Organic aerosols can exist in highly viscous or glassy phase states. A viscosity database for organic compounds with atmospherically relevant functional groups is compiled and analyzed to quantify the influence of number and location of functional groups on viscosity. For weakly functionalized compounds the trend in viscosity sensitivity to functional group addition is carboxylic acid (COOH) ≈ hydroxyl (OH) > nitrate (ONO) > carbonyl (CO) ≈ ester (COO) > methylene (CH). Sensitivities to group addition increase with greater levels of prior functionalization and decreasing temperature. For carboxylic acids a sharp increase in sensitivity is likely present already at the second addition at room temperature. Ring structures increase viscosity relative to linear structures. Sensitivities are correlated with analogously derived sensitivities of vapor pressure reduction. This may be exploited in the future to predict viscosity in numerical models by piggybacking on schemes that track the evolution of organic aerosol volatility with age.

A Dimer Coagulation, Isolation and Coalescence (DCIC) technique is used to probe the phase behaviour and glass transition temperatures of ternary aerosol mixtures. The DCIC technique is used to perform temperature and relative humidity dependent viscosity measurements at viscosities near 5 × 106 Pa s. Measurements include organic-organic and organic-inorganic mixtures composed of sucrose-citric acid and sucrose-sodium nitrate. The data reported here add additional insight into the wide discrepancies in glass transition temperatures reported for pure sodium nitrate. The phase diagram model used in the work of Rothfuss and Petters (Phys. Chem. Chem. Phys., 2017, 19, 6532-6545) is expanded to include multiple solute components. Data and model predictions of the mixtures are in good agreement with the modified model. These measurements are compared with values from Holographic Optical Tweezer (HOT) measurements taken at room temperature. Overall, the viscosities determined from the DCIC and HOT techniques are in good agreement.

Atmospheric aerosols can exist in amorphous semi-solid or glassy phase states. These states are determined by the temperature (T) and relative humidity (RH). New measurements of viscosity for amorphous semi-solid nanometer size sucrose particles as a function of T and RH are reported. Viscosity is measured by inducing coagulation between two particles and probing the thermodynamic states that induce the particle to relax into a sphere. It is shown that the glass transition temperature can be obtained by extrapolation to 10 Pa s from the measured temperature-dependent viscosity in the 10 to 10 Pa s range. The experimental methodology was refined to allow isothermal probing of RH dependence and to increase the range of temperatures over which the dry temperature dependence can be studied. Several experiments where one monomer was sodium dodecyl sulfate (SDS), which remains solid at high RH, are also reported. These sucrose-SDS dimers were observed to relax into a sphere at T and RH similar to those observed in sucrose-sucrose dimers, suggesting that amorphous sucrose will flow over an insoluble particle at a viscosity similar to that characteristic of coalescence between two sucrose particles. Possible physical and analytical implications of this observation are considered. The data reported here suggest that semi-solid viscosity between 10 and 10 Pa s can be modelled over a wide range of T and RH using an adapted Vogel-Fulcher-Tammann equation and the Gordon-Taylor mixing rule. Sensitivity of modelled viscosity to variations in dry glass transition temperature, Gordon-Taylor constant, and aerosol hygroscopicity are explored, along with implications for atmospheric processes such as ice nucleation of glassy organic aerosols in the upper free troposphere. The reported measurement and modelling framework provides a template for characterizing the phase diagram of other amorphous aerosol systems, including secondary organic aerosols.

Viscosity of atmospheric aerosol spans at least 15 orders of magnitude, from thin liquids to glassy solids, with possible concomitant impact on multiple processes of meteorological and/or climatological concern. Recently there has been interest in aerosol phase assessment techniques based upon dimer coalescence. Theoretical treatment suggests discernible reductions in dimer diameter begin when viscosity »10 8 Pa¢s and the dimer is spherical at »10 5 Pa¢s for submicron particles, or the middle range of the semisolid regime. A method using nanoparticle dimers synthesized by utilizing differential mobility analyzers of opposite polarity to produce monomers of opposite charge that subsequently undergo electrostatically mediated coagulation has been developed and is detailed in this work. This method was used to assess the aerosol phase state of several atmospherically relevant organic species and inorganic salts at relative humidity (RH) values ranging between 10% and 100%. Ammonium sulfate, monosodium a-ketoglutaric acid, sodium chloride, and sucrose all displayed RH-dependent phase state. These observed viscous transitions occurred at RH values less than existing deliquescence RH data, a result consistent with existing literature reports of RH-induced structural rearrangements. Fully coalesced and fully uncoalesced diameters could be fitted to single values, indicating that the presented technique is absolute. The method was also used to assess the phase state of dry sucrose aerosol at temperatures between 20 C and 70 C. A phase transition was noted at 63.7 C § 4.4 C, near the glass transition temperature, suggesting the presented method may also be useful for probing phase responses to temperature perturbations.