MERCURY DEPOSITION, PHOTOREDUCTION, AND GASEOUS ELEMENTAL MERCURY EMISSIONS FROM THE SEASONAL SNOW SURFACE AT CANAAN VALLEY, WV

Canaan Valley, WV is an Atmospheric Mercury Network (AMNet) Hg speciation, and continuous flux measurement site, operated by NOAA. The site is located within an intact 926 acre (375 hectare) research watershed of primarily deciduous hardwood forest. On average, Canaan Valley receives 3.7m of annual snowfall, with the seasonal snow pack existing 135 days, at an average depth of 28cm. Permanent mercury measurements consist of a Tekran speciation system, a continuous 7m gradient modified-Bowen ratio vertical flux system for gaseous elemental mercury, weekly total Hg in precipitation, and weekly surrogate surfaces to estimate mercury dry deposition.

Due to very low wintertime leaf area, oxidized mercury is primary wet and dry deposited to the snow surface. This snow surface, in turn, serves to isolate the deposited mercury from the litter and soil organics, and allows the majority of the deposited mercury to be photoreduced and emitted back to the atmosphere as gaseous elemental mercury.

During the snow covered period (averaging Nov 10^th to March 25^th) average daily emission fluxes peak at 1240 local solar time, with the peak daily gaseous elemental mercury in the near surface air, at 1320 local time, showing enhancements of 0.08 ng m-3 above the nighttime averages. While these are averages over the entire snow-covered period, snow surface emissions are strongly enhanced by warmer, sunny days when the snow pack experiences partial melting. From our ancillary measurements of soil, litter-fall, watershed stream output, surrogate deposition surfaces, and precipitation sampling, we also conclude that the net mercury deposition is at a yearly minimum during the annual snow covered period. In effect, at this location, the existence of the seasonal snow pack reduces the annual net deposition by roughly 15-25%.