Exploring the water structure on atmospheric particle surfaces

Water is prevalent on all atmospheric particle surfaces, independent of material, phase state or water vapor saturation ratio. Under subsaturated relative humidity conditions, water occurs as reversibly adsorbed layers on solid surfaces or as concentrated aqueous solutions in aqueous particles, exhibiting different composition at the interface than in the bulk. The hydrogen bonding structure at these interfaces is affected by interactions with the substrate on solid surfaces and by ions and ionic or neutral surfactants on aqueous solutions. The water structure and its interplay with hydrated substrate features, hydrated free ions or neutral solutes or surfactant head groups are playing a key role in many chemical and physical processes at these interfaces. We have developed X-ray photoelectron spectroscopy (XPS) and electron yield near edge X-ray absorption fine structure (NEXAFS) spectroscopy to explore the interfacial water structure in situ. We will present examples of the structure of adsorbed water on different Feldspar surfaces, on silver iodide and titanium dioxide. We will also show cases related to the water structure on aqueous solutions containing salts or different surfactant species.