The biogeochemical cycling of formic and acetic acids through the troposphere: an overview of current understanding

Despite remaining uncertainties, a general picture of the biogeochemical cycling of HCOOH and CH₃COOH is beginning to emerge. These acids are ubiquitous vapor- and aqueous-phase constituents of the global troposphere, contributing significant fractions of the natural acidity in precipitation and cloudwater, particularly in continental regions of the tropics and subtropics. The similarity of concentrations in remote and impacted regions indicates that anthropogenic emissions of carboxylic acids or precursors are probably not important sources over broad geographic areas. In continental regions, seasonally varying emissions of acids and hydrocarbon precursors from vegetation are thought to represent major sources for both acids. In marine areas, the seasonality of wet deposition and apparent lack of transport from continents support the hypothesis of a marine biogenic source. Ratios of total (aqueous + vapor) concentrations of HCOOH versus CH₃COOH in clouds vary as a function of H⁺. This observation is consistent with the hypothesis of a pH-dependent aqueous-phase source and sink for HCOOH in clouds which may act to stabilize aqueous-phase ratios against shifts in solution H⁺. Large die1 periodicities in the concentrations of vapor-phase species over continents, together with a lack of evidence for long-distance transport, suggest that HCOOH and CH₃COOH experience short atmospheric lifetimes of several hours to a few (at most) days. Because they are important controllers of free acidity and represent a potentially large sink for aqueous-phase OH radicals, the cycling of the carboxylic acids is expected to interact directly and indirectly in the chemical cycling of other atmospheric constituents.