The importance of snow scavenging of polychlorinated biphenyl and polycyclic aromatic hydrocarbon vapors

Recently, experimental data on the scavenging of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from the atmosphere by snow were interpreted assuming that the distribution of chemical between particles and dissolved phase measured in the meltwater reflects the state of the chemical during the scavenging process (Franz, T. P.; Eisenreich, S. J. Environ. Sci Technol. 1998, 32, 1771-1778). A consequence of this assumption is that vapor scavenging is found to be unimportant relative to particle scavenging. An alternative interpretation is presented that during melting repartitioning occurs from the dissolved phase to the particle-sorbed phase. Further, it is argued that a constant particle scavenging ratio may apply to all chemicals of the same class in the same precipitation event, and its value can be estimated from the scavenging characteristics of predominantly particle-sorbed, high molecular mass chemicals. This analysis suggests that for more volatile PCBs and PAHs vapor scavenging is an important, if not the dominating, snow scavenging process. Gas scavenging ratios obtained with this method are, as expected, negatively correlated with the vapor pressure of a substance, indicating that adsorption to the air-ice interface is the process responsible for vapor scavenging.