Chemical and Hygroscopic Characterization of Surface Salts in the Qaidam Basin: Implications for Climate Impacts on Planet Earth and Mars

Abstract

Salt particles play important roles in many atmospheric processes due to their high hygroscopicity. Saline lakes and playas are sources of salt particles, which are normally mixed with mineral dust that can be transported over long distances. In this study, salt samples collected from four saline lakes and surrounding playas in the Qaidam Basin are studied for their physicochemical properties, focusing on the chemical compositions and hygroscopicity. The salt samples include brines, lakebed salts, crust salts, and crystallized brines. Thermodynamic models are used to explain the precipitating and hygroscopic behaviors based on ionic compositions of dissolved saline solutions. Regarding the ionic compositions, the crystallized brines are similar to the complex mineral compositions of brines, while the natural solid salts, including lakebed salts and crust salts, show very distinct composition differences. The main difference between brines and natural solid salts is the presence of Mg2+ and SO4 2–, which are primarily found in brines but not in the solid phases. Moreover, all the crust salts are dominated by NaCl regardless of the chemical compositions of the nearby saline lakes. Positive matrix factorization is applied to the ionic concentrations, and the results show that solid salts and brines are governed by different factors. The pH of brines correlates with Mg2+ concentrations and is potentially influenced by ambient CO2 uptake. The hygroscopicity experimental results and thermodynamic model outcomes show that the water uptake of different salt types is controlled by different salt components, that is, crystallized brines are controlled by MgCl2 and natural salts are governed by NaCl and KCl. The characterizations of saline lake and playa salts improve the understandings of the roles that surface salts potentially play in the climate systems of both Planet Earth and Mars.

Surface solvation of Martian salt analogues at low relative humidities

“…In the latter case, reversible surface solvation was only observed when the RH approaches the deliquescence RH. Furthermore, no resolvable mass changes were observed for these RHs in a previously reported hygroscopicity study of the Qaidam salts, 9 which again supports the argument that the observed phenomena are surface specic.…”

“…In addition, no further changes are observed in the magnesium spectrum from RH = 5% to 20%, implying that even trace levels of water vapor (RH = 5%) are already enough to completely alter the physicochemical environment on the surface of the salt mixtures. Note that for these RHs, no detectable mass changes were reported from hygroscopicity measurements in a previous study, 9 indicating that this surface solvation is only occurring in the topmost surface layers.…”

“… 54 In a recent study on Qaidam salts, it was found that some saline lake brines contain relatively high contents of Mg 2+ and SO 4 2− but the naturally precipitated salts lack magnesium sulfates, which indicates that different mechanisms have influenced the salt compositions on Earth and Mars. 9 Chloride salt deposits were also found on the Martian surface, 55 but compared with Earth the coverages are smaller and mainly in or near craters. 56 This is because the chloride salts are concentrated in ponded evaporating brines owing to their high water solubility.…”

“… 57 Martian chloride is associated with Na, 56 similar to the NaCl dominated playa salt crusts in the Qaidam Basin. 9 One recently discovered phenomenon on Mars is the seasonal salty water flow, termed recurring slope lineae (RSL), 42 which is hypothesized to mainly be caused by chlorine-containing salts (particularly perchlorates) through deliquescence. 43 However, the required water vapor levels to allow deliquescence to occur are much higher than that in the present Martian atmosphere, and thus the RSL formation mechanism and the roles of salts need to be further understood.…”

“…Salt aerosol plays important roles in many processes related to atmospheric chemistry and the climate system, 1 especially as active components in aerosol and cloud formations 2,3 due to their high hygroscopicity. [4][5][6][7][8][9] An important and dynamic source of salt aerosols is saline lakes and playas, 10,11 which produce salt particles mixed with mineral dust that are transported over long distances. 5,7,[12][13][14][15][16][17][18] Furthermore, saline lakes and playas are becoming more relevant as they are continuously develop, in particular in the context of climate change.…”