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Abstract. Formic and acetic acids are ubiquitous trace gases in ambient air, but their sources remain to be fully understood. They originate fi'om photochemical oxidation of volatile organic compounds (VOC). Direct emissions of these carboxylic acids are supposedly negligible at North European conditions. Radiocarbon analysis of formic and acetic acids 04rovides infbrmation on the origin of the precursor VOC, since biogenic VOC have the same C/I•-C ratio as atmospheric carbon dioxide, while VOC emitted from Fossil fuels are devoid of •4C• The origin of atmospheric tbrmic and acetic acids was studied by accelerator mass spectrometry of air and rainwater samples. Sampling sites were selected at different distances from centers of anthropogenic activities in Europe, ranging from urban via rural to remote sites. The procedure Ibr preparation of samples was checked by several quality assurance samples, and no significant contamination fi'om atmospheric carbon dioxide was observed. The results show very high (55-100%) biogenic contributions to both carboxylic acids in all samples. Only in semiurban and urban areas were the biogenic fkactions less than 80%. The results indicate that biogenic VOL; ca• have substantial influence on tbrmation of photochemical oxidation products and thus photochemical oxidants.

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