A historical record of formate and acetate from a high-elevation Alpine glacier: Implications for their natural versus anthropogenic budgets at the European scale

High-resolution records of formate and acetate from a Col du Dome (CDD, 4250 m elevation, French Alps) ice core were used to investigate the impact of man-made activities on the midtropospheric levels of these species over Europe since 1925. Formate whose summer levels remained unchanged (80+/-20 ng g(-1)) over the 1925-1995 time period is the major monocarboxylate present in summer CDD snow layers. In contrast, the acetate summer level being close to 15 ng g(-1) prior to 1950 increased by a factor of 3 from 1950 to 1975 and declined during the 1980s. The difference in past changes of these monocarboxylates is due to recent acidification of the atmosphere which has lowered the scavenging efficiency in a larger extent for formate than for acetate. Formate levels corrected from effect of past acidity changes show a long-term trend similar to the acetate one. These past changes are likely related to vehicle emissions (direct emissions and secondary production from alkenes) which strongly increased from 1950 to 1980. Because of improvement of engines and more stringent emission standards the CDD summer levels of monocarboxylates declined in the 1980s, and the 1990 levels are only slightly higher than their preindustrial levels. Therefore, except at the end of the 1970s when the anthropogenic input was as high as the natural one, natural sources appear to dominate the budget of the two monocarboxylates in the European midtroposphere. It is shown that natural sources include direct emissions by vegetation, oxidation of isoprene and monoterpenes, and possibly in-cloud oxidation of formaldehyde into formic acid and gas phase oxidation of peroxyl acetyl radical into acetic acid.