James W. Grayson

Viscosity of α-pinene secondary organic material and implications for particle growth and reactivity

Particles composed of secondary organic material (SOM) are abundant in the lower troposphere. The viscosity of these particles is a fundamental property that is presently poorly quantified yet required for accurate modeling of their formation, growth, evaporation, and environmental impacts. Using two unique techniques, namely a “bead-mobility” technique and a “poke-flow” technique, in conjunction with simulations of fluid flow, the viscosity of the water-soluble component of SOM produced by α-pinene ozonolysis is quantified for 20- to 50-μm particles at 293–295 K. The viscosity is comparable to that of honey at 90% relative humidity (RH), similar to that of peanut butter at 70% RH, and at least as viscous as bitumen at ≤30% RH, implying that the studied SOM ranges from liquid to semisolid or solid across the range of atmospheric RH. These data combined with simple calculations or previous modeling studies are used to show the following: (i) the growth of SOM by the exchange of organic molecules between gas and particle may be confined to the surface region of the particles for RH ≤ 30%; (ii) at ≤30% RH, the particle-mass concentrations of semivolatile and low-volatility organic compounds may be overpredicted by an order of magnitude if instantaneous equilibrium partitioning is assumed in the bulk of SOM particles; and (iii) the diffusivity of semireactive atmospheric oxidants such as ozone may decrease by two to five orders of magnitude for a drop in RH from 90% to 30%. These findings have possible consequences for predictions of air quality, visibility, and climate.

Estimating the viscosity of a highly viscous liquid droplet through the relaxation time of a dry spot

We discuss in this paper a technique which enables the estimation of the viscosity of microscopic droplets, with application to particles suspended in the atmosphere. The principle of this technique is to deposit a droplet of material approximately 30–100 μm in diameter on a substrate and poke it with a sharp needle hence generating a hole. The amount of sample needed to perform such measurement allows the viscosity of small sample volumes (less than a microliter), such as those generated from atmospheric sampling, to be determined. We show here that the time required for the droplet to relax to its equilibrium shape can be related to the viscosity. We hereby present two mathematical models based on the lubrication approximation which are able to capture the droplet relaxation dynamics. One model is fully transient and resolves the dynamics of the wetting front using a disjoining pressure approach. The other is quasistatic and requires a relationship between the contact line velocity and the contact angle...

Viscosities, diffusion coefficients, and mixing times of intrinsic fluorescent organic molecules in brown limonene secondary organic aerosol and tests of the Stokes-Einstein equation

The authors report about viscosity and diffusivity measurements of a brown carbon containing limonene SOA produced by ozonolysis under high mass loading conditions with subsequent exposure to ammonia. They report an increase in viscosity by 3-5 orders upon changing the water activity from 0.9 to dry and use the measured diffusion coefficient to deduce the mixing times for atmospheric particles. Their result suggest that mixing times are below 1 hour for PBL-conditions. This is in contrast to previous studies looking at SOA under low mass loading conditions which report significantly C1