Interactions between climate and atmospheric chemistry in the US 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 Smog over Pittsburgh, ranked #1 city for particulate pollution in 2008 by American Lung Association Effects of climate change on air quality Effects of short-lived species on climate

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 ppm standard Number of people living in areas that exceed the national ambient air quality standards (NAAQS) in EPA’s Technical Support Document for the proposed finding on CO 2 as a pollutant. Cites the threat of climate change to air quality. Public hearings last week on EPA proposed finding in Detroit + NYC.

O3O3 O2O2 h O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CO, VOC NO 2 h STRATOSPHERE TROPOSPHERE 8-18 km Nitrogen oxide radicals; NO x = NO + NO 2 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 Stagnation promotes ozone production Chemistry of tropospheric ozone: oxidation of CO, VOCs, and methane in the presence of NOx

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. 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 Days Number of summer days with ozone exceedances, mean over sites in Northeast 1988, hottest on record

Low pressure systems (aka cyclones) cross southern Canada and sweep out ozone pollution from 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

Sample storm tracks, summer Correlation between cyclone number each summer in red and green boxes and number of US ozone episodes Strong anti-correlation of cyclone number and number of ozone episodes in eastern US: Leibensperger et al., 2008 Cyclone passage through southern Canada/Great Lakes region strongly affects frequency and duration of U.S. ozone episodes. 27 year record Fewer cyclones per summer in green box leads to more ozone episodes in US.

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? Emissions of ozone precursors have declined during this period. Mickley et al., 2004; Leibensperger et al., trend in JJA cyclones in S. Canada 0.14 yr yr -1 NCEP/NCAR obs

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 + cold fronts mean more persistent pollution episodes Emissions: reduced emissions means fewer episodes. NE ozone episodes cyclones trends Decline in emissions of ozone precursors from US mobile sources. Parrish 2006.

Ozone pollution days in the Northeast US days yr -1 Trend in pollution days due to decline in emissions We find that if cyclone frequency had remained constant, we would have had zero episodes over Northeast. Trend in pollution days due to decline in cyclone frequency If emissions had remained constant, decline in mid-latitude cyclone number over Canada would have meant more persistent stagnation episodes, more ozone. Climate response

Particulate matter (PM, aerosols) sources and processes SO 2 H 2 SO 4 NH 3 VOCs NO x RCO… HNO 3 nucleation coagulation condensation carbonaceous combustion particles soil dust sea salt oxidation cycling ultra-fine (<0.01  m) fine (  m) cloud (1-100  m) combustion biosphere volcanoes agriculture biosphere coarse (1-10  m) scavenging precursor gases wildfires

Observed correlations of total PM2.5 with meteorology Precipitation Stagnation Temperature Positive correlation with temperature occurs due to: Increased oxidation of SO 2 Greater biogenic emissions Results from EPA AQS database: sites sampled every 1-6 days from 1998 to Observed correlations provide means to test model simulations. Temperature Precipitation Stagnation Tai et al., ms.

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

change in max daily 8-hour average JJA ozone Climate penalty for air quality: Harvard model shows 2-12 ppb increase in surface ozone in East change in max daily 8-hour average JJA ozone Multi-model comparison Weaver et al., 2009 Most models agree that surface ozone will increase over the Northeast. Disagreement occurs elsewhere due to differences in chemistry and cloud cover change. How will US surface ozone change in a changing climate? ppb

Calculated response in surface PM to +2.5 o C temperature change applied uniformly for July. Dawson et al., 2007 Uncertainty in response of surface PM to changing meteorology is large. We can use present-day observations to test models. (μg/m 3 ) Observed correlation between surface temperature and surface PM concentrations. Tai et al, ms. in progress Positive correlation with T due to: Increased oxidation of SO 2 Greater biogenic emissions

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 CO 2. Globally averaged radiative forcing due to CO 2 is +1.7 Wm -2. warming 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 IPCC, 2007; Liao et al., 2004

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

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. Is the climate response to changing aerosols regional or global? 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. Caveats: No transport, only direct effect considered in this pilot study. GISS GCM

Removal of anthropogenic aerosols over US increases annual mean surface temperatures by 0.5 o C. Summertime temperatures increase as much as 1.5 o C. 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/ cooling due to zeroing of US aerosols Mickley et al., ms oCoC oCoC

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

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. Ongoing study: Perform realistic simulation of changing aerosol optical depths over the US, together with sensitivity studies. Climate response to aerosol trends over the US

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 NH 3, 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