Hygroscopic growth and cloud droplet activation of xanthan gum as a proxy for marine hydrogels. Issue 19 (13th October 2016)

• Record Type: Journal Article
• Authors: Dawson, K. W.; Petters, M. D.; Meskhidze, N.; Petters, S. Suda; Kreidenweis, S. M.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 19(2016)
• Page Start: 11, 803
• Page End: 11, 818
• Publication Date: 2016-10-13
• Subjects: aerosol water uptake -- cloud condensation nuclei -- aerosol hygroscopicity -- marine organic aerosol -- xanthan gum -- hydrogel; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2016JD025143
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Knowledge of the physical characteristics and chemical composition of marine organic aerosols is needed for the quantification of their effects on cloud microphysical processes and solar radiative transfer. Here we use xanthan gum (XG)—a bacterial biopolymer—as a proxy for marine hydrogels. Measurements were performed for pure XG particles and mixtures of XG with sodium chloride, calcium nitrate, and calcium carbonate. The aerosol hygroscopicity parameter ( κ ) is derived from hygroscopic growth factor measurements ( κ gf ) at variable water activity ( aw ) and from cloud condensation nuclei activation efficiency ( κ ccn ). The Zdanovskii, Stokes, and Robinson (ZSR) hygroscopicity parameter derived for multicomponent systems ( κ mix, sol ) is used to compare measurements of κ gf and κ ccn . Pure XG shows close agreement of κ gf (at aw = 0.9) and κ ccn of 0.09 and 0.10, respectively. Adding salts to the system results in deviations of κ gf (at aw = 0.9) from κ ccn . The measured κ gf and ZSR‐derived hygroscopicity parameter ( κ mix, sol ) values for different solutions show close agreement at aw > 0.9, while κ gf is lower in comparison to κ mix, sol at aw < 0.9. The differences between predicted κ mix, sol and measured κ gf and κ ccn values are explained by the effects of hydration and presence of salt ions on the structure of the polymer networks. Results from this study imply that at supersaturations of 0.1 and 0.5%, the presence of 30% sea salt by mass can reduce the activation diameter of pure primary marine organic aerosols from 257 to 156 nm and from 87 to 53 nm, respectively. Key Points: Pure XG particles activate as CCN consistent with kappa = 0.1 CCN‐derived kappa is about 40% higher compared to HTDMA derived kappa for xanthan gum salt mixtures at RH = 90% Polymer/salt cross‐links may suppress hygroscopicity relative to the ZSR prediction at low water activity