Seasonal variation of gaseous mercury exchange rate between air and water surface over Baihua reservoir, Guizhou, China

Mercury (Hg) exchange between air and water surface is recognized to be one of the most important processes governing the biogeochemical cycling of Hg in aquatic system. Baihua reservoir is seriously contaminated with Hg due to anthropogenic activities. As a part of Hg mass balance study of Baihua reservoir, we measured Hg exchange fluxes between air and water surface at different seasons by using a quartz flux chamber method coupled with high time resolved atmospheric Hg analysis technique. Hg evasion from water surface is the preominant process at all seasons, and the overall total gaseous Hg emission flux in warm season is significantly larger than that in cold season. Hg flux significantly correlated with the intensity of solar radiation at all sampling campaigns, but it only correlated with wind speed at cold season when the solar radiation was weak. Sunlight induced reduction of divalent Hg to Hg0 is the predominant process of dissolved gaseous Hg formation in surface water of Baihua reservoir, and the process is very fast. The annual Hg emission from Baihua reservoir to the air is 752 g, constituting of 3% of total Hg in water body of the reservoir. Our study implies that Hg emission process plays an important role in Hg cycling in Baihua reservoir.