First Continuous Measurement of Gaseous and Particulate Formic Acid in a Suburban Area of East China: Seasonality and Gas–Particle Partitioning

Being the most abundant monocarboxylic acid in the atmosphere, formic acid impacts secondary organic aerosol formation and acidity of wet deposition. Despite its significance in atmospheric chemistry, the sources/sinks and gas–particle partitioning of formic acid is not well-understood, especially in polluted environments. In this study, we simultaneously measured gaseous and particulate formic acid on an hourly basis, for the first time, in suburban Shanghai of East China. Both gaseous and particulate formic acid showed evident midday peaks in the summer, suggestive of photochemical production. Formic acid was dominated in the gas phase, with an average particulate fraction of 12.5 ± 4.9%. The thermodynamic model (E-AIM IV) significantly underestimated the concentrations of particulate formic acid, possibly as a result of the biased low effective Henry’s law constant and organic salt formation as well as the external mixing of formic acid with non-volatile cations. The partitioning of formic acid correlated well with the aerosol water content in the summer but not in the winter, which was possibly ascribed to the partitioning to the organic phase.