Abstract
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.
Highlights
Inorganic ions and salts are essential for life on Earth
To the best of our knowledge, similar sizedependent data does not exist for CaCl2 particles, but in contrast to pure NaCl, the data reported for marine chloride mixtures reports lower deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) values for 100 nm particles compared to supermicrometer particles.[16]
This study demonstrates novel use of so X-ray absorption spectroscopy to investigate hydration of free- ying salt NPs and their mixtures with amino acids
Summary
Inorganic ions and salts are essential for life on Earth. They play key roles in biological processes of living organisms, e.g. ClÀ regulates the osmotic pressure in the cell and Ca2+ is essential for intracellular signaling.[1]. We added phenylalanine (Phe) to the solution as a model biomolecule, representing an amino acid with a hydrophobic side essential for many processes in the human body,[30,31] and found in atmosphere.[32,33,34] By using X-ray photoelectron spectroscopy, we recently observed that the addition of an organic acid to an aerosolized salt solution can drastically change the surface composition of dried NPs.[35] Here, we observed that Phe and CaCl2 create mixed hydrated NPs. The XAS recorded at C 1s and O 1s edges agree in shape with previously reported solid state spectra of Phe, but relative energy shi s in bound and continuum resonance energies indicate interaction with a high ion concentration solvent and shortening of bond lengths
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