Hygroscopicity and Compositional Evolution of Atmospheric Aerosols Containing Water-Soluble Carboxylic Acid Salts and Ammonium Sulfate: Influence of Ammonium Depletion

Abstract

Water-soluble organic acid salts are important components of atmospheric aerosols. Despite their importance, it is still not clear how water-soluble organic acid salts influence interactions between aerosols and water vapor in the atmosphere. In this study, the hygroscopic behaviors and chemical compositions of aerosol particles containing water-soluble organic acid salt ((CH₂) _n(COONa)₂, n = 0, 1, 2) and (NH₄)₂SO₄ were measured using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The ammonium depletion due to release of gaseous NH₃ was found in mixed aerosols composed of (CH₂) _n(COONa)₂ ( n = 1, 2) and (NH₄)₂SO₄ upon dehydration. The ammonium loss could modify the aerosol composition, resulting in the formation of corresponding organic acid and monosodium dicarboxylate in mixed particles with high and low (NH₄)₂SO₄ content, respectively. Due to the weaker hydrolysis of oxalate anions, the ammonium depletion was not observed for the Na₂C₂O₄/(NH₄)₂SO₄ mixtures. The changes in the particle composition led to the decreased water uptake upon hydration as compared to that upon dehydration. Our findings reveal that interactions between water-soluble organic acid salts and (NH₄)₂SO₄ in aqueous aerosols may affect the repartition of NH₃ between the condensed and gas phases, thus modifying composition and physicochemical properties of aerosols as well as relevant chemical processes.