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Title: Extinction cross section measurements for single aerosol particles confined in a Bessel layer beam optical trap
Author: Cotterell, Michael Ian
ISNI:       0000 0004 6059 3776
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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The contribution of aerosol to the radiative forcing of Earth's atmosphere remains the largest uncertainty in climate models. Atmospheric aerosol particles scatter and absorb both solar and terrestrial radiation, in addition to perturbing the albedo and lifetime of clouds. The key property governing the amount of light that interacts with an aerosol particle is the particle extinction cross section (δext), commonly measured by probing an ensemble of particles using spectroscopic techniques. However, the uncertainties in particle size, distribution and number density associated with aerosol ensembles leads to imprecise characterisations of δext. By probing a single particle at a time, δext will be measured with higher precision. This thesis develops a new single particle cavity ring-down spectrometer (SP-CRDS) for measuring δext at λ = 405 nm, optically confining a single particle using a Bessel laser beam trap and probing this particle using CRDS. Moreover, a similar instrument performing CRDS at λ = 532 nm is developed. Using these SP-CRDS instruments, coupled with elastic light scattering measurements, the variations in δext with particle size and composition are determined for a variety of evaporating organic species or aqueous droplets containing atmospherically relevant, hygroscopic inorganic solutes. Particle refractive indices (RIs) are retrieved by fitting the elastic light scattering and δext measurements to light scattering models, such as Mie theory. The variations in RI are determined as a function of relative humidity for wavelengths across the visible spectrum. The RI retrieval precision from (Text data, or using light elastically scattered from a Gaussian beam, is shown to be better than 0.2%. However, the RI retrieval precision from elastic light scattering measurements using a Bessel beam, instead of a Gaussian beam, is degraded significantly to 0.5%. The accuracy in RI retrievals from (Text data is found to be ~0.07% for particle radii>1 μm, i.e. an accuracy exceeding the requirements for calculating aerosol radiative forcing to better than 1 %. Therefore, SP-CRDS measurements of δext will be important in reducing the uncertainties associated with aerosol radiative forcing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available