Atmospherically derived organic surface films along an urban-rural gradient

Atmospherically derived organic films have been found on an impervious surf ace along an urban-rural gradient in the Toronto, Ontario, Canada area. Fil m thickness and concentrations, expressed on an aerial basis, of Sigman-alk anes, Sigma PCB, Sigma PAH, and Sigma OC (organochlorine) pesticides decrea se along this gradient, coincident with lower atmospheric emissions (PCB an d PAH) and less accumulation in thinner rural films (OC pesticides). For PC Bs and some OC pesticides, patterns of chemical abundance also shift, indic ating a "fresh" pattern near emission sources (downtown) versus aged patter ns at rural locations that are indicative of atmospheric transport. Plant-d erived n-alkane concentrations were greater at urban than rural sites, and we hypothesize greater urban plant wax production and erosion due to air po llution. As expected, along the urban-rural gradient the concentration of p article-phase PAH decreased more rapidly than that of gas-phase compounds, but unexpectedly the contribution of alkylated PAH increased from urban to rural locations. Distances over which concentrations decline by 63% vary fr om 50 km for persistent gas-phase compounds (e.g., P1,2CB) to 10-20 km for reactive gas-phase compounds (e.g., naphthalene, fluoranthene) to <5-10 km for particle-phase compounds (e.g., P6-10CB, benzo[b]fluoranthene, benzo[e] pyrene).