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Molecular hydrogen (H 2 ) is an indirect greenhouse gas present at the trace level in the atmosphere. So far, the sum of its sources and sinks is close to equilibrium, but its large-scale utilization as an alternative energy carrier would alter its atmospheric burden. The magnitude of the emissions associated with a future H 2 -based economy is difficult to predict and remains a matter of debate. Previous attempts to predict the impact that a future H 2 -based economy would exert on tropospheric chemistry were realized by considering a steady rate of microbial-mediated soil uptake, which is currently responsible of ~ 80% of the tropospheric H 2 losses. Although soil uptake, also known as dry deposition is the most important sink for tropospheric H 2 , microorganisms involved in the activity remain elusive. Given that microbial-mediated H 2 soil uptake is influenced by several environmental factors, global change should exert a significant effect on the activity and then, assuming a steady H 2 soil uptake rate for the future may be mistaken. Here, we present an overview of tropospheric H 2 sources and sinks with an emphasis on microbial-mediated soil uptake process. Future researches are proposed to investigate the influence that global change would exert on H 2 dry deposition and to identify microorganisms involved H 2 soil uptake activity.

Mercury (Hg) contamination in tundra region has raised substantial concerns, especially since the first report of atmospheric mercury depletion events (AMDEs) in the Polar Regions. During the past decade, steady progress has been made in the research of Hg cycling in the Polar Regions. This has generated a unique opportunity to survey the whole Arctic in respect to Hg issue and to find out new discoveries. However, there are still considerable knowledge gaps and debates on the fate of Hg in the Arctic and Antarctica, especially regarding the importance and significance of AMDEs vs. net Hg loadings and other processes that burden Hg in the Arctic. Some studies argued that climate warming since the last century has exerted profound effects on the limnology of High Arctic lakes, including substantial increases in autochthonous primary productivity which increased in sedimentary Hg, whereas some others pointed out the importance of the formation and postdeposition crystallographic history of the snow and ice crystals in determining the fate and concentration of mercury in the cryosphere in addition to AMDEs. Is mercury re-emitted back to the atmosphere after AMDEs? Is Hg methylation effective in the Arctic tundra? Where the sources of MeHg are? What is its fate? Is this stimulated by human made? This paper presents a critical review about the fate of Hg in the Arctic tundra, such as pathways and process of Hg delivery into the Arctic ecosystem; Hg concentrations in freshwater and marine ecosystems; Hg concentrations in terrestrial biota; trophic transfer of Hg and bioaccumulation of Hg through food chain. This critical review of mercury fates and contamination in the Arctic tundra ecosystem is assessing the impacts and potential risks of Hg contamination on the health of Arctic people and the global northern environment by highlighting and "perspectiving" the various mercury processes and concentrations found in the Arctic tundra.

Air and precipitation samples were collected and analyzed for 91 pesticides or metabolites from May to September 2004 at St. Damase, an agricultural site located in Yamaska basin in Québec, Canada. A broad range of pesticides was detected during this experiment where 40 different compounds were measured. Most of the samples showed more than 10 simultaneous pesticide detections and sometimes reaching 19 simultaneous detections. The most detected pesticides in air were trifluralin, metolachlor and captan, which were found in all the samples during the 5 months of measurements. For the detected compounds, the average concentrations ranged from 4 pg/m3 to 8 ng/m3. Some of the pesticides detected in air were found in precipitation samples as well showing that wet deposition can occur and have an impact on aquatic ecosystems. The most important pesticides detected were related to corn and soya cultivations, the two main cultures in this region highlighting that the major sources come from local applications. On the other hand, the detection of some pesticides in precipitation which were undetectable in air implies that some compounds may have a long range transport origin.

This paper reports the first ebullitive fluxes of hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2) and total gaseous mercury (TGM) from the Cornwall Area of Concern (CAC). Although sediments were contaminated with mercury, bubbling was a negligible source of mercury for the global atmosphere. Indeed, the average emission of TGM through ebullition was 0.04 pg m(-2) h(-1). Measurements of H2, CO, CH4 and CO2 trapped gas concentrations and fluxes were used as indicators of diagenesis processes. The CAC represented a significant regional source of CH4 since the estimated ebullitive fluxes (3.5 mg m(-2) h(-1)) were similar to the CH4 emissions measured above typical flooded freshwater wetlands. As molecular diffusion is known as the main pathway of CO2 transport from water to the atmosphere, CO2 ebullitive fluxes were weak (0.39 mg m(-2) h(-1)). Bubbling of CO (1.6 microg m(-2) h(-1)) was 10 folds less important than CO fluxes measured over flooded freshwater wetlands. Finally, H2 emissions (0.001 microg m(-2) h(-1)) were negligible since the level of this trace gas is tightly regulated by microorganisms in anaerobic environments.

