Abstract
Abstract. 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 α-pinene at < 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τexp, flow) of SOM after poking the material with a needle. In the first set of experiments, we show that τexp, flow increased by a factor of 3600 as the RH increased from < 0.5 RH to 50 % RH, for SOM with a production mass concentration of 121 µg m−3. Based on simulations, the viscosities of the particles were between 6 × 105 and 5 × 107 Pa s at < 0.5 % RH and between 3 × 102 and 9 × 103 Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τexp, flow decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 µg m−3. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 µg m−3 was determined to be between 4 × 104 and 1.5 × 106 Pa s compared to between 6 × 105 and 5 × 107 Pa s for SOM with a production mass concentration of 121 µg 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.
Highlights
Aerosol particles are ubiquitous in the Earth’s lower atmosphere, of which a major source is the oxidation of volatile organic compounds (VOCs; Andreae and Crutzen, 1997)
Based on simulations of the poke-and-flow experiments the viscosities of the secondary organic material (SOM) samples are between 4 × 104 and 1.5 × 106 Pa s for SOM produced at a production mass concentration of 14 000 μg m−3 and between 6 × 105 and 5 × 107 Pa s for SOM produced at a production mass concentration of 121 μg m−3 (Fig. 5b)
The inverse relationship between viscosity and production mass concentration suggests the results determined here likely represent a lower limit of viscosity for SOM produced by the ozonolysis of α-pinene in the atmosphere
Summary
Aerosol particles are ubiquitous in the Earth’s lower atmosphere, of which a major source is the oxidation of volatile organic compounds (VOCs; Andreae and Crutzen, 1997). Grayson et al.: Effect of relative humidity and particle mass concentrations the atmosphere by providing solid or liquid phases for reactions (Hallquist et al, 2009). If SOM particles are solid or “glassy” in phase under atmospheric conditions they may provide a surface for ice nucleation (Baustian et al, 2013; Berkemeier et al, 2014; Knopf and Rigg, 2011; Ladino et al, 2014; Wang et al, 2012b; Murray et al, 2010; Schill et al, 2014). The results add to the few existing measurements of the effect of RH on the viscosity of SOM produced via the ozonolysis of α-pinene (Renbaum-Wolff et al, 2013; Bateman et al, 2015; Kidd et al, 2014; Zhang et al, 2015). The production mass concentrations of the SOM in the current study ranged from 121 to 14 000 μg m−3
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