Abstract
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.
Highlights
Volatile organic compounds emitted by the biosphere, as well as from anthropogenic activities, react in the atmosphere with oxidants to produce secondary, oxygenated species (Fehsenfeld et al, 1992; Hallquist et al, 2009)
Some of these products contribute to the mass concentration of the atmospheric particle population, as so-called secondary organic material (SOM) (Hallquist et al, 2009)
In addition to the results presented here, it is expected that the general method can be applied more broadly across other disciplines which seek to determine the viscosity of material bodies in the key size ranges of nanotechnologies and biological sciences
Summary
Volatile organic compounds emitted by the biosphere, as well as from anthropogenic activities, react in the atmosphere with oxidants to produce secondary, oxygenated species (Fehsenfeld et al, 1992; Hallquist et al, 2009). The viscosity and the diffusivity of SOM have emerged as important topics (Vaden et al, 2010; Virtanen et al, 2010; Abramson et al, 2013; Hosny et al, 2013; Pajunoja et al, 2013; Power et al, 2013; Renbaum-Wolff et al, 2013; Bateman et al, 2015; Kidd et al, 2014; Wang et at., 2014) These properties influence whether gases and the dynamic interplay between atmospheric particles are confined to the surface region of a particle or alternatively can proceed to the interior (Shiraiwa et al, 2013a, 2014), with potential important consequences for the growth, the reactivity, and the fate. In addition to the results presented here, it is expected that the general method can be applied more broadly across other disciplines which seek to determine the viscosity of material bodies in the key size ranges of nanotechnologies and biological sciences (i.e., from < 10 nm up to several microns)
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