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Studies of chemistry-climate interactions at Harvard
• 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, Cynthia Lin
Funding for this work: NASA, EPA, EPRI
I. Effects of climate change on air quality
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
Is there a connection between summertime cyclone frequency and
the number of ozone episodes?
Correlation of 1980-2006 JJA ozone
exceedances with storm tracks
Sample summertime storm tracks, 1979-81
weak
correlation
Strong
anti-correlation
NCEP/NCAR reanalysis
• Frequent cyclones plus associated cold
fronts mean fewer ozone episodes.
• Fewer cyclones mean more persistent
stagnation and more intense pollution.
6
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
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.
What does this trend mean for
ozone pollution in US?
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
means longer pollution episodes
Emissions: reduced emissions
means fewer episodes.
1980-2006 trends
cyclones
NE ozone episodes
Decline in emissions of
ozone precursors from
mobile sources,
Parrish 2006.
Mickley et al., 2004;
Leibensperger et al., 2008
Ozone pollution days in the Northeast US
Decline in mid-latitude cyclone
number over mid-latitudes leads
to more persistent stagnation
episodes, more ozone.
If cyclone frequency had
remained constant, we calculate
zero episodes over Northeast.
 d (exceedances)   d (exceedances) 
 d (exceedances) 





dt
dt
dt

 
emissions 
cyclones
d (exceedances) d (cyclones )
 d (exceedances) 

 (4.2)( 0.15)  0.63


dt
d (cyclones )
dt

 cyclones
 d (exeedances) 

  0.84 days yr-1
dt


 d (exceedances) 
 1.5


days yr-1
dt

 emissions
days yr-1
Trend in pollution days
due to decline in
cyclone frequency
Trend in pollution days due to
decline in emissions
II. Effects of aerosols on regional climate
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
Annual temperature difference between the two cases stays about
constant, but in fact the annual mean hides seasonal differences.
Mickley et al., ms
The regional surface temperature response to aerosol removal
varies according to season.
Temperature (oC)
Additional warming due to aerosol removal over Eastern US
winter
fall
spring
summer
9-year running means. Stars indicate a significance difference
between no-US-aerosols case and control :
Winter: Temperature response is initially very strong, then dies off.
Spring and Fall: Temperature response kicks in around 2030.
Summer: Moderate temperature response.
Mickley et al., ms
Working hypothesis for time-dependent responses,
varying by season.
Temperature (oC)
Additional warming due to aerosol removal over Eastern US
winter
fall
spring
summer
1. Initial temperature response is strongest in winter. In the other
seasons, excess surface heat can be more easily carried off through
convection.
2. Rise in greenhouse gases increases stability of atmosphere, inhibits
ventilation of warm surface air. Eventually surface warming in the noUS-aerosol case shows up in spring and fall.
3. In fall-winter-spring, the warming due to greenhouse gases eventually
swamps the warming due to aerosol removal.
Mickley et al., ms
Next steps: Perform more 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.
Calculated trend in surface sulfate concentrations, 1950- 2001.
1950
1960
Sequence shows
increasing sulfate from
1950-1980, followed by a
decline in recent years.
We will test the climate
effects of this and other
aerosol trends in the
GISS climate model.
1970
1990
1980
2001
Comparison to observed
sulfate concentrations
shows good agreement.