The seasonal variation of selected natural and anthropogenic halocarbons in the arctic troposphere

Weekly average concentrations of four natural halocarbons (bromoform, dibromochloromethane, bromochloromethane, chloroiodomethane) and three anthropogenic halocarbons (trichloroethylene, tetrachloroethylene, and trans-1,2 dichloroethylene) were measured at Alert, Canada, from January 1992 to June 1994. Air samples were collected on Tenax TA adsorbent, and were analyzed with capillary GC/MS (SIM). Most of the measured halocarbons showed significant seasonal variation. The winter/summer ratio of mixing ratios was as much as 60 for trichloroethylene and 3 for bromoform. The winter/summer differences are closely related to chemical reactivity in the atmosphere, and higher reactive loss in summer during long-range transport from mid-latitudinal sources.