Chemical and Hygroscopic Characterization of Surface Salts in Qaidam Basin: Implications for Climates

Abstract

<p>Salt particles from saline lakes and playas play important roles in many processes related to atmospheric chemistry and climate system, especially active in aerosol and cloud formations due to their high hygroscopicity and efficient ice nucleation ability. However, physiochemical natures of these salts are relatively poorly understood due to their chemical complexity. Deepened understandings on playa-origin aerosol particles are desired and are expected to improve current climate models. Only a few studies have investigated the climate impacts of the salts from saline lakes and playas, from the perspectives of hygroscopicity, cloud condensation nuclei activity and ice nucleation ability.^1-3</p><p>In this study, the investigated salts are collected from the Qaidam Basin, which is one of the largest regions of saline lakes and playas on Earth. Four saline lake areas (Chaka, Keke, Qarhan and Mang&#8217;ai)&#160;are selected as the sampling sites, and four forms of samples are compared, including lake brines, crystalized brines, lakebed salts and crust salts. The cations (Na⁺, K⁺, Mg²⁺ and Ca²⁺) and anions (Cl^&#8722;, SO₄^2&#8722;, and NO₃^&#8722;) are simultaneously measured by the ion chromatography. A vapor sorption analyzer is used to measure the hygroscopic properties. To thoroughly understand the hygroscopic behaviors, the AIOMFAC model&#160;is used to predict the deliquescence RH (DRH) based on the chemical matrix of each sample.</p><p>From the ionic composition perspective, the crystalized brines and the brines show similar ionic textures, indicating that the crystalized brines well reflect the complex mineral composition of brines. In contrast, the natural solid salts, including lakebed salts and crust salts, show distinct mineral compositions from the brines, i.e., mainly NaCl, regardless of chemical composition of nearby lakes, suggesting that halite is the prevailing salt on the massive landscape exposed to the atmosphere. The hygroscopicity experimental results are well described by the AIOMFAC model, based on the ionic composition. The results show that the water uptake by crystalized salts is initialized by MgCl₂ at RH 30-40%. For natural salts, the hygroscopic behavior is similar to NaCl except for the QH lakebed salt, which is co-influenced by both NaCl and KCl. The findings presented in this study improve our understandings of the physical and chemical properties of surface salts spread across the Qaidam Basin, and the implications to climate systems on Planet Earth and Mars are discussed.</p><p>&#160;</p><p><strong>Reference</strong></p><p>¹K. A. Koehler et al., Journal of Geophysical Research: Atmospheres <strong>112</strong> (2007)</p><p>²K. A. Pratt et al., J. Geophys. Res.-Atmos. <strong>115</strong>, D15301 (2010) 17.</p><p>³M. Tang et al., Journal of Geophysical Research: Atmospheres <strong>124</strong> (2019) 10844.</p>