Humidity-Driven Chloride Protonation and Surface Enrichment on Natural Salt Surfaces near Deliquescence.

Natural salts are ubiquitous in saline environments and atmospheric aerosols, where their surface chemistry influences multiphase reactions and chlorine activation. The release of chloride as reactive chlorine species affects tropospheric oxidation and ozone chemistry, yet the molecular-level processes governing chloride speciation near deliquescence remain poorly understood. Here, we investigate humidity-driven interfacial chemistry of a natural chloride-rich salt using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. By systematically varying relative humidity (RH) near the deliquescence threshold of MgCl2·6H2O, we observe reversible changes in chloride speciation prior to the formation of a fully deliquesced bulk phase. Hydration induces pronounced surface enrichment and redistribution of chlorine-containing species, accompanied by the emergence of molecular HCl at the interface. This enrichment increases the surface availability of HCl-like species, suggesting a greater potential for gas-phase release under fluctuating atmospheric conditions. Depth-resolved measurements show that these transformations are confined to the outermost nanometers and are fully reversible upon dehydration. Complementary NEXAFS measurements reveal the structural transition from a crystalline salt to a hydrated and ultimately aqueous phase. These results demonstrate that RH-driven phase transitions in hygroscopic components such as MgCl2 create reactive, transient interfaces that govern chloride chemistry and may enhance chlorine activation under atmospherically relevant humidity fluctuations.