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The Climate Change – Urban Pollution Relationship
• Effects of climate change on air quality
• Effects of aerosols on regional climate
Smog over Pittsburgh, ranked #1 city for
particulate pollution in 2008 by ALA
Loretta J. Mickley, Harvard University
also Shiliang Wu, Jennifer Logan, Dominick
Spracklen, Amos Tai, Rynda Hudman, Daniel
Jacob, Moeko Yoshitomi, Eric Leibensperger,
Havala Pye
Funding for this work: NASA, EPA, EPRI
Millions of people already live in areas of high pollution.
Number of people living in areas that
exceed the national ambient air quality
standards (NAAQS) in 2006.
0.08 ppm
65 mg/m3
EPA’s Technical Support Document for the
proposed finding on CO2 as a pollutant.
Cites the threat of climate change to air quality.
Calculated with old 0.08 ppm
standard. New standard of
0.075 ppm will push more
areas into non-attainment.
O2
hn
STRATOSPHERE
8-18 km
O3
Chemistry of tropospheric ozone:
oxidation of CO, VOCs, and methane in
the presence of NOx
TROPOSPHERE
Stagnation promotes
ozone production
hn
O3
NO2
NO
OH
HO2
hn, H2O
Deposition
CO, VOC
Nitrogen oxide radicals; NOx = NO + NO2
combustion, soil, lightning
Tropospheric
ozone
precursors
Methane
wetlands, livestock, natural gas
Nonmethane volatile organic compounds (VOCs)
vegetation, combustion, industry
CO (carbon monoxide)
combustion, VOC oxidation
H2O2
Weather plays a large role in ozone air quality.
Northeast
1988, hottest
on record
Probability
Days
Number of summer days with ozone
exceedances, mean over sites in Northeast
Probability
of ozone exceedance
vs. daily max. temperature
Curves include effects of
• Biogenic emissions
• Stagnation
• Clear skies
Southeast
Los Angeles
The total derivative d[O3]/dT is the sum
of partial derivatives (dO3/dxi)(dxi/dT).
Temperature (K)
x = ensemble of ozone forcing variables
that are temperature-related.
Lin et al., 2001
Cyclones crossing southern Canada affect ozone air quality in
Eastern US.
cold front
EPA ozone levels
L
Stalled high pressure
system associated with:
• increased biogenic
emissions
• clear skies
• weak winds
• high temperatures.
cold front
L
3 days later
Cold front pushes smog
poleward and aloft on a
warm conveyor belt.
Hazardous levels of
ozone
Decline in mid-latitude cyclone number over mid-latitudes leads
to more persistent stagnation episodes, more ozone.
1950-2006 trend in JJA cyclones in S. Canada
0.14 a-1
0.16 a-1
Trend in cyclones appears due in
part to weakened meridional
temperature gradients, reduction
of baroclinicity over midlatitudes.
1980-2006 trends
cyclones
Trend in emissions and trend in cyclones
have competing effects on surface ozone.
If cyclone frequency had remained constant,
we calculate zero episodes over Northeast.
Mickley et al., 2004; Leibensperger et al., 2008
NE ozone episodes
Particulate matter (PM, aerosols) sources and processes
ultra-fine
(<0.01 mm)
precursor gases
oxidation
SO2
nucleation
H2SO4
fine
(0.01-1 mm)
. . coagulation
.
. . .
cloud
(1-100 mm)
cycling
condensation
VOCs
NOx
RCO…
coarse
scavenging
(1-10 mm)
HNO3
wildfires
NH3
combustion
biosphere
volcanoes
agriculture
biosphere
carbonaceous
combustion
particles
soil dust
sea salt
Observed correlations of total
PM2.5 with meteorology
• Precipitation
• Stagnation
Precipitation
• Temperature
Positive correlation with temperature
occurs due to:
• Increased oxidation of SO2
• Greater biogenic emissions
Stagnation
Results from EPA AQS database: 1000+
sites sampled every 1-6 days from 1998 to
2007.
Temperature
Observed correlations provide means to
test model simulations.
E.g., observed sulfate response to
temperature is ~ +0.08 μg m-3/K,
4x greater than Dawson et al. 2007 model.
Tai et al., ms.
What do models project for future air quality?
We have developed GCAP (Global Change and Air Pollution).
GISS GCM
GEOS-Chem
Physics of the
atmosphere
met fields
Chemical scheme
Qflux ocean or
specified SSTs
met
fields
Emissions
Deposition
met
fields
Regional
climate model
Chemistry model driven
by GCM meteorology to
study influence of
climate on air quality.
chemistry
fields
Regional
chemistry
model
2000-2050 decrease in
cyclone frequency leads
to increased stagnation.
2050s
CO tracer
Northeast,
Jul-Aug
1990s
AIR QUALITY
Mickley et al., 2004
2000-2050 climate change increases JJA surface ozone:
1-5 ppb on average across US, 5-10 ppb during heat waves in Midwest
Max. 8-hr-avg ozone
Effect of climate change alone
Daily max 8h-avg ozone
averaged in JJA (ppb)
2000s conditions
2050s climate
2050s emissions
2050s climate & emis
Increase of
summer max8h-avg ozone
99th
percentile
Midwest
Cumulative probability (%)
We define the climate change penalty as the effort required
to meet air quality standards under future climate change.
