WATER-AIR AND SOIL-AIR EXCHANGE-RATE OF TOTAL GASEOUS MERCURY MEASURED AT BACKGROUND SITES

In order to evaluate and understand the processes of water-air and soi l-air exchanges involved at background sites, an intensive field measu rement campaign has been achieved during the summer of 1995 using high -time resolution techniques (10 min) at two sites (land and water) in southern Quebec (Canada). Mercury flux was measured using a dynamic fl ux chamber technique coupled with an automatic mercury vapour-phase an alyser (namely, Tekran(R)). The flux chamber shows that the rural gras sy site acted primarily as a source of atmospheric mercury, its flux m imicked the solar radiation, with a maximum daytime value of similar t o 8.3 ng m(-2) h(-1) of TGM. The water surface location (St. Lawrence River site located about 3 km From the land site) shows deposition and evasion fluxes almost in the same order of magnitude (-0.5 vs 1.0 ng m(-2) h(-1)). The latter is influenced to some extent by solar radiati on but primarily by the formation of a layer of stable air over the wa ter surface in which some redox reactions might promote evasion proces ses over the water surface. This process does not appear over the soil surface. As a whole, soil-air exchange rate is about 6-8 fold greater than the water-air exchange. (C) 1998 Elsevier Science Ltd. All right s reserved.