Surface migration of chloride ions on natural salts by ammonia and water vapor under low RH conditions

Natural salts are minerals formed through natural processes, generally consisting of complex mixtures of ions. In the atmosphere, natural salts are present as aerosol particles, originating from both marine and terrestrial sources. The interaction of these natural salt particles with trace gases, also termed ageing, can influence various atmospheric processes such as cloud properties or atmospheric chemistry. However, our understanding of these interactions, particularly at low relative humidity (RH) well below deliquescence conditions, when particles are in the solid state, remains limited. This study investigates phenomena occurring on surfaces of two polycrystalline solid natural salt samples upon exposure to ammonia (NH₃) and low RH conditions. Surface structures and chemical compositions, at nanometre-scale depths, were measured using X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy at a synchrotron-based ambient pressure XPS (APXPS) endstation. The findings indicate that the addition of NH₃ to a humidified atmosphere leads to an enrichment of chloride (Cl^-) on the surface of the solid natural salt samples. We propose that the formation of ammonium (NH₄⁺) from NH₃ uptake on the sample surfaces causes the migration of Cl^- to the surface, which appears to outcompete other anions such as sulfate (SO₄^2-) for cation-anion neutralization. This surface enrichment of Cl^- indicates potential dechlorination processes, highlighting how anthropogenic emissions, here NH₃, can affect the properties of solid natural salt particles.