Measurements and Predictions of Binary Component Aerosol Particle Viscosity

Young Chul Song, Allen Haddrell, Bryan Bzdek, Jonathan Reid, Thomas Bannan, David Topping, Carl J. Percival, Chen Cai

Research output: Contribution to journalArticle (Academic Journal)peer-review

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Organic aerosol particles are known to often absorb/desorb water continuously with change in gas phase relative humidity (RH) without crystallisation. Indeed, the prevalence of metastable ultraviscous liquid or amorphous phases in aerosol is well-established with solutes often far exceeding bulk phase solubility limits. Particles are expected to become increasingly viscous with drying, a consequence of the plasticising effect of water. We report here measurements of the variation in aerosol particle viscosity with RH (equal to condensed phase water activity) for a range of organic solutes including alcohols (diols to hexols), saccharides (mono-, di- and tri-) and carboxylic acids (di-, tri- and mixtures). Particle viscosities are measured over a wide range (10-3 to 1010 Pa s) using aerosol optical tweezers, inferring the viscosity from the timescale for a composite particle to relax to a perfect sphere following the coalescence of two particles. Aerosol measurements compare well with bulk phase studies (well-within an order of magnitude deviation at worst) over ranges of water activity accessible to both. Predictions of pure component viscosity from group contribution approaches combined with either non-ideal or ideal mixing reproduce the RH-dependent trends particularly well for the alcohol, di- and tri-carboxylic acid systems extending up to viscosities of 104 Pa s. By contrast, predictions over-estimate the viscosity by many orders of magnitude for the mono-, di-, and tri-saccharide systems, components for which the pure component sub-cooled melt viscosities are >>1012 Pa s. When combined with a typical scheme for simulating the oxidation of α-pinene, a typical atmospheric pathway to secondary organic aerosol (SOA), these predictive tools suggest that the pure component viscosities are less than 106 Pa s for ~97% of the 50,000 chemical products included in the scheme. These component viscosities are consistent with the conclusion that the viscosity of α-pinene SOA is most likely in the range 105 to 108 Pa s. Potential improvements to the group contribution predictive tools for pure component viscosities are considered.
Original languageEnglish
Pages (from-to)8123–8137
Number of pages5
JournalJournal of Physical Chemistry A
Issue number41
Early online date29 Sept 2016
Publication statusPublished - 20 Oct 2016


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