Interactions between climate and atmospheric
chemistry in the US
• Effects of climate change on air quality
• Effects of short-lived species on climate
Smog over Pittsburgh, ranked #1 city for
particulate pollution in 2008 by American
Lung Association
Loretta J. Mickley, Harvard University
Collaborators:
Rynda Hudman, Daniel Jacob, Eric Leibensperger, Jennifer Logan,
Havala Pye, Dominick Spracklen, Amos Tai, Shiliang Wu, Moeko
Yoshitomi
Funding for this work: NASA, EPA, EPRI
Part 1: Effects of climate change on air quality.
Millions of people in U.S. already live in areas of high
pollution. How will a changing climate affect pollution?
Calculated with new 0.075 ppm
standard
Number of people living in areas
that exceed the national ambient
air quality standards (NAAQS) in
2008.
EPA’s Technical Support Document for the
proposed finding on CO2 as a pollutant.
Cites the threat of climate change to air quality.
Public hearings last week on EPA proposed finding
in Detroit + NYC.
O2
hn
STRATOSPHERE
8-18 km
Chemistry of tropospheric ozone:
oxidation of CO, VOCs, and methane in
the presence of NOx
O3
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.
1988, hottest
on record
Northeast
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
Low pressure systems (aka cyclones) cross southern Canada and
sweep out ozone pollution from Eastern US.
cold front
EPA ozone levels
L
Stalled high pressure
system associated with:
• increased biogenic
emissions
• clear skies
• weak winds
• high temperatures.
Hazardous levels of ozone
cold front
L
3 days later
Cold front pushes smog
poleward and aloft on a
warm conveyor belt.
Cyclone passage through southern Canada/Great Lakes region strongly
affects frequency and duration of U.S. ozone episodes.
Sample storm tracks,
summer 1979-1981
Correlation between cyclone number
each summer in red and green boxes
and number of US ozone episodes
27 year
record
Strong anti-correlation of cyclone number and
number of ozone episodes in eastern US:
Fewer cyclones per summer in green box
leads to more ozone episodes in US.
Leibensperger et al., 2008
1950-2000 observed trend in cyclone frequency matches that in
climate model with increasing greenhouse gases.
1950-2006 trend in JJA cyclones in S. Canada
NCEP/NCAR obs
0.14 yr-1
0.16 yr-1
Trend in cyclones appears due in
part to weakened meridional
temperature gradients, reduction
of baroclinicity over midlatitudes.
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
Trend in emissions and trend in cyclones have competing
effects on surface ozone.
Cyclones: less frequent cyclones + cold
fronts mean more persistent pollution
episodes
Emissions: reduced emissions means
fewer episodes.
1980-2006 trends
cyclones
Decline in emissions of ozone
precursors from US mobile sources.
Parrish 2006.
NE ozone episodes
Mickley et al., 2004; Leibensperger et al., 2008
Ozone pollution days in the Northeast US
We find that if 1980-2006
cyclone frequency had
remained constant, we
would have had zero
episodes over Northeast.
Climate
response
If emissions had remained constant,
decline in mid-latitude cyclone
number over Canada would have
meant more persistent stagnation
episodes, more ozone.
 d (exceedances)   d (exceedances) 
 d (exceedances) 





dt
dt
dt

 
emissions 
cyclones
d (exceedances) d (cyclones )
 d (exceedances ) 

 (4.2)( 0.15)  0.63 days yr-1


dt
d (cyclones )
dt

 cyclones
 d (exeedances) 

