Influence of submicron particles on hydrophobic organic contaminants in precipitation .1. Concentrations and distributions of polycyclic aromatic hydrocarbons and polychlorinated biphenyls in rainwater

Concentrations of nonfilter-retained, particle-associated organic contaminants in rain are estimated as the difference between measured filtrate concentrations and those supported by gas exchange into raindrops, using field data resulting from the simultaneous collection of air and precipitation. The distributions of organic contaminants in four discrete precipitation events are presented for several polychlorinated biphenyl congeners and for polycyclic aromatic hydrocarbons. These contaminants are up to 80% bound to nonfilterable particulate material in rainwater and <9% ''truly'' dissolved, demonstrating that submicron particles significantly contribute to the overall removal of organic contaminants from the atmosphere by precipitation. In the precipitation events described in this paper, organic carbon-normalized filter-retained particle (greater than or similar to 0.5 mu m) partition coefficients for PCB congeners and PAHs are larger than those for nonfilter-retained particles, suggesting that organic contaminants adsorb differently to large and small (i.e., nonfilter-retained) particles. Partition coefficients to both large and submicron particles are weakly correlated with hydrophobicity, indicating that the speciation of PCBs and PAHs in rain is not predictable from these compounds' octanol-water partition coefficients. We attribute the poor correlation to slow desorption kinetics and to nonexchangeable fractions of contaminants within atmospheric particles.