Effects of inorganic salts and pH on the gas-aqueous partitioning of formic acid

Formic acid is one of the most abundant organic acids in the atmosphere. Formic acid is water-soluble and semi-volatile so it can partition to aqueous droplets and particles in the atmosphere. There is often a significant disparity between measurements and predictions of the abundance and gas-particle partitioning of formic acid. Inorganic salts in aqueous aerosols and cloud droplets may alter the partitioning of formic acid, potentially contributing uncertainty to these predictions. We conducted experiments using a dual mist-chamber setup to investigate the effects of salt identity, salt concentration, and pH on the partitioning of formic acid. We studied (NH₄)₂SO₄, MgSO₄, NaCl, MgCl₂, mixed salts ((NH₄)₂SO₄+ NaCl), and a sea water surrogate over the pH range 1 - 6. NaCl showed “salting-out” behavior at ionic strengths > 0.005 mol/kg and all pH values, while (NH₄)₂SO₄ exhibited “salting-in” at all pH and ionic strengths. In mixed salts, the additivity was found to be pH dependent: for pH > 2, sulfate salts appear to dominate while at pH < 2, chloride salts have a stronger effect. Monovalent and divalent cations also displayed non-uniform results that depend on the anion identity. For chloride salts, Mg²⁺ weakened the salting-out behavior in comparison to Na⁺, but for sulfate salts, we did not observe any effect of Mg²⁺ on formic acid partitioning compared with NH₄⁺. These observations demonstrate the importance of inorganic salts on the partitioning of organic compounds in the atmosphere, and also highlight the complexity of such organic-inorganic interactions.