VERTICAL PROFILES OF BROMOFORM IN SNOW, SEA-ICE, AND SEAWATER IN THE CANADIAN ARCTIC

Bromoform (CHBr3) was measured in vertical profiles from the snow surf ace through the snowpack, sea ice, and water column to the seafloor at Resolute Bay, Canada, in the spring of 1992. Elevated concentrations of bromoform were observed in both the ice (32-266 ng L-1 by liquid wa ter volume) and seawater (similar to 20 ng L-1) at the ice-water inter face, associated with bromoform emission from ice microalgae. A surpri sing finding was a second horizon of high bromoform concentrations (33 6-367 ng L-1) in sea ice at the snow-ice interface. Chlorophyll and sa linity were also elevated in this upper ice layer, although chlorophyl l was much lower than in the basal ice microalgal layer. We speculate that this upper bromoform-enriched layer may have originated from scav enging of the surface water layer by frazil ice during initial ice for mation in the preceding autumn. Equally unexpected was the occurrence of yet higher bromoform concentrations in snowpack immediately overlyi ng the sea ice (492-1260 ng L-1), declining in concentration (by about a factor of 2 or more) toward the snow surface. Snow of very recent o rigin, however, contained as little as 2 orders of magnitude less brom oform than the older snowpack. Possible origins for elevated bromoform in the snowpack include diffusion out of the bromoform-enriched upper ice layer and gradual concentration of bromoform out of the atmospher e by adsorption on to ice crystals. These are considered in turn. In o ne scenario, photolysis of bromoform from snow is considered, which mi ght help account for atmospheric bromine-ozone chemistry. The possible contributions from snow, sea ice, and seawater to atmospheric bromofo rm levels during both the winter and spring are also considered, and i t is concluded that surface seawater presents the most significant res ervoir for atmospheric bromoform.