A comprehensive study of hygroscopic properties of calcium-and magnesium-containing salts: Implication for hygroscopicity of mineral dust and sea salt aerosols

Abstract

Calcium- and magnesium-containing salts are important components for mineral dust and sea salt aerosols, but their physicochemical properties are not well understood yet. In this study, hygroscopic properties of eight Ca- and Mg-containing salts, including Ca(NO3)2 ĝš 4H2O, Mg(NO3)2 ĝš 6H2O, MgCl2 ĝš 6H2O, CaCl2 ĝš 6H2O, Ca(HCOO)2, Mg(HCOO)2 ĝš 2H2O, Ca(CH3COO)2 ĝš H2O and Mg(CH3COO)2 ĝš 4H2O, were investigated using two complementary techniques. A vapor sorption analyzer was used to measure the change of sample mass with relative humidity (RH) under isotherm conditions, and the deliquescence relative humidities (DRHs) for temperature in the range of 5-30 ĝ C as well as water-to-solute ratios as a function of RH at 5 and 25 ĝ C were reported for these eight compounds. DRH values showed large variation for these compounds; for example, at 25 ĝ C DRHs were measured to be ĝ1/4 28.5 % for CaCl2 ĝš 6H2O and 95 % for Ca(HCOO)2 and Mg(HCOO)2 ĝš 2H2O. We further found that the dependence of DRH on temperature can be approximated by the Clausius-Clapeyron equation. In addition, a humidity tandem differential mobility analyzer was used to measure the change in mobility diameter with RH (up to 90 %) at room temperature, in order to determine hygroscopic growth factors of aerosol particles generated by atomizing water solutions of these eight compounds. All the aerosol particles studied in this work, very likely to be amorphous under dry conditions, started to grow at very low RH (as low as 10 %) and showed continuous growth with RH. Hygroscopic growth factors at 90 % RH were found to range from 1.26 ± 0.04 for Ca(HCOO)2 to 1.79 ± 0.03 for Ca(NO3)2, and the single hygroscopicity parameter ranged from 0.09-0.13 for Ca(CH3COO)2 to 0.49-0.56 for Ca(NO3)2. Overall, our work provides a comprehensive investigation of hygroscopic properties of these Ca- and Mg-containing salts, largely improving our knowledge of the physicochemical properties of mineral dust and sea salt aerosols.