The effect of relative humidity on CaCl₂ nanoparticles studied by soft X-ray absorption spectroscopy

Ca- and Cl-containing nanoparticles are common in atmosphere, originating for example from desert dust and sea water. The properties and effects on atmospheric processes of these aerosol particles depend on the relative humidity (RH) as they are often both hygroscopic and deliquescent. We present here a study of surface structure of free-flying CaCl₂ nanoparticles (CaCl₂-NPs) in the 100 nm size regime prepared at different humidity levels (RH: 11-85%). We also created mixed nanoparticles by aerosolizing a solution of CaCl₂ and phenylalanine (Phe), which is a hydrophobic amino acid present in atmosphere. Information of hydration state of CaCl₂-NPs and production of mixed CaCl₂ + Phe nanoparticles was obtained using soft X-ray absorption spectroscopy (XAS) at Ca 2p, Cl 2p, C 1s, and O 1s edges. We also report Ca 2p and Cl 2p X-ray absorption spectra of an aqueous CaCl₂ solution. The O 1s X-ray absorption spectra measured from hydrated CaCl₂-NPs resemble liquid-like water spectrum, which is heavily influenced by the presence of ions. Core level spectra of Ca²⁺ and Cl^- ions do not show a clear dependence of % RH, indicating that the first coordination shell remains similar in all measured hydrated CaCl₂-NPs, but they differ from aqueous solution and solid CaCl₂.