Emissions, chemistry or bidirectional surface transfer? Gas phase formic acid dynamics in the atmosphere

Organic acids are among the many secondary components produced from organic compounds that can impact particulate matter composition and aerosol and cloud water acidity. The most abundant gas phase organic acid in the atmosphere is formic acid, which has been observed in concentrations in excess of 2.5 ppbV in rural areas. However, atmospheric model simulations of formic acid are typically biased low, potentially due to biases in their direct emissions and/or chemistry. In this study, we used the Community Multiscale Air Quality (CMAQ) to simulate gas phase formic acid in Yorkville, Georgia during an intensive campaign that lasted from September to October 2016. Similar to previous studies, the diurnal trend of simulated formic acid did not match observations, being biased low with a dissimilar diel profile. Potential reasons for mismatch were investigated. Additional gas phase reactions of isoprene and monoterpenes suggested by recent studies were added, leading to a small increase in acid concentrations, but having little impact on the diel profile. Surface reaction between OH and isoprene epoxydiols (IEPOX) was simulated, increasing the simulated concentration of formic acid by up to 150%; though the resulting simulated diurnal profile did not match the observed. Examination of the diel profile found that formic acid concentrations began to rise early in the morning before suspected precursors (e.g., isoprene, IEPOX and methanediol) and the hydroxyl radical, and that concentrations decreased rapidly in the afternoon. The rapid, early morning rise could be reproduced with increased emissions. Given the long lifetime of formic acid, and that the condensed phase formate concentrations are small, the evening decrease is explained mainly by rapid dry deposition. This suggests that formic acid underwent bidirectional deposition/emission, e.g., depositing rapidly as dew formed at night, with subsequent re-emission during the following day as the dew evaporated. This also led to observed decreases in concentrations with height.