SECONDARY ORGANIC AEROSOL FORMATION AND TRANSPORT .2. PREDICTING THE AMBIENT SECONDARY ORGANIC AEROSOL-SIZE DISTRIBUTION

The size distribution of atmospheric secondary organic aerosol is simu lated by a Lagrangian trajectory model that includes descriptions of g as-phase chemistry, inorganic and organic aerosol thermodynamics, cond ensation/evaporation of aerosol species, dry deposition and emission o f primary gaseous and particulate pollutants. The model is applied to simulate the dynamics of aerosol size and composition along trajectori es on 27-28 August 1987 during the Southern California Air Quality Stu dy (SCAQS). The secondary organic aerosol material is predicted to con dense almost exclusively on the submicron aerosol in agreement with th e available measurements, and its size distribution for Claremont, CA, is predicted to be unimodal with a mass mean diameter of roughly 0.2 mum. The distributions of the various secondary organic aerosol specie s are predicted to be essentially the same. The secondary organic aero sol (SOA) size distribution is found to depend crucially on the mass a nd size distribution of primary aerosol on which the secondary species condense and on the surface accommodation coefficient of the condensa ble species. The SOA size distribution is predicted not to be signific antly affected by diffusional dry deposition, sources and sinks of amm onia, emissions of VOC, and secondary aerosol yields from precursor hy drocarbons. A bimodal secondary organic aerosol size distribution is p redicted only if the submicron primary dust particles reside mainly in the 0.5-1.0 mum diameter size range, or if the condensable species ha ve a strong preference (an accommodation coefficient difference of two orders of magnitude) for the 0.5- 1.0 mum diameter particles. The sec ondary organic aerosol distribution in Claremont is predicted to shift slightly towards the larger aerosol particles during the nighttime ho urs with its mass mean diameter peaking around midnight at 0.21 mum an d having its minimum in early afternoon at 0. 18 mum. In coastal locat ions of the Los Angeles basin, secondary organic material exists in re latively smaller particles (mass mean diameter 0.16 mum) but in far in land locations it condenses on the available larger particles (mass me an diameter 0.23 mum).