Adsorptive and absorptive contributions to the gas-particle partitioning of polycyclic aromatic hydrocarbons: State of knowledge and recommended parametrization for modeling

Four contrasting descriptions of the gas-particle partitioning of SOCs are currently used: the Junge-Pankow adsorption model, the empirical Finizio organic matter (OM) absorption relationship, the Harner-Bidleman OM absorption model, and a dual black carbon (BC) adsorption and OM absorption model. Use of these four descriptions in a box model resulted in very different global fates, particularly for PAHs such as chrysene and benzo[a]pyrene. By reviewing published gas-particle distributions of PAHs, we found evidence for both absorptive and adsorptive contributions. Based on results from laboratory and controlled field studies we suggest that on average, octanol-air partitioning (K-oa) is a good approximation for the OM absorption of PAHs. However, higher concentrations in particles than could be explained by OM absorption were found in selected gas-particle partitioning field studies, which were corrected for gaseous adsorption to the filter. We argue that adsorption onto BC is responsible for most of the additional sorption. Apparent adsorption coefficients to BC, KBC-air, were derived from field studies and showed good agreement with those predicted by adsorption onto diesel soot. For atmospheric long-range transport models we suggest the use of a dual OM absorption and BC adsorption model, with BC properties being approximated by diesel soot: K-p = 10(-12) (f(om) 1/rho(oct) K-oa + f(BC) 1/rho(BC) Ksoot-air a(atm-BC)/a(soot)). We hypothesize that kinetic constraints related to shell-like particle structures might lead to deviations from sorption equilibrium and higher particle-borne fractions of PAHs in particular at remote sites.