Relative contribution of biogenic and anthropogenic sources to formic and acetic acids in the atmospheric boundary layer

Formic and acetic acids are ubiquitous trace gases in ambient air, but thei r sources remain to be fully understood, They originate fi om photochemical oxidation of volatile organic compounds (VOC). Direct emissions of these c arboxylic acids are supposedly negligible at North European conditions. Rad iocarbon analysis of formic and acetic acids provides information oil tale origin of the precursor VOC, since biogenic VOC have the same C-14/C-12 rat io as atmospheric carbon dioxide, while VOC emitted from fossil fuels are d evoid of C-14. The origin of atmospheric formic and acetic acids was studie d by accelerator mass spectrometry of air and rainwater samples. Sampling s ites were selected at different distances from centers of anthropogenic act ivities in Europe, ranging from urban via rural to remote sites. Tie proced ure for preparation of samples was checked by several quality assurance sam ples, and no significant contamination from atmospheric carbon dioxide was observed. The results show very high (55-100%) biogenic contributions to bo th carboxylic acids in ail samples. Only in semiurban and urban areas were the biogenic fractions less than 80%. The results indicate that biogenic VO C can have substantial influence on formation of photochemical oxidation pr oducts and thus photochemical oxidants.