Modelling urban and regional aerosols—II. Application to Californiaʹs South Coast Air Basin

A three-dimensional gas/aerosol atmospheric model is presented that predicts the size-resolved concentrations of all major primary and secondary components of atmospheric particulate matter (PM), including sulfate, nitrate, ammonium, chloride, sodium, elemental carbon, organic carbon, water, and crustal material. Aerosol size resolution is based on a sectional representation, typically extending from 0.01 to 10 μ for aerosols and from 0.01 to 30 μ when fog is present. The model is based on an internally mixed aerosol, wherein all particles in a specific size range are assumed to have the same chemical composition. Gas/aerosol equilibrium is computed based on the SEQUILIB algorithm of Pilinis and Seinfeld. An empirical fog model is included that approximates the effect of fogs on gas-phase photolysis rates, on aqueous-phase chemical reactions of sulfate and nitrate, and on the growth and shrinkage of the aerosol/fog droplet size distribution. The model is applied to simulate atmospheric conditions in the South Coast Air Basin of California during the 24–25 June 1987 episode of the Southern California Air Quality Study (SCAQS). The sensitivity of predicted aerosol levels to changes in source emissions is investigated.