Measurements of the water activity-dependent viscosity of aerosol particles from two techniques are compared, specifically from the coalescence of two droplets in holographic optical tweezers (HOT) and poke-and-flow experiments on particles deposited onto a glass substrate. These new data are also compared with the fitting of dimer coagulation, isolation, and coalescence (DCIC) measurements. The aerosol system considered in this work are ternary mixtures of sucrose-citric acid-water and sucrose-NaNO3-water, at varying solute mass ratios. Results from HOT and poke-and-flow are in excellent agreement over their overlapping range of applicability (∼103-107 Pa s); fitted curves from DCIC data show variable agreement with the other two techniques because of the sensitivity of the applied modeling framework to the representation of water content in the particles. Further, two modeling approaches for the predictions of the water activity-dependent viscosity of these ternary systems are evaluated. We show that it is possible to represent their viscosity with relatively simple mixing rules applied to the subcooled viscosity values of each component or to the viscosity of the corresponding binary mixtures.

Information on the rate of diffusion of organic molecules within secondary organic aerosol (SOA) is needed to accurately predict the effects of SOA on climate and air quality. Diffusion can be important for predicting the growth, evaporation, and reaction rates of SOA under certain atmospheric conditions. Often, researchers have predicted diffusion rates of organic molecules within SOA using measurements of viscosity and the Stokes-Einstein relation (D ∝ 1/η, where D is the diffusion coefficient and η is viscosity). However, the accuracy of this relation for predicting diffusion in SOA remains uncertain. Using rectangular area fluorescence recovery after photobleaching (rFRAP), we determined diffusion coefficients of fluorescent organic molecules over 8 orders in magnitude in proxies of SOA including citric acid, sorbitol, and a sucrose-citric acid mixture. These results were combined with literature data to evaluate the Stokes-Einstein relation for predicting the diffusion of organic molecules in SOA. Although almost all the data agree with the Stokes-Einstein relation within a factor of 10, a fractional Stokes-Einstein relation (D ∝ 1/ηξ) with ξ = 0:93 is a better model for predicting the diffusion of organic molecules in the SOA proxies studied. In addition, based on the output from a chemical transport model, the Stokes-Einstein relation can overpredict mixing times of organic molecules within SOA by as much as 1 order of magnitude at an altitude of ∼ 3 km compared to the fractional Stokes-Einstein relation with ξ = 0.93. These results also have implications for other areas such as in food sciences and the preservation of biomolecules.

Information on liquid-liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ∼ 70% to ∼ 100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the presence of an organic-rich outer phase at high-RH values can lower the supersaturation with respect to water required for cloud droplet formation. At ≤ 10%RH, the viscosity was ≥ 1×108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38 %-50%RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O V C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes-Einstein relation, at ≤ 10%RH diffusion coefficients of organics within diesel fuel SOA is ≤ 5:4×10-17 cm2 s-1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) is 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low-RH conditions and on timescales of minutes to hours. At 38 %-50%RH, the calculated organic diffusion coefficients are in the range of 5:4×10-17 to 1:8×10-13 cm2 s-1 and calculated τmixing values are in the range of ∼ 0:01 h to ∼ 50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.