Minor Components in Natural Inorganic Aerosols Dominate Surface Composition: A Contrast Between Surface and Bulk

Aerosol surface composition plays a critical role in atmospheric chemistry but remains poorly characterized, especially for inorganic systems lacking organic surfactants.While bulk composition is typically measured, key interfacial processes, including heterogeneous reactions, water uptake, and cloud activation, occur at the surface.Here, we present the first depth-resolved, in-flight X-ray photoelectron spectroscopy (XPS) analysis of natural salt aerosols generated from hypersaline lake brines in the Qaidam Basin, a pristine region on the Qinghai-Tibet Plateau.These organic-poor aerosols offer a unique view of inorganic interfacial behavior.Using a synchrotron-based in-flight aerosol delivery system, we performed real-time, surface-specific analysis of freely suspended particles.Results reveal strong surface enrichment of minor ions such as Mg 2+ and SO 4 2-and depletion of major bulk ions like Na + and Cl -.This divergence was consistent across aerosol types and confirmed by ion chromatography of the source brines.A simplified two-layer model indicates a subnanometer surface layer that substantially alters surface identity.Minor components, often overlooked in bulk analyses, dominate the interface and may influence reactivity and cloud activation potential.These findings underscore the need to further investigate the role of surface composition in atmospheric aerosol processes.