Interactions between Climate and Air Quality A.M. Fiore, H. Levy II*, Y. Ming, Y. Fang, L.W. Horowitz National Oceanic Atmospheric Administration Geophysical Fluid Dynamics Laboratory (NOAA GFDL), NJ, USA *Presenting author We present two examples of air pollutant contributions to climate forcing. First, oxidation of the potent greenhouse gas methane produces tropospheric ozone, another greenhouse gas and the primary constituent of ground-level smog. Methane emission controls are thus a “ win-win” strategy for jointly addressing air quality and climate goals, particularly given the availability of low-cost emission control options. Second is the “ win-lose” case of aerosol sulfate, where decreases improve air quality but lead to additional warming due to decreased scattering of solar radiation. We highlight the potential for aerosols to change the hydrologic cycle and key aspects of how climate change may affect air quality, underscoring a need for evaluating chemistry-climate models with observed relationships between meteorology and air pollutants to build confidence in future projections. 1. Introduction Ground-level smog, detrimental to human health and vegetation, is pervasive in populated world regions. In the United States, over 150 million people live in counties exceeding air quality standards for ozone (O 3 ) or particulate matter (aero- sols), the two major smog constituents (U.S. EPA, 2008). These air pollutants al- so influence climate, with tropospheric O 3 the 3 rd most important greenhouse gas after carbon dioxide (CO 2 ) and methane (CH 4 ), and aerosols exerting a net cooling influence (Forster et al., 2007). The major precursors to O 3 that fuel rapid photochemical build-up of O 3 during regional air pollution episodes are non-methane volatile organic compounds (NMVOC) and nitrogen oxides (NO x ), whereas the global burden of tropospheric O 3 is most sensitive to NO x and CH 4 (e.g., Fiore et al., 2002). As CH 4 and O 3 to- gether are estimated to have contributed nearly half as much radiative forcing from 1750 to 2005 as CO 2 (Forster et al., 2007), controls on CH 4 emissions could help to slow greenhouse warming (Hansen et al., 2000). Since CH 4 oxidation (in the presence of NO x ) contributes to the formation of tropospheric O 3 (Crutzen, 1973), including in surface air (Fiore et al., 2002), such controls would also de-