The presence of snow locally modifies the atmospheric concentrations of trace gases. This is confirmed by numerous and recent experimental observations that raised the interest of the scientific community on the physico-chemical processes involved in the air-snow interactions. Quantifying these mechanisms requires the knowledge of the Specific Surface Area (SSA) of snow, which is the surface area of snow accessible to gases. This basic parameter was poorly documented at the beginning of this study, essentially because it is very difficult to measure. The objective of this thesis consists in the measurement of the SSA of snow and in the understanding and modeling of its evolution during snow metamorphism (i.e. the morphological transformations of snow after deposition). A specific experimental setup was designed to measure the SSA by adsorption of methane on ice at 77K and a protocol of measurement was defined to fulfill the specific conditions imposed by the snow. More than 300 SSA values were obtained, which ranged from 100 to 1540 cm² g-1, with a reproducibility of 6% and an accuracy estimated at 12%. The 176 first results allowed to build a classification of snow as a function of the age and morphology of the snow crystals and the SSA was estimated in each class. This allows to evaluate the SSA within 40% uncertainty in the first confidence level, from a simple optical observation. The effect of metamorphism on the SSA of snow was studied in natural conditions. The SSA decreases with time, and this decrease is faster when the SSA or the temperature is high. Wind was observed to accelerates the SSA decrease. Optical and scanning electron microscopy observations were also made to understand the morphological changes associated to the decrase in SSA. Our observations confirm the usual theories of metamorphism and support the hypothesis that growth occurs by layer nucleation limited by the diffusion of water vapor in air. Unexpectedly, small, newly formed facets with sharp angles were detected, which was interpreted as sublimation initiated at emerging dislocations. The rate of SSA decrease was studied during isothermal experiments at -4, -10 and -15°C. It is very well fitted by a logarithmic law of the form SS=B-ALn(T+Δt) , where B is close to the initial SSA and A represents the rate of SSA decrease. A linear relationship has been found between A and B from 7 experiments at -15°C which suggests that the rate of SSA decrease of snow may be predicted by just knowing its initial SSA. The theories of transient Ostwald ripening explain the logarithmic law and this framework was used to build a mean-field model of isothermal metamorphism. This model allows the rapid calculation of the water vapor fluxes and the growth rate of the snow grains. It reproduces the evolution of the distribution of the radius of curvature of snow under isothermal conditions but it cannot predict the SSA because of a too cursory description of the geometry of snow.
