 2002
22
Methane emission controls reduce surface O3
everywhere
Decrease in July 1995 Mean Surface O3
concentrations from 50 reductions in global
anthropogenic CH4 emissions
23
Methane emission reductions shift entire summer
afternoon surface O3 frequency distribution over
U.S.
Probability ppbv-1
Ozone Concentration (ppbv)
24
Impacts of future changes in global anthrop.
emis. on U.S. air quality
Number of summer grid-square days with O3 gt 80
ppbv
Increase in U.S. pollution events despite
domestic decline in anthrop. emissions
1995 (base)
50 anthr. VOC
50 anthr. CH4
50 anthr. NOx
2030 A1
2030 B1
IPCC 2030 scenario Anthrop. NOx emissions Global U.S. Anthrop. NOx emissions Global U.S. Methane emissions
A1 80 -20 30
B1 -5 -50 12
25
Rising emissions from developing countries
lengthen the O3 pollution season in the United
States
1995 Base Case
Degradation of U.S. air quality from rise in
global emissions despite domestic reductions
2030 A1
26
CONCLUSIONS,
Public policy implications, and Next steps

• Background O3 is typically less than 40 ppbv
  even lower under polluted conditions
• High-O3 events at remote U.S. sites in spring can
  be explained largely by pollution from North
  America
• Hemispheric pollution enhances U.S. background

?health risk from O3 underestimated in present
EPA risk assessments

• compare additional model estimates of background
  (MOZART-2) for
• use in risk assessments

?these events do not represent U.S. background
conditions and should not be used to
challenge legitimacy of O3 NAAQS

• extend source attribution for background

• international agreements to reduce hemispheric
  background should
• improve air quality facilitate compliance w/
  more stringent standards

• feasibility of methane control West and Fiore,
  submitted AGU poster
• better characterize relationship between methane
  sources and surface
• ozone response