Influence of Submicron Particles on Hydrophobic Organic Contaminants in Precipitation. 2. Scavenging of Polycyclic Aromatic Hydrocarbons by Rain

Abstract

In the second of two papers documenting the importance of submicron particles in controlling the concentrations and distributions of organic contaminants in precipitation, we examine the percipitation scavenging of polycyclic aromatic hydrocarbons (PAHs) as a function of the size distribution of atmospheric particles. We argue that particle scavenging rather than gas scavenging is the dominant removal mechanism for PAHs from the atmosphere by precipitation and that previously reported PAH gas scavenging ratios were overestimated due to the presence of submicron particles in the operationally defined dissolved phase of precipitation samples. In a series of five storms in the Chesapeake Bay region during summer 1992, dimensionless PAH scavenging ratios for submicron particles range from 10 4 to 10 5 , while those for larger particles (>0.5 μm) range from 10 5 to 10 6 . In contrast, gas scavenging ratios, as predicted from temperature-corrected Henry's law constants, range from 10 to 500. The particle scavenging ratios of PAHs associated with large and small particles are similar to those measured for a variety of inorganic species and with those predicted for incloud scavenging of trace atmospheric species. Relative standard deviations of PAH particle scavenging ratios measured during the five storms are nearly 100%, suggesting that precipitation scavenging mechanisms are highly variable among the storms. This variability should be considered when using gas and particle scavenging ratios to model wet depositional fluxes of organic contaminants from the atmosphere.