Influence of the Salting-out Effect of Ammonium Sulfate on the Gas-Particle Partitioning of Organic Compounds in the Atmosphere

Abstract:

As one of the major salts in the atmosphere, ammonium sulfate ((NH₄)₂SO₄) influences significantly the partitioning of organic compounds into atmospheric aqueous phases. This salting out effect has important implications regarding the reactivity, transport and fate of organic compounds in atmospheric waters, particularly in aerosol particles because of the relatively large amount of salts present. However, very few studies have quantified the salting out effect of (NH₄)₂SO₄. For the first time, we measured salting-out coefficients (or Setschenow constants) (K_S [M^-1]) for a large and diverse group of organic compounds in (NH₄)₂SO₄ solutions at a wide range of salt concentrations, providing a reliable database for model development. K_S values for (NH₄)₂SO₄ are correlated with and always higher than K_S for sodium chloride (NaCl), suggesting a higher salting-out effect of (NH₄)₂SO₄. Three modeling approaches: a poly-parameter linear free energy relationship (pp-LFER), the quantum-chemical COSMOtherm software and the group contribution method AIMOFAC, were calibrated and evaluated in terms of their capability to predict K_S. Relative merits of the prediction methods were identified. These models can be applied to estimate Setschenow constants for atmospherically relevant compounds involved in secondary organic aerosol formation based on chemical structure alone.