ALBERT

Description: [1]  Airborne observations from the CalNex campaign in May and June 2010 are used to investigate the role of ammonia (NH 3 ) in fine particulate matter (PM 2.5 ) formation and surface air quality in California and test the key processes relevant to inorganic aerosol formation in the GEOS-Chem model. Concentrations of ammonia throughout California, sulfur dioxide (SO 2 ) in the Central Valley, and ammonium nitrate in the Los Angeles (LA) area are underestimated several-fold in the model. We find that model concentrations are relatively insensitive to uncertainties in gas-particle partitioning and deposition processes in the region. Conversely, increases to anthropogenic livestock ammonia emissions (by a factor of 5) and anthropogenic sulfur dioxide emissions in the Central Valley (by a factor of 3 - 10) and a reduction of anthropogenic NO x emissions (by 30 %), substantially reduces the bias in the simulation of gases (SO 2 , NH 3 , HNO 3 ) throughout California and PM 2.5 near LA, although the exact magnitudes of emissions in the region remain uncertain. Using these modified emissions, we investigate year-round PM 2.5 air quality in California. The model reproduces the wintertime maximum in surface ammonium nitrate concentrations in the Central Valley (regional mean concentrations are 3 times higher in December than in June), associated with lower planetary boundary layer heights and colder temperatures, and the wintertime minimum in the LA region (regional mean concentrations are 2 times higher in June than December) associated with ammonia-limitation. Year-round, we attribute at least 50 % of the inorganic PM 2.5 mass simulated throughout California to anthropogenic ammonia emissions.