Abstract
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 adsorption at conditions which are not accessible with typical aerosol instrumentation, we use surface-sensitive ambient pressure X-ray photoelectron spectroscopy (APXPS), which has a detection sensitivity starting at 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 establish the APXPS technique as a viable tool for studying chemical changes on the surfaces of atmospherically relevant aerosol particles which are not detected with typical online mass- and volume-based methods.
Highlights
The interaction between atmospheric particulate matter and water is one of the most important processes in Earth’s atmosphere
The main goal of this work has been to investigate the potential for obtaining meaningful results with the ambient pressure X-ray photoelectron spectroscopy (APXPS) technique for aerosol samples comprising simple but atmospherically relevant chemical components
We studied water adsorption onto Na 1s and Cl 2p (NaCl), sucrose and malonic acid aerosol particles deposited on silicon and gold substrates using the APXPS with a laboratory X-ray source (Al Kα and Mg Kα)
Summary
The interaction between atmospheric particulate matter and water is one of the most important processes in Earth’s atmosphere. Condensed water in the atmosphere critically influences both direct and indirect climate effects of aerosols, governed by aerosol growth and light scattering and via the activation of aerosol particles into cloud droplets (Kreidenweis and AsaAwuku, 2014). The surfaces of aerosol particles and droplets are distinct physical and chemical environments compared to their associated bulk phases. For cloud and fog systems where the interfacial region makes up a significant fraction of the condensed aqueous phase, the reaction rate at the surface can be the rate-limiting step in multi-phase OH oxidation involving surface-active organic species such as pinonic acid (Huang et al, 2018). Interfacial water molecules can promote reactions between organic acids and SO3, which are distinct from those that occur in the gas phase and important for heterogeneous formation of H2SO4 and subsequent new particle formation (Zhong et al, 2019; Lv and Sun, 2020).
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