Measurements and Predictions of Binary Component Aerosol Particle Viscosity.
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 (10-3to 1010Pa 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 104Pa 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 ≫1012Pa 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 106Pa 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 105to 108Pa s. Potential improvements to the group contribution predictive tools for pure component viscosities are considered.
School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
Young Chul
YC
Song
School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
Allen E
AE
Haddrell
School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
Bryan R
BR
Bzdek
School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
Jonathan P
JP
Reid
School of Earth, Atmospheric and Environmental Science, University of Manchester , Manchester, M13 9PL, United Kingdom.
Thomas
T
Bannan
School of Earth, Atmospheric and Environmental Science, University of Manchester , Manchester, M13 9PL, United Kingdom.|National Centre for Atmospheric Science, University of Manchester , Manchester, M13 9PL, United Kingdom.
David O
DO
Topping
School of Earth, Atmospheric and Environmental Science, University of Manchester , Manchester, M13 9PL, United Kingdom.
Carl
C
Percival
The Institute of Chemical Physics, Key Laboratory of Cluster Science, Beijing Institute of Technology , Beijing 100081, People's Republic of China.
Chen
C
Cai