The calculated effects of non-exchangeable material on the gas-particle distributions of organic compounds

The behavior of a chemical species in the atmosphere depends on how it partitions between the gas and aerosol particulate phases. Available evidence indicates that this partitioning can be described by a compound- and temperature-dependent constant K = A(TSP)/F, where A and F are the atmospheric concentrations measured in the gas and particulate phases, respectively (ng m⁻³). TSP is concentration (in μg m⁻³) of total suspended particulate matter. K values within some classes of compounds are known to correlate with the corresponding log p_o values where p_o is the vapor pressure of the compound. (For a solid compound, the vapor pressure of the sub-cooled liquid should be used.) Such correlations can break down when portions of the compounds are bound inside the particles. This paper develops equations for predicting the magnitude of this effect. The specific case of polycyclic aromatic hydrocarbons is used as an example. Values for log A(TSP)/F are predicted to be significantly undervalued in a variety of circumstances compared with what would be expected at full equilibrium, even when the fraction of non-exchangeable material is only a few per cent. Similarly, φ values = F/(A + F)] can be significantly overvalued. A knowledge of the magnitude of this effect will be necessary for accurate predictive modeling of the fates of chemical species in the atmosphere.