Studies of chemistry-climate interactions at Harvard 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 Smog over Pittsburgh, ranked #1 city for particulate pollution in 2008 by ALA Effects of climate change on air quality Effects of aerosols on regional climate Funding for this work: NASA, EPA, EPRI

I. Effects of climate change on air quality Weather plays a large role in ozone air quality. The total derivative d[O 3 ]/dT is the sum of partial derivatives (  O 3 /  x i )(  x i /  T). x = ensemble of ozone forcing variables that are temperature-related. 1988, hottest on record Days Number of summer days with ozone exceedances, mean over sites in Northeast Lin et al., 2001 Probability of ozone exceedance vs. daily max. temperature Curves include effects of Biogenic emissions Stagnation Clear skies Northeast Southeast Los Angeles Temperature (K) Probability

Cyclones crossing southern Canada affect ozone air quality in Eastern US. Stalled high pressure system associated with: increased biogenic emissions clear skies weak winds high temperatures. cold front EPA ozone levels 3 days later Cold front pushes smog poleward and aloft on a warm conveyor belt. cold front Hazardous levels of ozone L L

Is there a connection between summertime cyclone frequency and the number of ozone episodes? Correlation of JJA ozone exceedances with storm tracks weak correlation Strong anti-correlation Sample summertime storm tracks, NCEP/NCAR reanalysis 6 Frequent cyclones plus associated cold fronts mean fewer ozone episodes. Fewer cyclones mean more persistent stagnation and more intense pollution.

observed trend in cyclone frequency matches that in climate model with increasing greenhouse gases. 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., trend in JJA cyclones in S. Canada 0.14 a a -1

Trend in emissions and trend in cyclones have competing effects on surface ozone. Mickley et al., 2004; Leibensperger et al., 2008 Cyclones: less frequent cyclones means longer pollution episodes Emissions: reduced emissions means fewer episodes. NE ozone episodes cyclones trends Decline in emissions of ozone precursors from mobile sources, Parrish 2006.

Ozone pollution days in the Northeast US days yr -1 Trend in pollution days due to decline in emissions If cyclone frequency had remained constant, we calculate zero episodes over Northeast. Trend in pollution days due to decline in cyclone frequency Decline in mid-latitude cyclone number over mid-latitudes leads to more persistent stagnation episodes, more ozone.

Present-day radiative forcing due to aerosols over the eastern US is comparable in magnitude, but opposite in sign, to global forcing due to CO 2. Globally averaged radiative forcing due to CO 2 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 II. Effects of aerosols on regional climate

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. Is the climate response to changing aerosols collocated with regions of radiative forcing? 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. 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. GISS GCM

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

The regional surface temperature response to aerosol removal appears to persist for many decades in the model. Annual temperature difference between the two cases stays about constant, but in fact the annual mean hides seasonal differences. Temperature ( o C) No-US-aerosols case Control, with US aerosols Annual mean temperature trends over Eastern US Mickley et al., ms

The regional surface temperature response to aerosol removal varies according to season. Winter: Temperature response is initially very strong, then dies off. Spring and Fall: Temperature response kicks in around Summer: Moderate temperature response. Mickley et al., ms Temperature ( o C) spring winter fall summer Additional warming due to aerosol removal over Eastern US 9-year running means. Stars indicate a significance difference between no-US-aerosols case and control :

Working hypothesis for time-dependent responses, varying by season. 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 no- US-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 Temperature ( o C) spring winter fall summer Additional warming due to aerosol removal over Eastern US

GEOS-Chem chemistry transport model GISS GCM III climate model Calculation of cloud droplet number concentrations aerosol concentrations aerosol indirect effect We use historical/projected emissions of SO 2, NO x, BC, and OC to quantify the climatic role of US aerosols in the past and future Control simulation (EDGAR/Tami Bond historical emissions and A1B; includes rising U.S. aerosol sources until 1980 and subsequent decline) Sensitivity simulations: No US aerosols. Quantifies the past effect of U.S. anthropogenic sources on regional climate Constant US emissions Quantifies the warming effect from the projected decrease in U.S. emissions. Next steps: Perform more realistic simulation of changing aerosol optical depths over the US, together with sensitivity studies. Climate response to aerosol trends over the US

Calculated trend in surface sulfate concentrations, Comparison to observed sulfate concentrations shows good agreement. Sequence shows increasing sulfate from , followed by a decline in recent years. We will test the climate effects of this and other aerosol trends in the GISS climate model