Field observations of total gaseous mercury behaviour: Interactions with ozone concentration and water vapour mixing ratio in air at a rural site

Total Gaseous Mercury (TGM) data (n = 5125) measured from September 9 to November 30, 1994 in Southern Quebec with an automatic TGM analyser (Tekran(R)) are used to study the behaviour of TGM with ozone and the water vapour mixing ratio. Results show, as a whole, no straight-forward correlation between TGM and ozone concentrations. A significant correlation between TGM and water vapour mixing ratio was measured. Some back reduction of Hg(II) to Hg degrees in the aqueous phase and subsequent evaporation of Hg degrees according to Henry's Law is expected. The latter correlation seems to be partly ozone dependent since under 'clean conditions' (ozone < 30 ppbv) the slope of TGM vs the water vapour mixing ratio is twice that during 'polluted conditions' (ozone greater than or equal to 30 ppbv) which suggests some compensation effects under polluted conditions (counteracting the back-reduction of Hg(TT) to Hg degrees in the aqueous phase). Oxidation of TGM by ozone during high moisture conditions (water vapour mixing ratio similar to 14g/kg) seems to appear at the threshold ozone concentration greater than or equal to 30 ppbv.