Abstract: An extensive mercury study was conducted in April 2002 prior to and during the annual melting of a snowpack in a sub-arctic site along the Hudson Bay (Canada). Gas-phase measurements show that the snowmelt coincides with an elemental mercury (Hg°) pulse in the snowpack air far above ambient levels. Additional measurements of inorganic mercury (Hg2+) and methylmercury (MeHg+) in snow pits, in surface snow and in a meltwater sample clearly reveal that most of Hg is removed from the snow during the first days of snowmelt. We estimate that gas-phase exchanges contribute poorly to remove Hg from the snowpack; consequently during a snowmelt day more than 90% of Hg present in the snow surface is likely released with the meltwater. In arctic areas, where Hg accumulates at an accelerated rate in the snow surfaces [
Lu et al., 2001] during mercury depletion events (MDE), the discharge of this toxic and bio-accumulating pollutant in water systems could be a threat to ecosystems and local indigenous populations.

Abstract: Modeling the densification of polar firn is essential to estimate variations of close-off characteristics (density, close-off depth, delta age) in relation with past climate parameters (temperature and accumulation rates). Furthermore, the air composition in the firn is modified by gravitational and thermal fractionation, and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations. In this work, we propose a new model coupling firn densification and heat diffusion. In addition to the determination of the firn thickness and gas-ice age differences, the model allows a reconstruction of the time evolution of the temperature for different sites in Antarctica (Vostok) and Greenland (GISP2) and therefore the evolution of gravitational and thermal isotopic fractionations in firn. Under present-day conditions, the modeled profiles are in good agreement with the available temperature measurements in firn. For sites with low accumulation rates such as Vostok, the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur. The comparison of the modeled ?15N results to measurements allows to better constrain the ?18O-temperature relationship, used to infer the surface temperature history, and for GISP2, the model validates the long-term borehole-based temperature.

Abstract: Stratospheric ozone is enriched in the heavy isotopes (17O and 18O) relative to the ambient oxygen from which it is formed. This enrichment varies with altitude, attaining very high values between 30 and 40 km. A recent theory of Y. Q. Gao and R. A. Marcus explores the reasons for isotopic enrichment in the process of ozone formation and is particularly useful to understand the laboratory results. The stratospheric variations are sought to be explained in terms of temperature dependence of isotopic enrichment, but the magnitude of variation does not match with predictions accurately. We demonstrate here that isotopic enrichment in ozone generated by oxygen photolysis depends on the pressure of the oxygen reservoir and can have very high values (at about 15 torr), comparable to the highest observed stratospheric values. Analysis of the data shows that secondary enrichment through ozone dissociation can add to the primary enrichment associated with ozone formation. The effect of dissociation is found to be more pronounced in the pressure range of 15 to 50 torr, resulting in high enrichment. It is shown that the relative kinetics, pressure and temperature conditions of ozone formation and dissociation play an important role in determining the ultimate value of the enrichment. The results are particularly useful to understand the stratospheric data and resolve the observed discrepancy.

Abstract: This modeling study was motivated by the recent publication of year-round records of dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) in Antarctica, completing the available series of sulfate and methanesulfonic acid (MSA). Sulfur chemistry has been incorporated in the Laboratoire de Météorologie Dynamique-Zoom Tracers (LMD-ZT) Atmospheric General Circulation Model (AGCM), with high-resolution and improved physics at high-southern latitudes. The model predicts the concentration of six major sulfur species through emissions, transport, wet and dry deposition, and chemistry in both gas and aqueous phases. Model results are broadly realistic when compared with measurements in air and snow or ice, as well as to results of other modeling studies, at high- and middle-southern latitudes. Atmospheric MSA concentrations are underestimated and DMSO concentrations are overestimated in summer, reflecting the lack of a DMSO heterogeneous sink leading to MSA. Experiments with various recently published estimates of the rate of this sink are reported. Although not corrected in this work, other defects are identified and discussed: DMS concentrations are underestimated in winter, MSA and non-sea-salt (nss) sulfate concentrations may be underestimated at the South Pole, the deposition scheme used in the model may not be adapted to polar regions, and the model does not adequately reproduces interannual variability. Oceanic DMS sources have a major contribution to the variability of sulfur in these regions. The model results suggest that in a large part of central Antarctica ground-level atmospheric DMS concentrations are larger in winter than in summer. At high-southern latitudes, high loads of DMS and DMSO are found and the main chemical sink of sulfur dioxide (SO2) is aqueous oxidation by ozone (O3), whereas oxidation by hydrogen peroxide (H2O2) dominates at the global scale. A comprehensive modeled sulfur budget of Antarctica is provided.

Abstract: A detailed record of non-sea-salt calcium, a proxy for dust, and sea-salt sodium, a proxy for sea salt, covering the last 45 kyr is presented. It shows that in the first part of the transition from the last glacial period to the Holocene (18–15 kyr BP), the changes in dust flux mainly reflect changes at the dust source, namely vegetation cover and local climate. The changes in the later part of the transition (12–11 kyr BP) are similar in extent to the changes seen in sea salt and most likely reflect a reorganization of the atmospheric circulation. During the last glacial period, considerable variation of dust but not of sea salt is observed, pointing to climatic changes in Patagonia, the main dust source for Dome C. A comparison of the glacial records from Dome C and Taylor Dome suggests that similar influences controlled aerosol input at both sites during this period.