The weathering of aeolian dusts in alpine snows

The chemistry of precipitation (snow and rainfall), snow cover and meltwaters was studied at a French alpine site during the winter-spring seasons of 1986–1987 and 1987–1988. Both acid ( < pH 5.6) and alkaline ( > pH 5.6) deposition events occurred. The strong-acid anions, SO4 and BO3, contributed to the acidity of precipitation but NO3 was the principal anion associated with acidic snowfall. Many alkaline snowfalls are associated with airborne calcareous dusts from regional sources. The most alkaline snowfall, however, was associated with dust from the Sahara Desert. The weathering of dusts in the snow cover during melt leads to the consumption of acidity and an increase in the pH of meltwaters. The results of both field and laboratory experiments show that inputs of calcareous dusts by local aeolian erosion and transport can contribute significantly to the neutralization process. The laboratory experiments also show that the size and distribution of dusts in the snow cover have an effect on the degree of neutralization. Dust in the lower strata of snow cover is more efficient in neutralizing the acidity of meltwaters than dust in the upper strata. The relationship between the distribution of dust and its efficiency to neutralize the acidity of meltwaters can be explained by the kinetics of calcite dissolution under conditions of progressive decreases in the acidity of leached snow and the partial pressures of CO2 within the snow column during the melt process.