Fresh-snow samples were collected on an event basis on the Glacier de la Girose (3360 m above sea level (asl)) in the southern French Alps, during winters and early springs 1990 and 1991. In addition, a 13-m firn core was recovered in 1991 at the Col du Doˆme (4250 m asl), a cold glacier in the northern French Alps, offering the complete seasonal record of alpine precipitation during 3.5 years. All samples were analyzed for total formate and acetate and for major ions using ion chromatography. The acidity-alkalinity was accurately measured using a titration technique. An almost perfect ion balance was achieved for this data set. In absence of Saharan dust transport, the high alpine snow is slightly acid (H+~2--20 μEq L-1). HCOOT and CH3COOT are generally present in alpine acid snow at very low concentrations: 0.3--0.6 μEq L-1 in winter (January to February) and 0.6--2 μEq l-1 in early spring (March to April). At Col du Doˆme, total acetate concentrations of ~1 μEq L-1 are observed in summer It remains unclear from our results what the major sources of carboxylic acids are, and in particular of acetic acid, in the wintertime continental free troposphere, while it appears that formic and acetic acids are presumably mainly derived from natural sources in spring and summer.

The total contribution of formic and acetic acids to free acidity is, on average, less than 15--20%. Contrary to major ions which are present in wider concentration ranges and show large variations from one snowfall to the other, HCOOT and CH3COOT are surprisingly stable in acid alpine snow. The only significant deviation of HCOOT and CH3COOT from their mean values (up to 9 and 5 μEq L-1, respectively) are observed in case of Saharan dust transport, when precipitation pH is shifted from acid toward alkaline conditions. These observations suggest a pH partitioning effect between the aqueous and gas phases, formic and acetic acids being dissolved and neutralized as salts in alkaline cloudwater droplets. On a global scale, the scavenging by alkaline mineral dust could represent an important deposition process of carboxylic acids. ¿ American Geophysical Union 1994