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In order to better understand MDE chemistry, which are thought to happen in the presence of halogenated (chloride and bromide) species, we have led several kinetic experiments with gaseous mercury, ozone and chlorine in Teflon bags under polar atmospheric conditions. The chlorine was expected to produce, with natural solar radiation, the reactive chloride species assumed to play a role in the MDE mechanism. The aim of the first experiment was to study the reactivity between ozone and gaseous mercury: we obtained a rate constant of (13±6)x10 -20 cm 3 /molec/s, which is roughly comparable to constants given in the literature. Concerning the second experiment, for which chlorine has been introduced, a rate constant of (6.2 ±2.2))x10 -17 cm 3 /molec/s for the reactivity of gaseous mercury was obtained. This constant is too weak to recover the atmospheric lifetime of gaseous mercury during MDE. This difference is in discussion in this paper.

During a field experiment at Kuujjuarapik/Whapmagoustui (55°31N, 77°75W), Quebec, we have pointed out modifications in the size distribution of particles during a Mercury Depletion Event (MDE). We observed an increase of atmospheric particles concentrations in the ambient air, correlated with depletions of ozone and atmospheric mercury. These increases were followed by a change in the size particles distribution. Assuming that the divalent mercury species formed during the MDE could be adsorbed onto particles, this is likely to contribute to higher deposition fluxes of oxidised mercury on the snow pack.

This study reported the first synchronized automated high-time resolution mercury species fluxes over plant canopy. It is suggested that plants can uptake reactive gaseous mercury and release elemental gaseous mercury during daytime. This can be environmentally significant due to the bio-transfer (conversion) from a short transported mercury species (Hg (II)) to a long transport mercury species (Hg°).

The snowpack is a medium that greatly interacts with a variety of atmospheric gases. Its role in the mercury depletion events in arctic and sub-arctic regions seems to be crucial though it is poorly understood. We studied an environmental component of mercury that is interstitial gaseous mercury (IGM) present in the air of the snowpack at Kuujjuarapik (55°N, 77°W), Quebec in April 2002. Our data demonstrate that the snow-pack plays a major role in the global mercury cycle. Indeed, IGM concentrations exhibit a well-marked diurnal cycle with uninterrupted events of Hg° depletion and production within the snowpack. During snowmelt episode, high IGM concentrations (∼20 ng/m 3 at a depth of 37 cm) have been recorded. This study gives us the confirmation that Hg° is involved in oxidation processes occurring in the snowpack. Additionally, we assume that the impressive production of Hg° during the daytime may be the results of photoreduction and photo-initiated reduction of Hg(II) complexes.

The combined effect of light and hydrogen peroxide on the ability of surface snow to scavenge atmospheric mercury was investigated in Kuujjuarapik, Nunavik, Canada (latitude 55°N). Mercury concentrations in the snowpack increased five-fold upon spiking with hydrogen peroxide under solar irradiation. Anion analysis of the snow revealed the presence of chloride, which may stabilize Hg(II). Measurements of total inorganic and organic carbon showed that both kinds of carbon content decreased in the spiked irradiated snow samples.

Mercury may be one of the best-documented hazardous substances utilised by man. Still the uncovering of the global human impacts on the environment through the use and mobilisation of mercury cannot be considered complete. Recent mercury depletion events observed in the Arctic have opened the horizon to numerous new aspects on mercury fate and cycling in the environment. This chapter browses various aspects of the extremely complex cycling and fate of mercury in the perspective of mercury as a global pollutant and discusses some new emerging research fields in mercury.