Effects of Inorganic Salts on the Gas-Aqueous Partitioning of Formic Acid and Acetic Acid

Formic acid (FA) and acetic acid (AA) are abundant atmospheric carboxylic acids that play important roles in wet deposition acidity and secondary organic aerosol (SOA) formation. Despite their significance, observations of FA and AA gas-particle partitioning differ substantially from thermodynamic predictions, contributing to uncertainties in SOA burden estimates. In this study, we systematically investigate the effects of sodium chloride (NaCl) and ammonium sulfate ((NH4)2SO4) on the partitioning of FA and AA. Our results reveal a consistent salting-out effect for both compounds in NaCl solutions, with Setschenow constants of 0.031 ± 0.002 m-1 for FA and 0.066 ± 0.007 m-1for AA. In contrast, (NH4)2SO4 was found to have minimal impact on FA and AA partitioning at concentrations up to 2.5 mol kg–1. pH-dependent experiments show a modest effect of acidity on FA and AA partitioning in the range of pH 1.5–3.5, though pH is predicted to impart a strong effect at pH levels above 3.5. These findings demonstrate that both ionic strength and pH influence gas-aqueous partitioning of small organic acids; however, the magnitude of observed salting effects is insufficient to fully explain the discrepancy between measured and predicted gas-particle partitioning of FA and AA in the atmosphere.