Abstract: Ce travail qui n'est pas issu du programme 136 utilise des données climatologiques enregistrées Plaine Ampère et fournies par le 136

Abstract: Diamond dust (DD) refers to tiny ice crystals that form frequently in the Polar troposphere under clear sky conditions. They provide surfaces for chemical reactions and scatter light. We have measured the specific surface area (SSA) of DD at Barrow in March–April 2009. We have also measured its chemical composition in mineral and organic ions, dissolved organic carbon (DOC), aldehydes, H2O2, and the absorption spectra of water-soluble chromophores. Mercury concentrations were also measured in spring 2006, when conditions were similar. The SSA of DD ranges from 79.9 to 223 m2 kg-1. The calculated ice surface area in the atmosphere reaches 11000 (70%) μm2 cm-3, much higher than the aerosol surface area. However, the impact of DD on the downwelling and upwelling light fluxes in the UV and visible is negligible. The composition of DD is markedly different from that of snow on the surface. Its concentrations in mineral ions are much lower, and its overall composition is acidic. Its concentrations in aldehydes, DOC, H2O2 and mercury are much higher than in surface snows. Our interpretation is that DOC from the oceanic surface microlayer, coming from open leads in the ice off of Barrow, is taken up by DD. Active chemistry in the atmosphere takes place on DD crystal surfaces, explaining its high concentrations in aldehydes and mercury. After deposition, active photochemistry modifies DD composition, as seen from the modifications in its absorption spectra and aldehyde and H2O2 content. This probably leads to the emissions of reactive species to the atmosphere.

Abstract: The snowpack is a photochemically active medium which produces numerous key reactive species involved in the atmospheric chemistry of polar regions. Formaldehyde (HCHO) is one such reactive species produced in the snow, and which can be released to the atmospheric boundary layer. Based on atmospheric and snow measurements, this study investigates the physical processes involved in the HCHO air-snow exchanges observed during the OASIS 2009 field campaign at Barrow, Alaska. HCHO concentration changes in a fresh diamond dust layer are quantitatively explained by the equilibration of a solid solution of HCHO in ice, through solid-state diffusion of HCHO within snow crystals. Because diffusion of HCHO in ice is slow, the size of snow crystals is a major variable in the kinetics of exchange and the knowledge of the snow specific surface area is therefore crucial. Air-snow exchanges of HCHO can thus be explained without having to consider processes taking place in the quasi-liquid layer present at the surface of ice crystals. A flux of HCHO to the atmosphere was observed simultaneously with an increase of HCHO concentration in snow, indicating photochemical production in surface snow. This study also suggests that the difference in bromine chemistry between Alert (Canadian Arctic) and Barrow leads to different snow composition and post-deposition evolutions. The highly active bromine chemistry at Barrow probably leads to low HCHO concentrations at the altitude where diamond dust formed. Precipitated diamond dust was subsequently undersaturated with respect to thermodynamic equilibrium, which contrasts to what was observed elsewhere in previous studies.

Abstract: We report on dual isotopic analyses (δ15N and Δ17O) of atmospheric nitrate at daily time-resolution during the OASIS intensive field campaign at Barrow, Alaska, in March–April 2009. Such measurements allow for the examination of the coupling between snowpack emissions of nitrogen oxides (NOx = NO + NO2) and their involvement in reactive halogen-mediated chemical reactions in the Arctic atmosphere. The measurements reveal that during the spring, low δ15N values in atmospheric nitrate, indicative of snowpack emissions of NOx, are almost systematically associated with local oxidation of NOx by reactive halogens such as BrO, as indicated by 17O-excess measurements (Δ17O). The high time-resolution data from the intensive field campaign were complemented by weekly aerosol sampling between April 2009 and February 2010. The dual isotopic composition of nitrate (δ15N and Δ17O) obtained throughout this nearly full seasonal cycle is presented and compared to other seasonal-scale measurements carried out in the Arctic and in non-polar locations. In particular, the data allow for the investigation of the seasonal variations of reactive halogen chemistry and photochemical snowpack NOx emissions in the Arctic. In addition to the well characterized peak of snowpack NOx emissions during springtime in the Arctic (April to May), the data reveal that photochemical NOx emissions from the snowpack may also occur in other seasons as long as snow is present and there is sufficient UV radiation reaching the Earth's surface.

Abstract: Since January 1994, continuous surface O3 measurements have been carried out at Amsterdam Island (37°S, 77°E) in the southern Indian Ocean using a UV absorption analyzer. Mean O3 concentrations and standard deviations are recorded every 5 min. This paper shows and discusses the O3 time series for a 3-year period (1994–1996). During this period, O3 shows a seasonal variation with maxima around 30 ppbv during winter (July – September) and minima around 13 ppbv during summer (December – February). O3 levels at Amsterdam Island are close to those observed at Cape Grim (Tasmania, 41°S, 145°E) for the period of 1991 to 1995. In order to improve the understanding of the mechanisms controlling the O3 seasonal variation at Amsterdam Island, our observations have been analyzed by using the global three-dimensional climatological Model of the Global Universal Tracer Transport in the Atmosphere (MOGUNTIA). The model reproduces well the observed O3 mixing ratios and their seasonal cycle. The seasonal variation of stratospheric origin O3 calculated by MOGUNTIA shows a maximum of 18 ppbv in September and a minimum of 13 ppbv in April. The oxidation of continentally emitted O3 precursors during their transport to Amsterdam Island contributes to the photochemical production of O3 by up to 12 ppbv during austral summer. From this amount, 3–7 ppbv are directly linked to nonmethane hydrocarbon oxidation chemistry under relatively high NOx conditions. In particular, biomass burning emissions contribute up to 5.5 ppbv to the observed O3 levels in September. Both photochemical production by biomass burning and stratospheric influx of O3 lead to a maximum in O3 mixing ratios during late winter to early spring in agreement with the observations. Minimum O3 mixing ratios observed during austral summer are related to photochemical O3 depletion.