Snow chemistry of high altitude glaciers in the French Alps

Snow samples were collected as snowcores in the accumulation zone of four high altitude glaciers (2980–3540 m.a.s.l.) from each of the 4 highest mountain areas of the French Alps, during 3 consecutive years: 1989, 1990 and 1991. Sampling was performed in spring (~ May), before the onset of late spring—summer percolation. The accumulated snow therefore reflects winter and spring conditions. A complementary sampling of fresh-snow was performed on an event basis, on one of the studied glaciers, in 1990 and 1991. All samples were analysed for major ions (but also for total formate and acetate in fresh-snow samples) using ion chromatography. The acidity-alkalinity was accurately determined with a titration technique. The ion balance of alpine snow has been achieved from those analyses. High alpine snow is slightly acid (H⁺~ 3–20 μeq 1^-1), but is episodically affected by alkaline saharan dust events. The different sources (pollution, seasalt and soil dust) affecting the impurity content of snow were identified using principal component analysis. The measured free acidity, mainly from anthropogenic origin, originates from nitric acid scavenging while sulfuric acidity is partially neutralized by atmospheric ammonia and by alkaline soil dust derived species, the contribution of hydrochloric acid being negligible. All ions exhibit higher concentrations in spring than in winter snow, indicating most likely the influence of increased vertical transport from the lower troposphere at this time. The transport of saharan dust is described through three major events reaching the Alps during March 1990 and 1991. Very high concentrations of Ca²⁺ and HCO^3- were measured in corresponding samples, indicating that the solubilisation of CaCO₃ represents the major influence of saharan dust on the impurity content of alpine snow, shifting the pH from acid towards alkaline values. Chemical analysis suggests that during their transport, mineral alkaline particles can react through acid-base neutralization processes, mainly with sulfur, but also with nitrogen compounds, leading to increased deposition rates of SO₄ ^2-. The potential of deposited alkaline snow layers to neutralize the acidity stored in the snowpack, at the onset of late spring—summer percolation is also assessed.