Spectroscopic measurement of the absorption and scattering properties of single aerosol particles

Abstract

The absorption of solar radiation by atmospheric aerosol represents one of the largest uncertainties in predictions of current and future climate. Accurate and sensitive measurements of the optical properties of light-absorbing aerosols will improve the characterisation of aerosol-light interactions. The extinction cross-section is an important optical property of aerosol particles that governs the amount of light with which a particle interacts. This thesis demonstrates a new approach coupling cavity ring- down spectroscopy (CRDS) with a linear electrodynamic quadrupole trap, that allows the first direct spectroscopic quantification of the evolving extinction cross-sections of single, light-absorbing aerosol particles as their size and composition change. The evolving real and imaginary components of the complex refractive index are revealed for micrometer-scale droplets composed of a mixture of a non-absorbing organic species with a strongly light-absorbing dye, by fitting the measured cross-sections to light scattering models (e.g., Mie theory). Further experiments use a combination of single particle CRDS and UV/visible spectroscopy (for beaker-scale solutions) to determine the real and imaginary components of the complex refractive index for aqueous droplets containing an atmospherically relevant, light-absorbing organic carbon species, and the dependence of these refractive indices on optical wavelength, relative humidity, and droplet pH.
The accuracy with which the real and imaginary components of the complex refractive index are retrieved using the single particle CRDS approach is quantified using a combination of experimental measurements and numerical methods. The assessments account for fundamental noise in the CRDS measurements, the effects of particle motion within the LEQ trap, and particle light-absorption strength. Importantly, the evaluations find that the real and imaginary components of the complex refractive index are retrieved to an accuracy better than 0.005 and 0.002, respectively, for almost all particle absorption strengths studied (with imaginary refractive indices in the range 0–0.1).

Date of Award	23 Jan 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael I Cotterell (Supervisor) & Andrew J Orr-Ewing (Supervisor)