Articles | Volume 15, issue 14
https://doi.org/10.5194/acp-15-7819-2015
https://doi.org/10.5194/acp-15-7819-2015
Research article
 | 
16 Jul 2015
Research article |  | 16 Jul 2015

Changing shapes and implied viscosities of suspended submicron particles

Y. Zhang, M. S. Sanchez, C. Douet, Y. Wang, A. P. Bateman, Z. Gong, M. Kuwata, L. Renbaum-Wolff, B. B. Sato, P. F. Liu, A. K. Bertram, F. M. Geiger, and S. T. Martin

Abstract. The change in shape of atmospherically relevant organic particles is used to estimate the viscosity of the particle material without the need for removal from aerosol suspension. The dynamic shape factors χ of particles produced by α-pinene ozonolysis in a flow tube reactor, under conditions of particle coagulation, were measured while altering the relative humidity (RH) downstream of the flow tube. As relative humidity was increased, the results showed that χ could change from 1.27 to 1.02, corresponding to a transition from aspherical to nearly spherical shapes. The shape change could occur at elevated RH because the organic material had decreased viscosity and was therefore able to flow to form spherical shapes, as favored by the minimization of surface area. Numerical modeling was used to estimate the particle viscosity associated with this flow. Based on particle diameter and RH exposure time, the viscosity dropped from 10(8.7±2.0) to 10(7.0±2.0) Pa s (two sigma) for an increase in RH from < 5 to 58 % at 293 K. These results imply that the equilibration of the chemical composition of the particle phase with the gas phase can shift from hours at mid-range RH to days at low RH.

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Short summary
The present work estimates the viscosity of submicron organic particles while they are still suspended as an aerosol without further post-processing techniques that can possibly alter the properties of semi-volatile materials. Results imply that atmospheric particles, at least those similar to the ones of this study and for low- to middle-RH regimes, can reach equilibrium or react rather slowly with the surrounding gas phase on time scales even longer than the residence time in the atmosphere.
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