Improving the understanding of the gas-particle partitioning of organic aerosol

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Aerosols play major roles in controlling the climate, through the scattering and absorption of light and
the seeding of clouds, and in the transport of microorganisms for pollination and disease transmission.
They are also used in a wide range of industries, including agricultural, pharmaceutical and cosmetic
due to their favourable properties relative to the bulk equivalent. From an atmospheric perspective,
a few key properties govern the formation, transformation and lifetime of aerosol. Of particular
interest to this thesis are the physicochemical and transport properties of multicomponent organic
aerosol.
In this work, experimental methods for estimating key properties that determine the gas-particle
partitioning kinetics of organic aerosol are introduced. Binary organic droplets of known chemical
speciation are suspended in an electrodynamic balance with a gas flow of controlled temperature and
relative humidity passing over them. Droplet size change as a function of time is measured via analysis
of light scattering through the droplet, which is then used to infer the kinetics of partitioning, which is
related to the properties of the organic components.
An experimental methodology for estimating organic activity coefficients in binary organic aerosol is
introduced. A binary droplet containing a volatile and non-volatile component is trapped and optically
sized in an electrodynamic balance in a dry (0% relative humidity) gas flow. The measured evaporation
rate is compared to an isothermal ideal evaporation model and estimation of the volatile component
activity coefficient is performed. Validation of the method is conducted by comparing the
experimental activity coefficients of a range of simple organics to an activity coefficient predictor
called “AIOMFAC”, where good agreement between modelled and measured values is observed. A
series of more functionally complex binary organic systems ranging in functionality and molecular
weight are tested, including diols, ethers, esters, dicarboxylic acids, aromatic acids and nitroaromatics.
Estimated activity coefficients for dicarboxylic acids are in good agreement with AIOMFAC, however,
agreement is varied for other functionalities, likely due to secondary interactivity that is unaccounted
for in the fitting set of AIOMFAC. The largest discrepancy between modelled and measured values is
seen in nitroaromatic compounds, which may be due to a combination of poor representation of the
nitro group and the lack of accounting for secondary interactions by AIOMFAC.
A methodology for experimentally estimating the bulk diffusion of solutes at the transition between
gas and bulk-diffusion limited evaporation is introduced. As in the previous method described, binary
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organic droplets are suspended in an electrodynamic balance in dry conditions, and sized to estimate
the evaporation rate as a function of composition. The bulk diffusion coefficients of non-volatile
sugars are estimated from binary droplet evaporation using this methodology, and the estimated
values are compared with the Stokes-Einstein relation. All diffusion coefficients were greater than 3
orders of magnitude higher than predicted by the Stokes-Einstein relation, with increasing discrepancy
with increasing viscosity at the transition between gas and bulk diffusion-limited evaporation.
A bulk vapour generator used in the calibration of instruments for chemical vapour detection is
characterised. The vapour and aerosol produced by the generator is sampled, and the temperature
and sample preparation are changed to investigate the effect on partitioning. The deposition of
vapour and aerosol in the experimental setup is modelled to investigate the potential for aerosol to
be used as a vector for the transport of semi and low-volatility vapours in future chemical detection
procedures.
Date of Award19 Mar 2024
Original languageEnglish
Awarding Institution
  • The University of Bristol
SponsorsDefence Science and Technology Laboratory & EPSRC Centre in Doctoral Training in Aerosol Science
SupervisorJonathan P Reid (Supervisor)

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