| Summary: | In this thesis, an indirect effect of sulfuric acid particles on Arctic climate during the cold season, from November to May, is investigated. Specifically, the research is focused on the alteration of the air mass dehydration rate due to sulfuric acid aerosols. These anthropogenic aerosols differ from natural aerosols by their poor ability to nucleate ice, their strong solubility and their reduced homogeneous freezing temperature when internally mixed with other compounds. Simulations performed with three column models and analysis of observed data at Alert (1991--94) are used to investigate an indirect effect of these aerosols on climate: the dehydration - greenhouse feedback. Each model covers different levels of physical basis and realism of their simulation. Two scenarios have been compared in the simulations: an acid aerosols scenario and a natural aerosols scenario. Results show that aerosol acidification leads to a depletion of the ice crystal number concentration and an increase of their mean size. As a result, clear sky precipitation (CSP) occurs more frequently than ice fogs during Arctic haze episodes. This result is in agreement with observations that indicate an increase by more than 50% of the weekly mean CSP frequency, when the proportion of sulfuric acid is greater than the mean observed value of 20%. Consequently, the sedimentation flux of ice crystals and the dehydration rate of the lower troposphere are accelerated. The radiative effect is a weaker atmospheric emissivity in the boundary layer, up to the height corresponding approximately to the top of the ice crystal layer. As a result, the infrared flux reaching the surface and the greenhouse effect are decreased. Simulations performed for the period 1991 to 1994 at Alert show a negative radiative forcing of about --3 W/m2 at the surface between November and May. The net result is a strengthening of the surface-based temperature inversion of 1.3°C, with a surface cooling of 0.4°C and a warming of 0.9°C at 800 hpa. The indirect effect of the dehydration - greenhouse feedback due to anthropogenic acid aerosols can explain in part the observed strengthening of the surface-based temperature inversion in the Arctic (Kahl et al., 1993).
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