Reconciling atmospheric water uptake by hydrate forming salts

• Published in: Environmental science--processes & impacts Vol. 22; no. 8; pp. 1759 - 1767
• Main Authors: Rosati, Bernadette, Paul, Andreas, Iversen, Emil Mark, Massling, Andreas, Bilde, Merete
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 01.08.2020
• Subjects: Aerosols; Aqueous solutions; Atmospheric aerosols; Atmospheric models; Atmospheric water; Calcium; Calcium chloride; Growth factors; Hydrates; Hygroscopicity; Magnesium; Magnesium chloride; Nanoparticles; Salts; Supersaturation; Thermodynamic models; Water uptake
• ISSN: 2050-7887; 2050-7895
• DOI: 10.1039/d0em00179a

Magnesium and calcium chloride salts contribute to the global atmospheric aerosol burden via emission of sea spray and mineral dust. Their influence on aerosol hygroscopicity and cloud forming potential is important but uncertain with ambiguities between results reported in the literature. To address this, we have conducted measurements of the hygroscopic growth and critical supersaturation of dried, size selected nano-particles made from aqueous solution droplets of MgCl 2 and CaCl 2 , respectively, and compare experimentally derived values with results from state-of-the-art thermodynamic modelling. It is characteristic of both MgCl 2 and CaCl 2 salts that they bind water in the form of hydrates under a range of ambient conditions. We discuss how hydrate formation affects the particles' water uptake and provide an expression for hydrate correction factors needed in calculations of hygroscopic growth factors, critical super-saturations, and derived κ values of particles containing hydrate forming salts. We demonstrate the importance of accounting for hydrate forming salts when predicting hygroscopic properties of sea spray aerosol. Magnesium and calcium chloride salts contribute to the global atmospheric aerosol burden via emission of sea spray and mineral dust.