Further discussion of the octanol/air partition coefficient Koa as a correlating parameter for gas/particle partitioning coefficients

The pure-compound liquid vapor pressure (poL) has been used extensively as a log–log correlating parameter for gas/particle partitioning constants. When the partitioning to the aerosol particle phase involves partitioning into an organic material (om) phase, the success of poL in this context relies on compound-to-compound constancy of the molecular activity coefficient ζom in the om phase. While ζom for a particular type of aerosol may indeed tend to remain constant within a given compound class, significant class-to-class variations in ζom are certain. This paper discusses the theory underlying the advantages of using the octanol/air partition coefficient Koa as a correlating parameter for gas/particle partitioning constants. In particular, it is far more likely that different compounds will exhibit similar values of the ratio than it is that they will exhibit similar values of ζom. The conclusion that the octanol-based partition coefficient Koa has significant advantages over the pure-compound partition coefficient poL when parameterizing gas/particle partitioning has a direct analog in the selection of the octanol/water partition coefficient Kow as a correlating parameter for soil/water partition coefficients like the organic carbon/water partition coefficient Koc. The nature of this analogy is discussed in detail. The acknowledged success of log Kow in this regard presages wide application of log Koa as a correlating parameter for gas/particle partition coefficients.