Wu et al., 2007
Change in annual mean surface inorganic aerosol from
2000-2050 climate change (no change in emissions)
Increase in Northeast due to
increased temperature and
accelerated oxidation rates
Decrease in Southeast due
mainly to increased precipitation.
Calculation of future aerosol levels is
challenging because of uncertainty in
future rainfall over mid-latitudes.
Present-day annual average
Also, mix of aerosol species is expected
to change, so sensitivity to climate will
also change.
Pye et al., 2009
sulfate
nitrate
ammonium
Projected increase in wildfires could affect air quality in the US.
Area burned / 106 Ha
May-Oct area burned
observations
in Pacific Northwest
0.5
R2=52%
We predict future wildfires using
observed relationships between
meteorology and area burned for
different ecosystems.
model
0.25
1980
1990
1990
2000
Perturbation due to
climate change only
2000-2050
changes in fire
season surface
ozone.
Spracklen et al., 2009
Hudman et al, ms.
Present-day radiative forcing due to aerosols over the
eastern US is comparable in magnitude, but opposite in
sign, to global forcing due to CO2.
Globally averaged
radiative forcing due to
CO2 is +1.7 Wm-2.
warming
Over the eastern US, radiative forcing
due to sulfate aerosols is -2 Wm-2.
cooling
IPCC, 2007; Liao et al. , 2004
Is the climate response to changing aerosols
collocated with regions of radiative forcing?
Recent US Climate Change report says NO:
Trends in short-lived species (such as aerosols)
affect global, but not regional, climate.
“Regional emissions control strategies for short-lived pollutants
will . . . have global impacts on climate.”
– U.S. Climate Change Science Program, Synthesis
and Assessment Product 3.2
Harvard’s work to date says YES:
Removal of the aerosol burden over the
eastern US will lead to regional warming,
in a way that the US Climate Change
report would not have recognized.
Calculated present-day aerosol
optical depths
What is the influence of changing aerosol on regional climate?
In pilot study, we zero out aerosol optical depths over US.
GISS GCM
For pilot study, 2 scenarios were simulated:
Control: aerosol optical depths fixed at 1990s levels.
Sensitivity: U.S. aerosol optical depths set to zero
(providing a radiative forcing of about +2 W m-2 locally over
the US); elsewhere, same as in control simulation.
Each scenario includes an ensemble of 3 simulations.
Removal of anthropogenic aerosols over US leads to a 0.5-1o C
warming in annual mean surface temperature.
Warming due to 2010-2025
trend in greenhouse gases.
Annual mean surface temperature
change in Control.
Mickley et al., ms.
Additional warming due to
zeroing of aerosols over the US.
Mean 2010-2025 temperature difference:
No-US-aerosol case – Control
White areas signify no significant difference.
Results from an ensemble of 3 for each case.
The regional surface temperature response to aerosol removal
appears to persist for many decades in the model.
Temperature (oC)
Annual mean temperature trends over Eastern US
No-US-aerosols case
Control, with US aerosols
Temperature response is initially strongest in winter.
Summertime temperature response kicks in around 2030.
Mickley et al., ms
Implications for policymakers
• Policymakers need to consider “climate change penalty,” i.e., the additional
emission controls necessary to meet a given air quality target.
• Efforts to clear the air of anthropogenic aerosol over the US may
exacerbate regional warming.
Directions for future research
Understand causes in interannual variability of air quality.
Investigate model sensitivity of pollutants to meteorology, and compare to observations.
Understand chemistry of biogenic species, e.g. isoprene
Improve emission inventories for recent past/future, especially for NH3, black carbon,
organic carbon, mercury
Improve global and regional models of mercury
Understand secondary organic aerosols: sources, chemistry.
For cities: improve modeling of fine scale features, investigate how best to downscale
meteorology from global climate models, test effects of land use change.
Understand aerosol-cloud interactions, characterize aerosol composition
Extra slides
Ozone exceedances in eastern US correlate
with frequency of cyclone passage through
southern Canada/Great Lakes region.
Storm tracks for 3 years
(NCEP, summer 1979-1981)
Storm tracks calculated with cyclone tracker tool,
applies cyclone criteria to observations or model
output.
Leibensperger et al., 2008
Correlation between cyclone
number in red and green boxes
and US ozone exceedances
over 27-year record, JJA
Projected increase in wildfires could affect air quality in the US.
2000-2050 change in JJA
surface organic aerosol
due to increased wildfires
We have developed a fire prediction tool
based on observed relationships between
meteorology and area burned.
Applying these relationships to GCM
meteorology, we predict area burned and
future emissions of wildfire pollutants.
mg m-3
Perturbation due to
climate change only
Changes in JJA
surface ozone
concentrations
Spracklen et al., 2009
Hudman et al, ms.
We define the climate change penalty as the effort required to
meet air quality goals in the future atmosphere.
40% cut in NOx +
2050s climate
present-day NOx
emissions + climate
}
climate change
penalty
40% cut in NOx +
present-day climate
50% cut in NOx +
2050s climate
Midwest surface ozone
Wu et al., 2007
2000–2050 climate
change implies an
additional 25%
effort in NOx
emission controls
to achieve the
same ozone air
quality.