Interactions between climate and atmospheric chemistry in the US

1. 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

2. Part 1: Effects of climate change on air quality.

3. 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. 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.

4. 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

5. Northeast 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 Probability Days 1988, hottest on record Southeast Los Angeles Temperature (K) Weather plays a large role in ozone air quality. 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

6. 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.

7. Cyclone passage through southern Canada/Great Lakes region strongly affects frequency and duration of U.S. ozone episodes. Correlation between cyclone number each summer in red and greenboxes and number of US ozone episodes Sample storm tracks, summer 1979-1981 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

8. 0.14 yr-1 0.16 yr-1 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 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. NCEP/NCAR obs Mickley et al., 2004; Leibensperger et al., 2008

9. 1980-2006 trends cyclones NE ozone episodes 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. Decline in emissions of ozone precursors from US mobile sources. Parrish 2006. Mickley et al., 2004; Leibensperger et al., 2008

10. 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. If emissions had remained constant, decline in mid-latitude cyclone number over Canada would have meant more persistent stagnation episodes, more ozone. Climate response Trend in pollution days due to decline in cyclone frequency days yr-1 days yr-1 Trend in pollution days due to decline in emissions days yr-1

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

12. 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: 1000+ sites sampled every 1-6 days from 1998 to 2007. Observed correlations provide means to test model simulations. Temperature Tai et al., ms.

13. 2000-2050 decrease in cyclone frequency leads to increased stagnation. 2050s CO tracer Northeast, Jul-Aug 1990s AIR QUALITY 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, well-mixed GHGs GEOS-Chem Chemical transport model chemistry, emissions met fields 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. Mickley et al., 2004

14. 2000-2050 change in max daily 8-hour average JJA ozone 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 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

15. 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

16. 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.

17. 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

18. 1950 1960 1970 1980 1990 2001 Trend in aerosols over United States suggests cleaner skies, possible warming? Calculated trend in surface sulfate concentrations, 1950- 2001. Sequence shows increasing sulfate from 1950-1980, followed by a decline in recent years. Comparison to observed sulfate concentrations shows good agreement. Leibensperger et al., ms

19. Is the climate response to changing aerosols regional or global? Recent US Climate Change report suggests more global than regional response. “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 suggests more regional than global response at least for US. 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

20. 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.

21. 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 oC Annual mean surface temperature change in Control. 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. Mickley et al., ms. 2009

22. No-US-aerosols case Temperature (oC) Control, with US aerosols The regional surface temperature response to aerosol removal persists for many decades in the model. Annual mean temperature trends over Eastern US Bottom line: Efforts to clear the air of anthropogenic aerosol over the US may exacerbate regional warming. Mickley et al., ms

23. Ongoing study: Perform realistic simulation of changing aerosol optical depths over the US, together with sensitivity studies. • 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. GEOS-Chem chemistry transport model aerosol concentrations Calculation of cloud droplet number concentrations aerosol indirect effect GISS GCM IIIclimate model Climate response to aerosol trends over the US

24. 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