  0.84 days yr-1
dt


 d (exceedances ) 
 1.5


days yr-1
dt

 emissions
Trend in pollution days
due to decline in
cyclone frequency
Trend in pollution days due to
decline in emissions
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.
Observed correlations provide means to
test model simulations.
Temperature
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
Qflux ocean, wellmixed GHGs
met
fields
Chemical transport
model
chemistry, emissions
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 change in max daily 8-hour average JJA ozone
How will US surface ozone
change in a changing climate?
Climate penalty for air quality:
Harvard model shows 2-12 ppb
increase in surface ozone in East
2000-2050 change in max daily 8-hour average JJA ozone
Most models agree that
surface ozone will
increase over the
Northeast.
Disagreement occurs
elsewhere due to
differences in chemistry
and cloud cover change.
ppb
Multi-model comparison
Weaver et al., 2009
Uncertainty in response of surface PM to changing meteorology is
large. We can use present-day observations to test models.
Calculated response in surface
PM to +2.5 oC temperature change
applied uniformly for July.
Dawson et al., 2007
(μg/m3)
Observed correlation between
surface temperature and
surface PM concentrations.
Positive correlation with T due to:
• Increased oxidation of SO2
• Greater biogenic emissions
Tai et al, ms. in progress
Part 2: Effects of short-lived species on climate.
Case study of US aerosols and regional climate change.
Radiative forcing:
• Easily calculated metric
of climate change
• Suggests the relative
magnitude of surface
temperature response to
a given perturbation.
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.
Due to short lifetime, forcing due to
aerosols is not uniform across globe.
Over the US, radiative forcing due to
sulfate aerosols is -2 Wm-2.
cooling
Globally averaged radiative
forcing due to CO2 is +1.7 Wm-2.
warming
IPCC, 2007; Liao et al. , 2004
Trend in aerosols over United States suggests cleaner skies,
possible 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.
Comparison to observed
sulfate concentrations
shows good agreement.
1990
2001
Leibensperger et al., ms
Is the climate response to changing aerosols regional
or global?
Recent US Climate Change report suggests
more global than regional response, but the
report looks at an ensemble of short-lived
species all over the globe.
– U.S. Climate Change Science Program, Synthesis
and Assessment Product 3.2
Harvard’s work to date suggests more
regional than global response at least for
US aerosols.
Decline in 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.
Caveats: No transport, only direct effect considered in this
pilot study.
Removal of anthropogenic aerosols over US increases annual mean
surface temperatures by 0.5 o C.
Summertime temperatures increase as much as 1.5 oC.
Warming due to 2010-2050
trend in greenhouse gases.
Additional warming/ cooling due to
zeroing of US aerosols
oC
Annual mean surface temperature
change in Control.
Mickley et al., ms. 2009
oC
Mean 2010-2050 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
persists for many decades in the model.
Temperature (oC)
Annual mean temperature trends over Eastern US
No-US-aerosols case
Control, with US aerosols
Bottom line: Efforts to clear the air of anthropogenic aerosol
over the US may exacerbate regional warming.
Mickley et al., ms
Ongoing study: Perform realistic simulation of changing aerosol
optical depths over the US, together with sensitivity studies.
GEOS-Chem
chemistry transport model
aerosol
concentrations
Calculation of cloud droplet
number concentrations
aerosol indirect
effect
GISS GCM III
climate model
Climate response to aerosol trends
over the US
We use historical/projected
emissions of SO2, NOx, BC, and
OC to quantify the climatic role of
US aerosols in the past and future.
1950-2050 Control simulation
(EDGAR/Tami Bond historical
emissions and A1B; includes rising
U.S. aerosol sources until 1980
and subsequent decline)
Sensitivity simulations:
• 1950-2050 No US aerosols.
Quantifies the past effect
of U.S. anthropogenic sources on
regional climate.
• 2010-2050 Constant US
emissions
Quantifies the
warming effect from the projected
decrease in U.S. emissions.
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
Understand secondary organic aerosols: sources, chemistry.
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
Observed Correlations of PM2.5 with Meteorology
Multiple linear regression to fit 1998-2008 deseasonalized EPA/AQS data for PM2.5:
9
y   0    k xk  interactio n terms
xk
Meteorological Parameter
(NCEP/NCAR Reanalysis 1
& NOAA CPC)
x1
Surface temperature (K)
x2
Relative humidity (10%)
x3
Precipitation (cm/d)
x4
Cloud cover (10%)
x5
Geopotential height at 850
hPa (100 m)
x6
dSLP/dt (hPa/d)
x7
Wind speed (m/s)
x8
E-W wind direction indicator
(cosθ)
x9
N-S wind direction indicator
(sinθ)
k 1
 Observed +ve correlation with sulfate is larger by 10x  stagnation and air


mass origins
Strong +ve correlation with nitrate in the west and north  agricultural NH3 and
soil NOx emissions
Strong +ve correlation with OC  VOC emissions and fires
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
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.
2000-2050 JJA surface temperature change
Models tend to agree
on 2000-2050 changes
in surface temperature
over the United States.
2000-2050 change in max daily 8-hour average ozone
Most models agree that
surface ozone will increase
over the Northeast.
Disagreement occurs
elsewhere due to
differences in chemistry
and cloud cover change.
Weaver et al., 2009
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.
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.
Observations of a possible relationship between trends in aerosol
optical depths and surface temperature.
Smoothed monthly mean AOD for
sites in Europe
Pinatubo
Annual mean fluxes, temperatures
Anomaly of clear
sky shortwave
downward radiation
at surface
Surface temperature
anomalies averaged
over all sites,
excluding 2003.
+0.42 C /decade
Ruckstuhl et al., 2008