Gas-particle partitioning of low-molecular-weight organic acids in suburban Shanghai: Insight into measured Henry's law constants dependent on relative humidity

Low-molecular-weight (LMW) organic acids are among the most abundant water-soluble organic compounds, but their gas-particle partitioning mechanism remains unclear. In the present study, LMW organic acids were measured using a URG 9000D Ambient Ion Monitor in suburban Shanghai. The average concentrations of formic acid, acetic acid, oxalic acid, and methanesulfonic acid (MSA) in PM2.5 were 405 ± 116, 413 ± 11, 475 ± 266, and 161 ± 54 ng m−3, respectively. The particle fraction exceeded 30 % for formic acid and acetic acid. Model predictions underestimated the particle-phase monocarboxylic acids (MCAs) from the factor of 102 at the highest RH to 107 at the lowest RH. The average measured intrinsic Henry's law constants (Hmea) for formic acid, acetic acid, oxalic acid, and MSA were 3.8 × 107, 4.5 × 107, 8.7 × 108, and 3.4 × 107 mol L−1 atm−1, respectively, approximately four orders of magnitude higher than their literature-based intrinsic Henry's law constants (Hlit) for MCAs and approximately four orders of magnitude lower than Hlit, MSA. The ratio of Hmea /Hlit for MCAs ranged over three orders of magnitude, depending on relative humidity. The strong deviations at low RHs are attributed to the dominance of absorption by the organic phase. The discrepancy at the highest RH possibly relates to surfactant effects and dimer formation. We used Hmea as a model input for the first time to estimate the phase partitioning of particulate MCAs, finding that >80 % of MCAs resided in the organic phase under dry conditions. We propose parameterizing Hmea as model input to predict the multiphase partitioning of MCAs. [Display omitted] •Measured Henry's law constant is proposed as model input to predict the partitioning of organic acids in the atmosphere.•We quantify the multiphase distribution of aerosol formic and acetic acids for the first time.•Over 80 % of aerosol formic and acetic acids reside in the organic phase under dry conditions.