Causes, Magnitudes and Atmospheric Implications of Kinetically Limited Diffusion in Organic Aerosol

  • Stephen T Ingram

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

In order for a molecule to evaporate from an aerosol particle, first it must diffuse to the surface. Similarly, once a molecule has condensed onto a droplet, it is compelled by a gradient in chemical potential to diffuse inwards. These processes can be the rate limiting step in the equalising of concentrations between the gas and condensed phases. Here, several novel studies have been conducted, both computationally and in a laboratory, exploring different aspects of this phenomenon and the rate at which it proceeds. The experiments have been designed to reduce the current uncertainties in atmospheric science, by relying on proxies for common atmospheric molecules.
Firstly, molecular dynamics simulations have investigated the process by which a single water molecule diffuses through a dehydrated organic structure. The course taken was found not to be continuous, but rather proceeded by a random walk of hops between cavities. Displacement was
integrated and the diffusion coefficients were found to be consistent with existing experimental literature.
A methodology is presented for the extraction of diffusion coefficients from the growth and shrinkage rates of levitated organic droplets. The method was validated using sucrose aerosol, revealing that water equilibration is generally fast, although it can exceed several hours when particle viscosities approach that of a glass.
Once water diffusion was parameterised at different humidities, attention was turned to the evaporation of organic molecules, assuming the water flux would be unchanged by the presence of another solute. Several aerosol systems were studied, each with differing viscosities, volumes and volatilities of the evaporating molecule. Kinetic modelling was found to predict evaporation effectively when semivolatile organics were assumed to obey the Stokes-Einstein relation. This result is deemed highly significant to the modelling of gas-particle partitioning.
Finally, an atmospherically relevant oxidation reaction was initiated in the presence of a single seed droplet. By tracking resonances in the Raman spectrum, it was observed that mass accreted onto the droplet surface. Several experiments were conducted investigating the proportion of the coating that evaporates at different humidities.
Date of Award6 Dec 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJonathan P Reid (Supervisor) & David R Glowacki (Supervisor)

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