Effect of varying experimental conditions on the viscosity of alpha-pinene derived secondary organic material

Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of alpha-pinene at < aEuro-5aEuro-% relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (tau(exp, flow)) of SOM after poking the material with a needle. In the first set of experiments, we show that tau(exp, flow) increased by a factor of 3600 as the RH increased from < aEuro-0.5 RH to 50aEuro-% RH, for SOM with a production mass concentration of 121aEuro-A mu gaEuro-m(-3). Based on simulations, the viscosities of the particles were between 6aEuro-aEuro parts per thousand x aEuro-10(5) and 5aEuro-aEuro parts per thousand x aEuro-10(7)aEuro-PaaEuro-s at < aEuro-0.5aEuro-% RH and between 3aEuro-aEuro parts per thousand x aEuro-10(2) and 9aEuro-aEuro parts per thousand x aEuro-10(3)aEuro-PaaEuro-s at 50aEuro-% RH. In the second set of experiments we show that under dry conditions tau(exp, flow) decreased by a factor of 45 as the production mass concentration increased from 121 to 14aEuro-000aEuro-A mu gaEuro-m(-3). From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14aEuro-000aEuro-A mu gaEuro-m(-3) was determined to be between 4aEuro-aEuro parts per thousand x aEuro-10(4) and 1.5aEuro-aEuro parts per thousand x aEuro-10(6)aEuro-PaaEuro-s compared to between 6aEuro-aEuro parts per thousand x aEuro-10(5) and 5aEuro-aEuro parts per thousand x aEuro-10(7)aEuro-PaaEuro-s for SOM with a production mass concentration of 121aEuro-A mu gaEuro-m(-3). The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere.