Gas- and aqueous-phase formic and acetic acids at a tropical cloud forest site

Abstract Atmospheric gas-phase and aqueous-phase (dew and fog) formic and acetic acids were measured over a cloud forest in Venezuela. The gaseous acids showed diurnal cycles, with higher mixing ratios during daytime. Higher concentrations were observed during the dry season (HCOOH 1.7 +/−0.5 ppb; CH 3 COOH 1.4+/−0.6 ppb) in comparison with the rainy season (HCOOH 0.79+/−0.24 ppb; CH 3 COOH 0.54+/−0.20 ppb). Liquid-phase concentrations in dew and fog are of the same order and range from 8.1 to 69.5 μM for HCOOH and 4.3 to 15.3 μM for CH 3 COOH. The field-observed Henry's Law coefficients, calculated from the simultaneous measurements of gas- and liquid-phase acids, do not show a significant trend with the pH of the solution, in contrast to theoretical considerations. Dry deposition velocities to the nighttime dew are 1.1+/−0.6 and 0.68+/−0.42 cm s −1 for formic and acetic acids, respectively. A loss of 0.054 ppb HCOOH and 0.022 ppb CH 3 COOH from the atmospheric boundary layer to the dew is produced nightly.