Precipitation scavenging of polychlorinated biphenyls and polycyclic aromatic hydrocarbons along an urban to over-water transect.

Concentrations of semi-volatile organic compounds in dissolved and particulate forms in precipitation are related to those measured concurrently in air measured at multiple locations along the tracks of several storms. In 14 paired (air and precipitation) samples collected at urban, over-water, and downwind stations around southern Lake Michigan, compound-specific total (gas + particle) precipitation scavenging ratios range from 180 to 8.2 x 10(7) (fluorene and benzo[a]pyrene, respectively) for PAHs and from 0.5 to 1.1 x 10(7) for PCBs (PCB26 and PCB194, respectively). Particle scavenging, rather than gas scavenging, is the dominant removal mechanism for both PAHs and PCBs in all samples collected along the urban to over-watertransect. Variations in measured total scavenging ratios within the 14 samples are large (from 69 to 1.00 x 10(6) for sigma-PAHs and from 290 to 88,000 for total sigma-PCBs), with larger variation between samples of differing storms collected at a single location than among samples of the same storm collected along the urban to rural transect. This minimal variance of scavenging ratio during the transport of a storm across multiple locations holds in two of three storms measured at multiple locations along the urban to over-water to rural storm track. This suggests that scavenging mechanisms are largely variable between storms and that the relative importance of each mechanism can occasionally also vary greatly during the progression of the storm from the urban to downwind locations. Furthermore, precipitation scavenging appears to be more variable between storms and generally less variable during progression of a storm down transect, indicating that meteorologic and precipitation characteristics, generally referred to here as storm type, control precipitation scavenging.