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

Wang, Na, Jing, Bo, Wang, Pan, Wang, Zhen, Li, Jiarong, Pang, Shufeng, Zhang, Yunhong, Ge, Maofa
Environmental science & technology 2019 v.53 no.11 pp. 6225-6234
Fourier transform infrared spectroscopy, aerosols, ammonia, ammonium, ammonium sulfate, anions, chemical composition, dicarboxylic acids, hydrolysis, hygroscopicity, oxalates, reflectance, water solubility, water uptake, water vapor
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.