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Title: Aerosol cavity ring down spectroscopy : from ensemble to single particle measurements
Author: Mason, Bernard James
ISNI:       0000 0004 5355 0997
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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Aerosol particles are prevalent in the atmosphere and impact the Earth's energy balance through scattering and absorption of incoming and outgoing radiation. Such particles represent one of the largest uncertainties when trying to characterise the anthropogenic causes in the Earth's changing radiation balance. This thesis describes the development of laboratory based techniques for measuring aerosol optical and microphysical properties that are atmospherically relevant. A single particle trapping technique that uses a Bessel-beam propagating counter to a' gas flow (Bessel-beam/gas-flow) is presented. The changing radius of the trapping particle is determined from the collected elastically scattered light. The fluctuating position of a particle trapped along the Bessel beam length is shown to be directly related to the particle radiation pressure efficiency and thus to its radius and refractive index counter to what is expected from , liquid phase optical chromatography measurements. It is shown that ensemble aerosol particle fractionation using a Bessel-beam/gas-flow instrument is not possible. An aerosol ensemble cavity ring down spectrometer (AE-CRDS) was used to determine the refractive index of hygroscopic sodium nitrate aerosols at different relative humidities, A comparison is made between the refractive index retrieved using AE-CRDS and the refractive index retrieved using a single particle, optical tweezers instrument. The accuracy of the optical tweezers refractive index measurement is found to be significantly higher due to the poorly defined size distribution of the aerosol ensemble in the cavity ring down technique. The development of the single particle cavity ring down spectroscopy (SP-CRDS) technique for highly accurate measurements of aerosol extinction efficiencies is presented. The SP-CRDS instrument uses a Bessel-beam/gas-flow optical trap to control the position of a particle within a cavity ring down spectrometer. A new method of accurately obtaining the real part of the refractive index using this technique is described. Measured extinction efficiencies are compared to Mie simulated extinction efficiencies to obtain the refractive index of single component aerosol particles to an accuracy of better than ± 0.1%.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available