Improving Our Understanding of the Optical Properties of Atmospheric Aerosols

  • Rose Willoughby

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The interaction of atmospheric aerosols with radiation remains a significant uncertainty in radiative forcing. This work aims to elucidate this uncertainty through the measurement of the optical properties of atmospherically relevant single aerosol particles.

Single particles were confined and spectroscopically probed while the environmental conditions were varied, thus, the evolving optical properties were studied. A Bessel laser beam and counterpropagating humidity-controlled gas flow facilitated confinement of single particles. Concurrent measurements of the extinction cross section with cavity ring down spectroscopy (CRDS) and the variation in intensity of elastic light scattering, or phase functions (PFs), gave the refractive index and radius. Single particles composed of aqueous inorganic solutes were probed at several illumination wavelengths and relative humidities (RHs). Repeat measurements provided the instrument precision, with nλ ± 0.004 and nλ ± 0.005 for CRDS and PFs respectively. The data was parameterised in a Cauchy optical dispersion model which enabled the refractive index to be calculated at any given visible wavelength and RH; the most comprehensive and precise description of inorganic optical properties to date.

Mixing rules are often used to estimate the refractive index of multicomponent aerosols; however, with limited comparisons to measured data, the validity of such rules remains unclear. Organic aerosols composed of increasing complexity were probed to investigate any limitations of mixing rules. The group contribution theory provided remarkable predictions, with uncertainties comparable to typical ensemble techniques. The molar refraction prediction was highly dependent on the solute solubility.

This thesis also presents a new method to measure the absorption of aerosol through irradiation from a near-infrared source (NIR). Weakly absorbing particles were studied over a range of RHs, particle radii, and NIR laser power. The heating-induced size-change at varied NIR beam intensities, gave the change in the imaginary component of the refractive index; establishing absorption varies with RH.
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJonathan P Reid (Supervisor) & Andrew J Orr-Ewing (Supervisor)

Cite this

Improving Our Understanding of the Optical Properties of Atmospheric Aerosols
Willoughby, R. (Author). 23 Jan 2019

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)