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Insight into the Role of Water-Soluble Organic Solvents for the Cloud Condensation Nuclei Activation of Cholesterol

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journal contribution
posted on 31.07.2019, 15:08 by Farima Barati, Qi Yao, Akua A. Asa-Awuku
Organic aerosol is ubiquitous, and partially soluble organic particles can uptake water, form droplets, and act as cloud condensation nuclei (CCN). Cholesterol is a well-known organic aerosol. Cholesterol is insoluble in water (<0.002 g in 100 mL of H2O at 293 K) but readily dissolves in organic solvents. In this study, we examine the ability of cholesterol generated in 7.2%, 10.4%, and 18.9% (by volume) dilutions of three water-soluble organic solvents (ethanol, 2-propanol, and acetone) to act as CCN. The measured apparent particle hygroscopicity, κ, can vary over 2 orders of magnitude, from ∼0.001 to 0.1. We use statistical analysis of variance (ANOVA) to quantify experimental design factors, not explicitly addressed in traditional theory, that modify κ-values. Results show that (i) particle sizes (electrical mobility and aerodynamic diameters) are important for apparent hygroscopicity κ-values and that (ii) atomized aerosol formed in the presence of organic solvents modifies particle size and droplet surface tension at the air–water interface and promotes droplet formation. As the volume of water (dilution) in the atomized solution increases, the organic solvent decreases and κ decreases. Increases in organic solvent concentration decrease droplet surface tension and change aerosol shape. Thus, the apparent κ is corrected with surface tension and shape factor data. The results imply that miscible atmospheric organic solvents that readily adsorb and solvate in water enhance the droplet forming potential of cholesterol and may thus extend to other atmospheric water-insoluble organic particles.