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 cont aminants in rain are estimated as the difference between measured filt rate concentrations and those supported by gas exchange into raindrops , using field data resulting from the simultaneous collection of air a nd precipitation. The distributions of organic contaminants in four di screte precipitation events are presented for several polychlorinated biphenyl congeners and for polycyclic aromatic hydrocarbons. These con taminants are up to 80% bound to nonfilterable particulate material in rainwater and <9% ''truly'' dissolved, demonstrating that submicron p articles significantly contribute to the overall removal of organic co ntaminants from the atmosphere by precipitation. In the precipitation events described in this paper, organic carbon-normalized filter-retai ned particle (greater than or similar to 0.5 mu m) partition coefficie nts for PCB congeners and PAHs are larger than those for nonfilter-ret ained particles, suggesting that organic contaminants adsorb different ly to large and small (i.e., nonfilter-retained) particles. Partition coefficients to both large and submicron particles are weakly correlat ed with hydrophobicity, indicating that the speciation of PCBs and PAH s in rain is not predictable from these compounds' octanol-water parti tion coefficients. We attribute the poor correlation to slow desorptio n kinetics and to nonexchangeable fractions of contaminants within atm ospheric particles.