Formation of Sodium\nChloride on the Surface of Sulfate-Rich\nGobi Desert Salt in Response to Water Adsorption

Dust storms in arid regions transport desert salts and\ndust, affecting\ngeochemical processes, atmospheric chemistry, climate, and human health.\nThis study examines how the gas–salt interface composition\nof desert salt changes with varying relative humidity (RH), using\nambient pressure X-ray photoelectron spectroscopy (APXPS), near-edge\nX-ray absorption fine structure (NEXAFS) spectroscopy, and molecular\ndynamics (MD) simulations. Ion chromatography analysis of desert salt\nindicates it is predominantly composed of sulfate, sodium, and magnesium\nions, with traces of calcium, chloride, nitrate, and potassium ions.\nAPXPS and NEXAFS surface analyses show that, at 0% RH, the gas–salt\ninterface primarily features Na<sub>2</sub>SO<sub>4</sub>, with smaller\namounts of MgSO<sub>4</sub> and a trace of NaCl on the top layers.\nAs humidity increases, the composition at the gas–salt interface\nchanges, notably with Mg<sup>2+</sup> binding to SO<sub>4</sub><sup>2–</sup> ions and a dominant NaCl formation throughout the\nstudied surface depth. This shift indicates a transition from a sulfate-\nto a chloride-rich surface as humidity increases, contradicting MD\nsimulations that predicted that on salt crystals covered by a submonolayer\nof water with electrolytes, chloride ions migrate toward the liquid–solid\ninterface. This discrepancy indicates that other factors, like enhanced\nionic mobility at grain boundaries, might drive the accumulation of\nchloride ions at the gas interface. The study emphasizes the crucial\nrole of adsorbed water in ion migration and surface composition transformation\nof desert salts, affecting geochemical processes in arid regions.