Probing of the Changing Shapes and Viscosity of Suspended Organic Particles as a Function of Relative Humidity
Friday, 19 December 2014
Aerosol particles of secondary organic material (SOM) were produced by α-pinene ozonolysis in a flow tube reactor. The aerosol flow was passed into a chamber with a long residence time where coagulation of primary particles occurred. An experimental apparatus, consisting of a differential mobility analyzer coupled to a particle mass analyzer (DMA-APM), was used to classify coagulated particles by particle electric mobility diameter (52.4 to 190.0 nm) and then to measure associated particle mass. From these data, the dynamic shape factor was determined for particles of known material density. Experiments were conducted for variable relativity humidity (RH). The results showed that the dynamic shape factor depended on post-coagulation particle number concentration, particle diameter, and relative humidity. For some particle number concentrations, coagulation occurred between particles of similar diameters under dry conditions (< 5% RH), thereby forming non-spherical particles. The dynamic shape factors were observed to change from 1.24 to 1.02 between 5 and 35% RH, and 1.27 to 1.03 between 20% to 60% RH， implying a transformation from non-spherical to round shapes. The shape change arose from decreased viscosity at elevated RH, allowing the material to flow and thereby form a spherical shape (i.e., as favored by minimization of surface area). Numerical modeling was used to estimate the particle viscosity associated with this flow. Based on the particle size and exposure time to elevated RH, the viscosity was determined from 109 Pa s down to 107 Pa s from 3% RH to 65% RH. The experiments establish a method for estimating the viscosity of suspended submicron aerosol particles based on changes in particle shape.