Atmospheric formic and acetic acids: An overview

Abstract

Interest in the role of organic acids as chemical constituents in troposphere has been growing rapidly over the past couple of decades. In addition to their presence in the atmosphere in a variety of phases, organic acids are important constituents of the global troposphere and contribute a large fraction (~25%) to the nonmethane hydrocarbon atmospheric mixture. They contribute significantly to the acidity of precipitation and cloud water, especially in remote regions. In this review, we consider the information presently available on concentration distribution of formic and acetic acids in multiple phases and their sources in different geographical locations, i.e., midlatitude continental, tropical continental and marine sites. Photochemical reactions (i.e., ozone-olefin reaction, isoprene oxidation, gas phase reaction of formaldehyde with HO2, and aqueous phase oxidation of formaldehyde) are important sources of these acids. In midlatitude continental regions, possible sources of formic and acetic acids, in addition to photochemical reactive vehicular emission, are direct emission from vegetation and biomass burning. In tropical continental sites, direct emission from vehicles, ants, soil, vegetation, and biomass burning are the important source of these species. The probable sources at marine locations are photochemical reactions, biogenic emissions, and long-range transport from continental sites.