Measurements and Predictions of Binary Component Aerosol Particle Viscosity

Abstract

Organic aerosol particles are known to often absorb/desorb water continuously with change in gas phase relative humidity (RH) without crystallization. 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 plasticizing 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 to 10 Pa s) using aerosol optical tweezers, inferring the viscosity from the time scale 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 nonideal or ideal mixing reproduce the RH-dependent trends particularly well for the alcohol, di-, and tricarboxylic acid systems extending up to viscosities of 10 Pa s. By contrast, predictions overestimate the viscosity by many orders of magnitude for the mono-, di-, and trisaccharide systems, components for which the pure component subcooled melt viscosities are ≫10 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 10 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 10 to 10 Pa s. Potential improvements to the group contribution predictive tools for pure component viscosities are considered.

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

“…To correct for a possible bias caused by a plasticizing effect of PEG-4, rather than comparing data points of the same RH, the Stokes-Einstein comparison presented here is based on the sucrose mole fraction x suc . This is motivated by a recent study of Song et al (2016) who investigated the viscosity of mixtures and tested amongst other approaches the validity of a mixing rule presented by Bosse (2005), which is mole fraction based and uses pure component viscosities to predict viscosities of binary mixtures. Using ideal mixing as in Bosse (2005) yields good results for aqueous sucrose and suggests that, if applicable to our ternary system, sucrose provides the dominant contribution to the mixture viscosity due to its very high "pure component" viscosity, and the contribution of PEG-4 can be treated as water in first approximation.…”

“…Using ideal mixing as in Bosse (2005) yields good results for aqueous sucrose and suggests that, if applicable to our ternary system, sucrose provides the dominant contribution to the mixture viscosity due to its very high "pure component" viscosity, and the contribution of PEG-4 can be treated as water in first approximation. Based on this approach, our 19.5 • C data correspond to a viscosity range of 10 5 -10 8 Pa s. The room temperature RH-based viscosity parametrization presented by Song et al (2016; which includes the Power et al, 2013) data corresponding to water activities as low as 0.3) was plotted as a function of x suc using the Zobrist et al (2011) a w parametrization (Stokes-Einstein-derived diffusivities given as purple curve). In comparison to our 19.5 • C measurement, Stokes-Einstein underestimates D PEG by about a factor of ∼ 140-600 over the whole experimental range.…”

“…The break-down of Stokes-Einstein seems much more pronounced if the measured diffusivities are compared to the temperature dependent Williams-Landel-Ferry (WLF) parametrization by Quintas et al (2006) (∼ 3-4 orders of magnitude). Note that, in contrast to the Song et al (2016) parametrization, we extrapolate far beyond the scope of the experimental data the parametrization is based on (see blue, 10 • C, and red, 20 • C, hollow symbols in Fig. 8).…”

α-pinene secondary organic aerosol at low temperature: Chemical composition and implications for particle viscosity

“…Observations of particle shape transformations , coalescence times (Pajunoja et al, 2014), and the particle bounce factor (BF) (Virtanen et al, 2010;Pajunoja et al, 2015) are other parameters used to indicate the phase 90 state and viscosity of particles. At dry conditions and at temperatures close to room temperature, the viscosity of α-pinene SOA is assumed to range from 10 5 to (higher than) 10 8 Pa s (Song et al, 2016;Renbaum-Wolff et al, 2013;Pajunoja et al, 2014), which corresponds to a semisolid state (Shiraiwa et al, 2011), whereas at a RH of about 90 % and room temperature its consistency is comparable to that of honey (~10 Pa s) (Renbaum-Wolff et al, 2013). Generally, SOA is more viscous in cool and dry conditions (shown e.g.…”

“…Generally, SOA is more viscous in cool and dry conditions (shown e.g. for α-pinene SOA at temperatures ranging from 235 K to 295 K and RH ranging from 35 95 to 90 %, Song et al, 2016;Shiraiwa et al, 2011;Wang et al, 2015;Kidd et al, 2014).…”