Pre-deliquescent water uptake in deposited nanoparticles observed with <i>in situ</i> ambient pressure X-ray photoelectron spectroscopy

Abstract. We study the adsorption of water onto deposited inorganic sodium chloride and organic malonic acid and sucrose nanoparticles at ambient water pressures corresponding to relative humidities (RH) from 0 to 16 %. To obtain information about water uptake at conditions where not accessible with typical aerosol instrumentation, we use surface-sensitive ambient pressure X-ray photoelectron spectroscopy (APXPS), which has a detection sensitivity from parts per thousand. Our results show that water is already adsorbed on sodium chloride particles at RH well below deliquescence, and that the chemical environment on the particle surface is changing with increasing humidity. While the sucrose particles exhibit only very modest changes on the surface at these relative humidities, the chemical composition and environment of malonic acid particle surfaces is clearly affected. Our observations indicate that water uptake by inorganic and organic aerosol particles could already have an impact on atmospheric chemistry at low relative humidities. We also conclude that the APXPS technique is a viable tool for studying chemical changes on the surfaces of atmospherically relevant aerosol particles which are not accessible with typical online mass- and volume-based methods.