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Concentrations of fine particulate matter (PM2.5) presently pose a serious health hazard to tens of millions of people in the U.S. While a significant portion of this PM2.5 is governed by the gas-phase precursors NH3 and NOx, current efforts to ascertain and control the influence of NH3 and NOx on air quality are hindered by large uncertainties in emissions inventories and compounded by complexities of the chemical and physical processes by which these species ultimately form PM2.5. The overarching objective of this proposal is to utilize Earth science products for improved air quality management at the U.S. Environmental Protection Agency (EPA). Key personnel at the EPA's Office of Air Quality Planning and Standards and the Atmospheric Modeling Division will support the proposed integration of satellite observations, surface measurements and sophisticated modeling tools into two main existing decision making activities. First, recently develop ed adjoints of both regional (CMAQ) and global (GEOS-Chem) air quality models will be used for inverse modeling of NH3 and NOx sources in North America to support upcoming updates to the EPA's National Emissions Inventory utilizing a novel combination of observations of NO2 from remote sensing instruments (OMI, SCIAMACHY) with speciated PM2.5 measurements from surface air quality monitoring networks. Second, in support of the Regulatory Impact Analysis (RIA) for the EPA's 2012 review of the PM2.5 National Ambient Air Quality Standards (NAAQS), adjoint models will be applied to estimate how regulating specific sources of PM2.5 precursors can most effectively reduce NAAQS nonattainment, considering both present day and mid-century climate. These efforts address multiple aspects of the NRA's air quality priority topic 1.3.2.b, North American emissions inventories, as well the air quality and public health applications' climate change topics, 1.3.2.a and 1.3.6.b, and will ultimately advance decision making processes for ongoing air quality management.