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 &gt; 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 &gt; 2, sulfate salts appear to dominate while at pH &lt; 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.

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

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