One fate of gaseous elemental mercury (GEM) in the Arctic has been identified as gas phase oxidation by halogen-containing radicals, leading to abrupt atmospheric mercury depletion concurrent with ozone depletion. Rapid deposition of oxidized mercury leads to snow enrichment in mercury. In this report, we describe experiments that demonstrate the ability of snow to directly scavenge atmospheric mercury. The study was conducted at Kuujjuarapik, Québec, Canada (latitude 55 degrees 17'N). A mercury depletion event (MDE) caused the mercury concentration in the surface snow of the coastal snowpack to double, from (9.4+/-2.0) to (19.2+/-1.7) ng/L. Independent of the MDE, mercury concentrations increased five-fold, from (10.0+/-0.1) to (51.4+/-6.0) ng/L, upon spiking the snow with 500 microM hydrogen peroxide under solar irradiation. Total organic carbon in the spiked irradiated snow samples also decreased, consistent with the formation of strongly oxidizing species. The role of the snowpack in releasing GEM to the atmosphere has been reported; these findings suggest that snow may also play a role in enhancing deposition of mercury.

Eight years of gaseous elemental mercury (GEM) concentration measurements from Alert, Nunavut, Canada (between 1995 and 2002) is presented. The annual time series shows a distinct repeating seasonal pattern with an overall annual median concentration for this time period of 1.58 (S.D.=0.04 ng m(-3)). Strong seasonal variation was observed throughout the years with springtime displaying strong variability in the GEM and overall lower median concentrations due to the so-called mercury depletion events (MDEs). Summer concentrations are higher than the annual average and show a decrease in variability. Fall and winter concentrations are distributed around the annual median concentrations and show little variability. The relationship between the springtime depression and the summer increase shows a change in the behaviour of mercury between 1995 and 2002. Preliminary results suggest that during this period an increasing amount of the mercury lost from the atmosphere in the spring is not returned to atmosphere in summer. A comparison of GEM concentration data from three sites--Alert (Canada), Amderma (Russia) and Kuujjuarapik (Canada)--demonstrated similar monthly distribution of GEM between Alert and Amderma, with the latter not showing as high summer concentrations. Monthly distribution of GEM at Kuujjuarapik varied considerably from the other two sites. MDEs were found to occur at each site in the spring yet displayed different characteristics. MDEs appear to start at Alert shortly after polar sunrise but in Amderma their initiation is delayed approximately 2 months following polar sunrise. MDEs occur in Kuujjuarapik in the springtime despite an incomplete development of the polar day-night cycle. In spring, as soon as air temperature attained temperatures consistently above 0 degrees C, MDEs ended immediately at all three sites. Continued studies into MDEs are warranted, but clearly an important component of future studies must focus on the origins of the variation of GEM behaviour at different sites.

The main objectives of this paper were to report total gaseous mercury (TGM) concentrations at four stations along the St. Lawrence River in 1998 and to give some insights regarding their fate. The TGM network stretches from latitudes 45 degrees N to 50 degrees N. TGM and ozone concentrations (used as complementary pollutant) were presented. The median TGM concentrations throughout the network varied from 1.62 to 1.79 ng/m3. Sites close to Montreal had larger TGM concentrations. Montreal surroundings appeared to be a source of TGM. The TGM concentrations were maximum in wintertime and minimum in summertime throughout the sites. However, the diurnal variations were site specific. St. Anicet and L'Assomption were more variable in terms of large concentrations, whereas Mingan and Villeroy were more variable in terms of low concentrations. Ozone and TGM concentrations were positively correlated at some sites (Villeroy, Mingan and St. Anicet) whereas they were not at L'Assomption (closest site to Montreal). Local TGM and ozone precursor sources would affect the correlation.

The paper presents one year of results (1995–1996) of an on-going research study in southern Quebec with regard to mercury in precipitation. A modified rain water collector (MIC-B1) coupled to a data logger is used, in order to be able to select individual events from a remote site. A peristaltic pump coupled with an enclosed Teflon system train separates `in situ' the particulate and the dissolved phases of mercury. QA/QC sampling protocol including memory effect, percentage of recovery as well as field blanks are applied to maintain the high standards of quality. Total mercury concentrations in both phases are measured by Cold Vapour Atomic Fluorescence Spectrophotometry (CVAFS). The volume-weighted mean of total Hg concentration is 6.98 ng.l−1 (the median concentration is 6.58 ng.l−1 and the arithmetic mean is 8.26 ng.l−1). The annual total mercury deposition is about 7.6 μg.m−2. The particulate fraction accounts for a median of 26% of the total mercury concentration. According to Henry's Law, the truly dissolved mercury accounts for less than 0.5% whereas most of the dissolved fraction is expected to be in molecular complexes or colloidal form (