Origins and variations of light carboxylic acids in polar precipitation
Authors: Legrand Michel , De Angelis Martine 
Published: February 2004

Central Greenland and East Antarctic ice cores have been studied to investigate the carboxylic acid (acetate, CH3COO−; formate, HCOO−; glycolate, C2H3O3−; and oxalate, C2O4−−) content of high‐latitude precipitation. The two records cover the end of the last glacial age, the last great climatic change having occurred between 10,000 and 15,000 years B.P., and the Holocene period which started some 10,000 years ago. Carboxylic acids were measured using ion chromatography. These measurements are more difficult and require more stringent precautions to prevent sample contamination than inorganic trace species measurements. Carboxylic acids account for up to 25% of the free acidity in Greenland and are one order of magnitude more abundant in Greenland than in Antarctic precipitation. In Greenland precipitation deposited under present climatic conditions, formic and acetic acids are both present at the 10 ng g−1level and are the most abundant carboxylic acids, while glycolic and oxalic acids with concentrations close to 1 ng g−1represent minor carboxylic acid species. The level of formic, and to a lesser extent acetic, acid is also found to be stronglypH dependent. There is less formic acid at depths corresponding to periods when the acidity of the atmosphere was enhanced by a volcanic activity. Over the last 12,700 years, the carboxylic acid level of Greenland precipitation has often been sporadically enhanced by several orders of magnitude. Such large perturbations, which are accompanied by large increases of NH4+concentrations, are probably caused by biomass‐burning events which occurred at high northern latitudes. A particular chemical feature appears in snow layers corresponding to biomass‐burning events which occurred during the Younger Dryas (11,550 to 12,700 years B.P.). During this cold stage, the precipitation was alkaline and the events were accompanied by an input of nitrite suggesting that peroxyacetyl nitrate (PAN) was present in the atmosphere and was hydrolyzed in alkaline cloud water. Formate and acetate profiles indicate that background levels of these carboxylic acids were 5 and 2 times lower, respectively, during the last glacial maximum (15,000 to 34,000 years B.P.) than during the Holocene stage in Greenland precipitation. The increases of the acetate and formate snow contents in response to the glacial‐interglacial climatic transition exhibit a time lag of some 5000 years. In particular, the formic acid increase follows perfectly the timing of the retreat of the Laurentide ice sheet from 18,000 years B.P. to the mid‐Holocene stage (6000 years B.P.). Our data suggest therefore that carboxylic acids in Greenland precipitation are mainly linked to emissions from the high‐latitude continental biosphere. In contrast, the incloud oxidation of formaldehyde produced from methane oxidation under acidic conditions likely represents the main source of formic acid in East Antarctica. Finally, the study of recent Greenland snow deposits indicates that the expected trend of carboxylic acid concentrations due to man‐made activities is counteracted by a simultaneous increase of the acidity related to growing fossil fuel combustion, which leads to a less efficient uptake of carboxylic acids into precipitation.