An absorption model of the gas/aerosol partitioning involved in the formation of secondary organic aerosol

Abstract

A partitioning model is developed to allow the modeling of the dynamics of secondary organic aerosol (SOA) formation. The gas/aerosol partitioning is assumed to be governed by equilibrium partitioning into an absorptive, well-mixed liquid (or at least amorphous) organic matter (om) phase. The model is represented using a set of coupled linear equations. It may be especially applicable when photochemical smog is being formed in the summer. The model permits (indeed, it requires) partitioning of a given compound i to occur even when i is present at a level below its saturation vapor pressure. During early periods of SOA formation, to determine the partitioning for each compound of interest, the model must be solved iteratively for each time and location of interest. Iteration is required because the partitioning is assumed to be governed by mole fraction concentrations in the om phase, and because prior to solving the problem, one does not know the total number of mols of condensed compounds in the om phase. During later stages of SOA formation, if the amount and general composition of the SOA begin to become constant, the partitioning coefficient of each of the compounds will also stabilize, and an iterative solution will be less needed. © 2007 Elsevier Ltd. All rights reserved.