Articles | Volume 15, issue 14
Atmos. Chem. Phys., 15, 7819–7829, 2015
Atmos. Chem. Phys., 15, 7819–7829, 2015

Research article 16 Jul 2015

Research article | 16 Jul 2015

Changing shapes and implied viscosities of suspended submicron particles

Y. Zhang1, M. S. Sanchez1,2, C. Douet1,3, Y. Wang1,4, A. P. Bateman1, Z. Gong1, M. Kuwata1,5, L. Renbaum-Wolff6, B. B. Sato1,7, P. F. Liu1, A. K. Bertram6, F. M. Geiger8, and S. T. Martin1,9 Y. Zhang et al.
  • 1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
  • 2Department of Chemical Engineering, University of São Paulo, São Paulo, Brazil
  • 3Department of Energy and Environment, National Institute of Applied Science of Lyon, Villeurbanne, France
  • 4School of Public Health, Harvard University, Boston, Massachusetts, USA
  • 5Earth Observatory of Singapore, School of Physical and Mathematical Sciences, College of Sciences, Nanyang Technological University, Singapore, Singapore
  • 6Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada
  • 7Department of Chemical Engineering, Federal University of Sao Carlos, Sao Carlos, São Paulo, Brazil
  • 8Department of Chemistry, Northwestern University, Evanston, Illinois, USA
  • 9Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA

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.

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.
Final-revised paper