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Interactions Between Air Quality and Climate Change
over the Eastern United States: An Investigation of
Climate Change in Our Own Backyard
Loretta J. Mickley
Harvard University
Daniel J. Jacob, Eric M. Leibensperger,
Amos P.K.A. Tai, Shiliang Wu
Haze over Boston on May 31, 2010
EPA Science Forum, March 14, 2012
Wildfires in Quebec the same day.
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Our work focuses on interactions of short-lived gases and
particles in the atmosphere and climate change.
Lifetimes in atmospheric
chemistry
Pollution over Hong Kong
Centuries: SF6, some CFCs
Decades: many greenhouse gases:
CO2, N2O, . . .
9-10 years: CH4 (methane,
precursor to ozone and
greenhouse gas)
Days-weeks: O3 (ozone),
particulate matter (PM)
Seconds: OH, NO
Air pollution over Hong Kong reached dangerous
levels one of every eight days in 2009
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Air pollution in the United States: Ozone and fine particulate
matter (PM2.5) are the two main pollutants
75 ppb (8-h average)
15 mg m-3 (1-y av.)
Millions of people living
in areas in violation of
the standards.
Counties
violating the EPA
standards
Ozone
PM2.5
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The greenhouse effect keeps the Earth warm.
Greenhouse Effect
• Radiation from the earth’s
surface is absorbed and
re-emitted by clouds and
greenhouse gases: CO2. . .
• This process warms the
earth.
CO2
CO2
CO2
Change in CO2 since mid-1800s:
280 ppm to 390 ppm.
IPCC, 2007
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Observed trends in surface
temperature, 1880-2011.
Global mean temperature
increase is ~0.5 oC since 1950s.
Boston/ Logan Airport
Record annual mean temperature
Over Boston, we see lots of yearto-year variability, but a significant
trend towards warming.
NASA Goddard Institute for Space Studies
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Climate models show that observed global warming
trends cannot be explained without human influence.
Observed trend
Models with
human activity.
Models with just
natural processes
Observed trend
IPCC, 2007
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Basic working of climate models
All climate models depend on basic physics to describe motions and
thermodynamics of the atmosphere:
E.g., vertical structure of pressure is described by hydrostatic equation
P( z )  P( z  dz )   a gdz

dP
  a g
dz
Climate models also depend on parameterizations for
many processes.
E.g., microphysics of cloud droplet formation,
vegetation processes.
Input
Tilt of earth,
geography,
greenhouse
gas content
Climate model
Physics +
Parameterized
processes
Output
Weather +
Climate
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Simulations of future climate depend on the path
of socio-economic development.
Global mean surface temperature anomalies
Different scenarios follow
different socio-economic
paths for developed and
developing countries.
A2 = heavy fossil fuel
B1 = alternative fuels
A1B = mix of fossil +
alternative fuels
IPCC 2007
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What do climate models predict for future (2100) climate?
Annual mean
Winter
Summer
Temperature increases
everywhere, especially
at high latitudes.
Annual mean
Winter
Summer
Precipitation
changes are more
complicated.
Precipitation will likely increase over high latitudes but decrease
over low latitudes.
In between, the trend is not clear!
Much uncertainty in future precipitation.
Christensen et al., 2007
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How do particles affect regional climate?
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Particles affect solar radiation directly…and also indirectly by
modifying cloud properties.
California fire plumes
Pollution off U.S. east coast
Light-colored particles reflect sunlight
and cool the earth’s surface.
Black carbon
Aircraft contrails and cirrus over Europe
cooler
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Clearing trend in particles over United States
since 1980s suggests possible recent warming.
Calculated trend in surface sulfate concentrations, 1950- 2001.
1950
1960
1970
1980
Sequence shows increasing
sulfate from 1950-1980,
followed by a decline in
recent years.
Most of aerosol has
already cleared by 2010.
Comparison to observed
sulfate concentrations
shows good agreement.
1990
2001
Leibensperger et al., 2011
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Contiguous US
Observed US surface temperature trend
oC
No trend between 1930 and
1980
Warming trend after 1980
Change in temperatures over
60 year period, 1930-1990
Is the U.S. “warming hole” a
signature of cooling due to
particles?
GISTEMP [2010]
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We can examine the influence of particles on
regional climate using a climate model.
