[1] To help quantify exchanges between the atmosphere and the snow cover, we have measured the specific surface area (SSA) of 176 snow samples taken from the seasonal snowpack in the Alps, Svalbard, and the Canadian high Arctic around Alert. A volumetric method was used, and the adsorption isotherm of CH 4 on snow at 77 K was recorded. The data were analyzed by the Brunauer-Emmett-Teller method to yield SSA and ÁQ CH4 , the mean heat of adsorption of the first CH 4 monolayer. SSA values obtained were between 100 and 1580 cm 2 /g. The reproducibility of the method is estimated at 6%, and the accuracy is estimated at 12%. We propose that ÁQ CH4 = 2240 ± 200 J/mol should be used as a criterion of reliability of the measurement. The method is described in detail to promote its use. Aged snow samples have lower SSA than fresh ones. The lowest values were found for faceted crystals and depth hoar, and the highest values were found for fresh rimed dendritic snow. A method that field investigators can use to estimate SSA from a visual examination of the snow and from a density measurement is suggested. Snow samples are classified into 14 types based on snow age and crystal shapes. Within each type, a density versus SSA correlation is determined. Our data indicate that, depending on snow type, SSA can then be estimated within 25 to 40% at the 1s confidence level with the method proposed. Preliminary data suggest that SSA spatial variability of a given snow layer is low (<5%), but metamorphism can increase it.
Quantifying the specific surface area (SSA) of snow and its variation during metamorphism is essential to understand and model the exchange of reactive gases between the snowpack and the atmosphere. Isothermal experiments were conducted in a cold room to measure the decay rate of the SSA of four snow samples kept in closed systems at −15 °C. In all cases, a logarithmic law of the form SSA=B−A ln(t+Δt) fits the SSA decrease very well, where A, B and Δt are adjustable parameters. B is closely related to the initial SSA of the snow and A describes the SSA decay rate. These and previous data suggest the existence of a linear relationship between A and B so that it may be possible to predict the decay rate of snow SSA from its initial value. The possibility that grain coarsening theories could explain these observations was investigated. The logarithmic equation was shown to be an approximation of a more general equation, that describes the time evolution of the mean grain radius R in most grain coarsening theories, such as Ostwald ripening: R̄n−R̄0n=Kt. R0¯ is the initial mean grain radius, R̄ is the mean grain radius, n and K are the growth exponent and the growth rate, respectively. Values of n between 2.8 and 5.0 are found. It is concluded that snow metamorphism and Ostwald ripening processes are governed by similar rules. Ostwald ripening theories predict that a steady-state regime is reached after a transient stage, but our results suggest that the steady-state regime is not reached after a few months of isothermal snow metamorphism. This last feature makes is difficult to predict the rate of decrease of snow SSA using the theory of Ostwald ripening.
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