1. 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

2. 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
Calculated with old 0.08 ppm standard. New standard of ppm will push more areas into non-attainment.
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.

3. O2 hn
Chemistry of tropospheric ozone: oxidation of CO, VOCs, and methane in the presence of NOx O3
STRATOSPHERE
8-18 km
TROPOSPHERE
Stagnation promotes ozone production hn
NO2 NO O3
hn, H2O OH
HO2
H2O2
Deposition
CO, VOC
Nitrogen oxide radicals; NOx = NO + NO2
combustion, soil, lightning
Methane
wetlands, livestock, natural gas
Nonmethane volatile organic compounds (VOCs)
vegetation, combustion, industry
CO (carbon monoxide)
combustion, VOC oxidation
Tropospheric
ozone
precursors

4. Weather plays a large role in ozone air quality.
Probability
of ozone exceedance
vs. daily max. temperature
Curves include effects of
Biogenic emissions
Stagnation
Clear skies
Northeast
Southeast
Los Angeles
Temperature (K)
1988, hottest on record
Days
Number of summer days with ozone exceedances, mean over sites in Northeast
The total derivative d[O3]/dT is the sum of partial derivatives (dO3/dxi)(dxi/dT).
x = ensemble of ozone forcing variables that are temperature-related.
Lin et al., 2001

5. Cyclones crossing southern Canada affect ozone air quality in Eastern US.
cold front L
EPA ozone levels
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

6. Decline in mid-latitude cyclone number over mid-latitudes leads to more persistent stagnation episodes, more ozone.
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.
trends
NE ozone episodes
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

7. Particulate matter (PM, aerosols) sources and processes
ultra-fine
(<0.01 mm)
fine
( mm)
cloud
(1-100 mm)
precursor gases
oxidation
nucleation
coagulation
cycling .
SO2
H2SO4
condensation
VOCs
RCO…
coarse
(1-10 mm)
scavenging
NOx
HNO3
wildfires
NH3
carbonaceous
combustion
particles
combustion
biosphere
volcanoes
agriculture
biosphere
soil dust
sea salt

8. Observed correlations of total PM2.5 with meteorology
Precipitation
Stagnation
Temperature
Positive correlation with temperature occurs due to:
Increased oxidation of SO2
Greater biogenic emissions
Precipitation
Stagnation
Results from EPA AQS database: sites sampled every 1-6 days from 1998 to 2007.
Observed correlations provide means to test model simulations.
E.g., observed sulfate response to temperature is ~ μg m-3/K,
4x greater than Dawson et al model.
Temperature
Tai et al., ms.

9. AIR QUALITY GISS GCM GEOS-Chem
What do models project for future air quality?
We have developed GCAP (Global Change and Air Pollution).
GISS GCM
Physics of the atmosphere
Qflux ocean or specified SSTs
GEOS-Chem
Emissions
Chemical scheme
Deposition
met fields
2050s
1990s
decrease in cyclone frequency leads to increased stagnation.
CO tracer Northeast, Jul-Aug
met fields
chemistry fields
met fields
Regional chemistry model
Regional climate model
Chemistry model driven by GCM meteorology to study influence of climate on air quality.
AIR QUALITY
Mickley et al., 2004

10. Daily max 8h-avg ozone averaged in JJA (ppb)
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
2000s conditions
2050s climate
2050s emissions
2050s climate & emis
Increase of summer max-8h-avg ozone
99th percentile
Cumulative probability (%)
Midwest
Daily max 8h-avg ozone averaged in JJA (ppb)
We define the climate change penalty as the effort required to meet air quality standards under future climate change.
Wu et al., 2007

11. Change in annual mean surface inorganic aerosol from 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.
Also, mix of aerosol species is expected to change, so sensitivity to climate will also change.
Present-day annual average
Pye et al., 2009
sulfate nitrate ammonium

12. Projected increase in wildfires could affect air quality in the US.
1980
2000
1990
R2=52%
Area burned / 106 Ha
0.5
0.25
observations
model
May-Oct area burned in Pacific Northwest
We predict future wildfires using observed relationships between meteorology and area burned for different ecosystems.
Perturbation due to climate change only
changes in fire season surface ozone.
Spracklen et al., 2009
Hudman et al, ms.

13. 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

14. 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

15. What is the influence of changing aerosol on regional climate
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.

16. Warming due to 2010-2025 trend in greenhouse gases.
Removal of anthropogenic aerosols over US leads to a 0.5-1o C warming in annual mean surface temperature.
Warming due to trend in greenhouse gases.
Additional warming due to zeroing of aerosols over the US.
Annual mean surface temperature change in Control.
Mean temperature difference:
No-US-aerosol case – Control
White areas signify no significant difference.
Results from an ensemble of 3 for each case.
Mickley et al., ms.

17. Annual mean temperature trends over Eastern US
The regional surface temperature response to aerosol removal appears to persist for many decades in the model.
Annual mean temperature trends over Eastern US
Temperature (oC)
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

18. 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

19. Extra slides

20. Ozone exceedances in eastern US correlate with frequency of cyclone passage through southern Canada/Great Lakes region.
Correlation between cyclone number in red and green boxes and US ozone exceedances over 27-year record, JJA
Storm tracks for 3 years
(NCEP, summer )
Storm tracks calculated with cyclone tracker tool, applies cyclone criteria to observations or model output.
Leibensperger et al., 2008

21. Projected increase in wildfires could affect air quality in the US.
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.

22. We define the climate change penalty as the effort required to meet air quality goals in the future atmosphere.
40% cut in NOx s climate
present-day NOx emissions + climate }
climate change penalty
40% cut in NOx + present-day climate
50% cut in NOx s climate
2000–2050 climate change implies an additional 25% effort in NOx emission controls to achieve the same ozone air quality.
Midwest surface ozone
Wu et al., 2007