Eastern US
• US manmade particles can
Model without US particles
explain the “warming hole.”
Standard model
• Warming since 1990s can
be attributed to reductions
in particle sources.
Observations
Most of the warming from reducing particle sources
has already been realized.
Leibensperger et al., 2012
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How does climate change affect smog
episodes?
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Roles of ozone (O3) in the atmosphere.
O2
O3
EPA
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Weather plays a large role in ozone air quality.
Days
Number of summer days with ozone exceedances,
mean over sites in Northeast
1988, hottest on
record
A very hot summer can mean
more ozone exceedances,
even if emissions of ozone
precursors are declining.
Lin et al., 2001
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In the Northeast, cold fronts sweep out pollution.
During heat wave:
• Clear skies
• Faster chemical reactions
• Greater biogenic emissions
• Little ventilation of pollutants
Can climate change affect the frequency
of cold fronts?
Fewer cold fronts could mean more
persistent smog episodes.
Leibensperger et al., 2008
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The number of cold fronts coming through the Northeast has
already showed a signal of decline.
Trend in summer low-pressure systems in S. Canada
observations
0.14 /year
model
with increasing GHGs
model
with constant GHGs
0.16 /year
Trend in low-pressure systems
and associated cold fronts is
linked to rapid warming at high
latitudes.
What does this trend mean for
ozone pollution in US?
Emissions of ozone precursors
have declined during this period.
Mickley et al., 2004; Leibensperger et al., 2008
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Trend in emissions and trend in cold fronts
have competing effects on surface ozone.
Emissions
Ozone Episodes
Cold fronts
per summer
Ozone Episodes
More heat waves, less ventilation,
more persistent smog.
Smog over
Baltimore in 2002.
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Observed trends of ozone pollution and cold fronts in Northeast US
Number of ozone episode days (O3> 80 ppb) +
Number of cold fronts, 1980-2006
Ozone episodes
Cold fronts
each summer
Ozone episodes,
constant climate
Ozone exceedance days in Northeast dropped from 30 in 1980 to 10 in 2006,
but would have dropped to ≈ zero in the absence of cold-front trend.
Leibensperger et al. [2008]
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Most models agree that surface ozone will increase over
the Northeast in a warming climate.
Change in max daily 8-hour average JJA surface
ozone in 2050s, relative to present-day.
Climate penalty for air
quality:
Harvard model shows 3-7
ppb increase in surface
ozone in Midwest US in
2050s climate.
During heatwaves,
increases in ozone reach
10 ppb in future climate.
ppb
Wu et al., 2008
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Take home messages:
1. Reductions in particles (PM2.5) over the Eastern US has likely
contributed to rapid warming in recent years.
2. Smog episodes in the East are sensitive to many meteorological
variables, especially the frequency of cold fronts.
3. The climate penalty: climate change will likely worsen ozone air quality
over the East.
Contiguous US
Acknowledgments:
oC
Rapid warming after 1980
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O2
hn
O3
STRATOSPHERE
8-18 km
Lifecycle of tropospheric ozone: production is via oxidation
of CO, VOCs, and methane in the presence of NOx.
Many processes affected by climate
TROPOSPHERE
Soup of chemical reactions
NOx
Ozone is produced in the
atmosphere in sunlight.
O3
Deposition
VOCs
NOx
CO
NOx
VOCs
emissions
Fires
•Nonmethane volatile organic compounds (VOCs)
•NOx = NO + NO2
NOx
VOCs
CH4
VOCs
VOCs
NOx
CO
CH4
NOx
Biosphere
Human activity
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Climate change affects many
processes, including gasparticle partitioning.
Life cycle of particulate matter (PM, aerosols)
precursor gases
ultra-fine
(<0.01 mm)
nucleation
Soup of
chemical
reactions
fine
(0.01-1 mm)
. . coagulation
.
. . .
condensation
cloud
(1-100 mm)
cycling
coarse
scavenging
(1-10 mm)
SO2
NOx
SO2
NOx
VOCs
combustion
volcanoes
NOx
VOCs
VOCs
VOCs
VOCs
NH3
NOx
NOx
agriculture
biosphere
wildfires
combustion
soil dust
sea salt27
Exceedances of the ozone
air quality standard
Current standard (75 ppb)
EPA-recommended range for revision
of standard (60-70 ppb)
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