Abstract
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⚫4H2O, Mg(NO3)2⚫6H2O, MgCl2⚫6H2O, CaCl2⚫6H2O, Ca(HCOO)2, Mg(HCOO)2⚫2H2O, Ca(CH3COO)2⚫H2O and Mg(CH3COO)2⚫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 ∘C as well as water-to-solute ratios as a function of RH at 5 and 25 ∘C were reported for these eight compounds. DRH values showed large variation for these compounds; for example, at 25 ∘C DRHs were measured to be ∼ 28.5 % for CaCl2⚫6H2O and >95 % for Ca(HCOO)2 and Mg(HCOO)2⚫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.
Highlights
Mineral dust, mainly emitted from arid and semiarid regions with an annual flux of ∼ 2000 Tg, is one of the most abundant types of aerosols in the troposphere (Textor et al, 2006; Ginoux et al, 2012)
We further found that the dependence of deliquescence relative humidities (DRHs) on temperature can be approximated by the Clausius–Clapeyron equation
First we investigated the effect of temperature on the DRH of Ca(NO3)2 q4H2O, Mg(NO3)2 q6H2O and MgCl2 q6H2O, which are the most stable forms of corresponding salts for the temperature range (5–30 ◦C) considered in this work (Kelly and Wexler, 2005)
Summary
Mainly emitted from arid and semiarid regions with an annual flux of ∼ 2000 Tg, is one of the most abundant types of aerosols in the troposphere (Textor et al, 2006; Ginoux et al, 2012). Mineral dust particles contain substantial amounts of carbonates, including CaCO3 (calcite) and CaMg(CO3) (dolomite) (Nickovic et al, 2012; Formenti et al, 2014; Jeong and Achterberg, 2014; Journet et al, 2014; Scanza et al, 2015) These carbonates are largely insoluble and have very low hygroscopicity (Sullivan et al, 2009; Tang et al, 2016a); their reactions with acidic gases in the troposphere can form Ca- and Mg-containing salts with higher hygroscopicity (Gibson et al, 2006; Liu et al, 2008b; Sullivan et al, 2009; Tang et al, 2016a), such as Ca(NO3) and Mg(NO3). The CCN activity of saline mineral dust was explored (Gaston et al, 2017), and good correlations were found between the CCN activities of saline mineral dust particles and the abundance of the soluble components (e.g., CaCl2) they contained
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