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

The size distribution of atmospheric secondary organic aerosol is simulated by a Lagrangian trajectory model that includes descriptions of gas-phase chemistry, inorganic and organic aerosol thermodynamics, condensation/evaporation of aerosol species, dry deposition and emission of primary gaseous and particulate pollutants. The model is applied to simulate the dynamics of aerosol size and composition along trajectories on 27-28 August 1987 during the Southern California Air Quality Study (SCAQS). The secondary organic aerosol material is predicted to condense almost exclusively on the submicron aerosol in agreement with the 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 species are predicted to be essentially the same. The secondary organic aerosol (SOA) size distribution is found to depend crucially on the mass and size distribution of primary aerosol on which the secondary species condense and on the surface accommodation coefficient of the condensable species. The SOA size distribution is predicted not to be significantly affected by diffusional dry deposition, sources and sinks of ammonia, emissions of VOC, and secondary aerosol yields from precursor hydrocarbons. A bimodal secondary organic aerosol size distribution is predicted only if the submicron primary dust particles reside mainly in the 0.5-1.0 mum diameter size range, or if the condensable species have a strong preference (an accommodation coefficient difference of two orders of magnitude) for the 0.5- 1.0 mum diameter particles. The secondary organic aerosol distribution in Claremont is predicted to shift slightly towards the larger aerosol particles during the nighttime hours with its mass mean diameter peaking around midnight at 0.21 mum and having its minimum in early afternoon at 0. 18 mum. In coastal locations of the Los Angeles basin, secondary organic material exists in relatively smaller particles (mass mean diameter 0.16 mum) but in far inland locations it condenses on the available larger particles (mass mean diameter 0.23 mum).