Source contributions to the size and composition distribution of atmospheric particles: Southern California in September 1996

An air quality model that follows the evolution of single particles in the atmosphere has been combined with new emissions measurements and then used to predict the size distribution and chemical composition of the airborne f ine particle mixture observed at Long Beach, Fullerton, and Riverside, CA, during September 1996. Model predictions shaw good agreement with ambient m easurements of particle size and chemical composition at all three air moni toring sites. The air quality model is used to separately track individual particles released from different sources as they evolve over time. Four ma jor classes of particles are observed: (1) large mineral dust and road dust particles that accumulate only small amounts of secondary aerosol products ; (2) primary combustion particles (released initially from diesel vehicles , noncatalyst gasoline-powered vehicles, and food processing) that grow by accumulation of secondary reaction products; (3) sea salt particles that ar e almost completely transformed by conversion from NaCl to NaNO3 during tra nsport across the air basin; and (4) sulfate-containing nonsea salt backgro und particles advected into the air basin from upwind over the ocean. The s ulfate-containing nonsea salt background particles have an initial PM2.5 co ncentration of only 8 mu g m(-3), but they accumulate significant secondary aerosol reaction products to produce a largely nitrate-containing aerosol having a PM2.5 concentration of 40 mu g m(-3) by the time that the air mass es studied here reach Riverside, CA.