Ammonia (NH3) is the dominant form of reduced nitrogen in the atmosphere and contributes to both the formation of particulate matter (PM) and the deposition of reactive nitrogen to the environment. PM degrades air quality and is one of the most uncertain contributors to global climate forcing. Ammonium aerosol is formed when ammonia reacts with sulfuric nitric acids to form ammonium salts. While ammonia neutralization is not required for sulfate formation, it is critical to the formation of nitrate aerosol. Inorganic PM formation is therefore a complex combination of emissions and thermodynamic formation processes. These processes are not well understood and while previous studies have suggested that ammonia emission controls may be a cost-effective means of reducing PM loading in the U.S., the paucity of measurements of ambient ammonia has limited our examination of this system. In this project we will use an unprecedented suite of in situ measurements of NH3 (as well as inorganic aerosol and related precursors) from ground sites and aircraft during the CalNex field campaign, as well as satellite retrievals of ammonia from the IASI instrument to study PM formation in the Western United States. This project will synthesize these measurements with the GEOS-Chem model to address the following objectives: A. To examine our understanding of ammonia emission sources in California B. To investigate the role of ammonia in PM loading in the Western US. These objectives will be explored through several specific tasks: 1. Use a constrained thermodynamic model to examine whether thermodynamic equilibrium applies to both surface and aircraft observations during CalNex and to investigate the relative role of different ions in inorganic aerosol formation. 2. Investigate whether the GEOS-Chem model reproduces with fidelity the spatial and temporal distribution of observed ammonia. From this, investigate any discrepancies considering both inaccuracies in emission inventories and deficiencies in the description of specific processes. 3. Use the IASI satellite observations to place the in situ CalNex measurements in the larger context of ammonia loading in the Western U.S. through 2010. 4. Use the GEOS-Chem model to assess the contribution of ammonia to PM loading in California, and the Western U.S.