Atmospheric geochemistry of formic and acetic acids at a mid‐latitude temperate site

We have determined the atmospheric concentrations of formic and acetic acid in gas and particulate phases and in precipitation over a 15‐month time period at a site in eastern Virginia. These atmospheric species occurred principally in the gas phase (≥98%), with a marked annual seasonality. Atmospheric concentrations of formic and acetic acid in the gas phase averaged 1890±1235 and 1310±910 ppt during the growing season, compared to 695±405 and 700±375 ppt over the nongrowing season, respectively. Our data support the hypothesis that biogenic emissions from vegetation are important sources of atmospheric formic and acetic acid during the local growing season. The same general time series trends were also evident in precipitation, but the seasonality was not defined nearly as well as in the gas phase. Atmospheric concentrations of aerosol formate and acetate showed substantial temporal variability and generally remained in the 5‐ to 25‐ppt range throughout the year. Our measurements indicate that the near‐surface (≤10 m altitude) diurnal behavior of formic and acetic acids in the gas phase is as follows: the concentrations are lowest prior to sunrise, increase steadily thereafter, and begin to decline by late afternoon, again reaching very low levels in the early morning hours. We have observed a diurnal variation with an amplitude of up to tenfold in atmospheric concentration for formic acid. This pronounced diurnal variation is presumably related to mixed layer dynamics, in addition to daytime biogenic and photochemical source inputs and gaseous dry deposition at nighttime. The removal of organic acids from the nocturnal boundary layer may be facilitated by uptake of their vapors into dew. A striking feature of our data is the distinct differences in both gas phase and precipitation ratios of formic‐to‐acetic acid between the growing and nongrowing seasons. In the gas phase this ratio had a mean value of 1.33 during the growing season, decreasing to 0.90 in the nongrowing season. Likewise, the ratio averaged 1.43 and 1.09 in precipitation at this site during the growing and nongrowing season, respectively. Our measurements indicated that direct emissions from motor vehicles and biomass combustion processes are important sources of atmospheric acetic acid and that these sources have formic‐to‐acetic ratios much less than 1.0. We hypothesize that the seasonal variability in formic‐to‐acetic acid ratios in ambient air is due to a shift in the relative dominance of biogenic versus anthropogenic sources. During the nongrowing season, anthropogenic inputs appear to be the principal source of acetic acid to the atmosphere. However, it remains unclear as to what the major source of formic acid is to the wintertime atmosphere.