Evolution of the specific surface area and of crystal morphology of Arctic fresh snow during the ALERT 2000 campaign

Metamorphism of surface snow results in a decrease in specific surface area (SSA), which causes the release of adsorbed trace gases. This process contributes to the impact of the snowpack on atmospheric chemistry and must be quantified. The SSA evolution of fresh snow layers was measured in February and April 2000 during the ALERT 2000 campaign at Alert (Canadian Arctic: 82°29′94″N, 62°20′55″W). Measurements were made using CH4 adsorption at 77 K. Numerous photomacrographs of snow crystals were also taken to interpret observed SSA decreases. Five snowfalls were studied. Four snowfalls consisted of fine snow called diamond dust, whose initial SSA ranged from 1460 to 770 cm2/g, and decreased to 500 cm2/g in winter, and to 300 cm2/g in spring after 10 days. The fifth one consisted of rimed dendritic crystals, whose SSA decreased from 1540 to 450 cm2/g in 3 windy days. The continuous growth of surface hoar at the surface of the snowpack, diluted surface diamond dust and contributed to their SSA decrease. Our data indicate that the total surface area of surface snow layers is sufficient to sequester adsorbable gases. The example of HNO3 shows that the release of this gas by SSA decrease may indeed lead to a significant increase in its atmospheric concentration.