An absorption model of gas/particle partitioning of organic compounds in the atmosphere

Abstract

An equation that can successfully parameterize gas-particle partitioning is K p(m 3 μg −1) = ( F/ TSP)/ A, where K p is a partitioning constant, TSP (μg m −3) is the concentration of total suspended particulate material, and F (ng m −3) and A (ng m −3), respectively, are the particulate-associated and gaseous concentrations of the compound of interest. By itself, this equation does not indicate whether the partitioning is primarily adsorptive to surfaces, primarily adsorptive into some phase(s), or some combination thereof. Much past research has indicated that plots of log K p vs log p L o (compound vapor pressure) tend to be highly correlated, with slopes near —1. Such correlation is consistent with (though not proof of) adsorption. The governing equations are now developed for absorptive partitioning into an organic material (om) phase making up part of the TSP. The equations indicate that absorptive partitioning could well be an important sorption mechanism in the urban environment. Plots of log K p vs log p L o are predicted to be linearly correlated when the sorption is absorptive, just as earlier theoretical work predicted such correlatability when the sorption is adsorptive. In the general case, observed K p values may contain contributions from both mechanisms. Regardless of the relative importance of adsorptive vs absorptive partitioning in any given situation, a plot of K p values vs log p L o values will be correlated with a slope of near —1.