Modeling Studies of the Contribution of Regional Transport to the Formation of Secondary Aerosol over the Western United States

The formation of ammonium nitrate particles from nitrogen oxides, volatile organic compounds and ammonia produces a large fraction of secondary aerosol particles over the Western United States. The three largest source regions in California are Los Angeles, the San Francisco Bay Area and the San Diego Region. Simulations were made with a 3-d air quality model, the Comprehensive Air Quality Model with eXtensions (CAMx), to estimate the effect of emissions from these urban areas on secondary aerosol concentrations over the Western United States. The emissions were found to increase aerosol production over much of California but the effect was smaller for cities such as Las Vegas. Preliminary simulations showed that secondary aerosol concentrations reached peak values, up to 11.5 mg m^-3, during the early morning hours in Las Vegas. The mass fractions of nitrate, ammonium, sulfate and organic aerosol in the secondary aerosol were calculated to be 71.6, 22.4, 5.0 and 1.1%, respectively. Eliminating the VOC and NOx emissions from Los Angeles resulted in a maximum reduction of total secondary aerosol near 25% in Las Vegas with an average reduction of 8.9%. Eliminating emissions from the San Francisco Bay Area resulted in a maximum reduction near 10% with an average reduction of 3.0% and eliminating emissions from the San Diego Region resulted in a near 10% reduction with an average reduction of 3.3%. Improved simulations are in progress with improved emissions inventories and models, including the MM5-Chem model.