Considerations regarding sources for formic and acetic acids in the troposphere

Abstract

HCOOH and CH 3 COOH are important chemical constituents of cloud water and precipitation, but sources for these compounds in the atmosphere are at present unknown. The question of source identification was addressed through the analysis of 465 samples of precipitation collected at 14 continental and marine locations around the world. Continental precipitation during growing seasons contained, relative to marine precipitation and to continental precipitation during nongrowing seasons, higher absolute concentrations of organic acids and higher ratios of HCOO T (HCOOH aq + HCOO − ) to CH 3 COO T (CH 3 COOH aq + CH 3 COO − ). The concentrations of HCOO T and CH 3 COO T in precipitation at most locations were also highly correlated. These results support the hypothesis that organic acidity in precipitation may originate with two major sources, volatile vegetative constituents over continents and a second weaker source in both continental and marine regions. Relative to the similar ratios of HCOO T to CH 3 COO T in the aqueous phase, differences in precipitation p H resulted in large regional differences in calculated equilibrium vapor phase concentrations. The mechanism(s) by which proportionate concentrations of HCOO T and CH 3 COO T are maintained in the aqueous phase remains an open question. Comparisons between precipitation in impacted and remote regions indicate that although possibly important near large population and industrial centers, anthropogenic emissions are probably not major sources for organic acids in precipitation over broad geographic regions.