THE WEATHERING OF AEOLIAN DUSTS IN ALPINE SNOWS

The chemistry of precipitation (snow and rainfall), snow cover and mel twaters was studied at a French alpine site during the winter-spring s easons 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 NO3, contributed to the acidity of precipitation but NO3 was the princ ipal anion associated with acidic snowfall. Many alkaline snowfalls ar e associated with airborne calcareous dusts from regional sources. The most alkaline snowfall, however, was associated with dust from the Sa hara Desert. The weathering of dusts in the snow cover during melt lea ds to the consumption of acidity and an increase in the pH of meltwate rs. The results of both field and laboratory experiments show that inp uts of calcareous dusts by local aeolian erosion and transport can con tribute significantly to the neutralization process. The laboratory ex periments also show that the size and distribution of dusts in the sno w cover have an effect on the degree of neutralization. Dust in the lo wer 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 s now and the partial pressures of CO2 within the snow column during the melt process.