Abstract
Atmospheric aerosol affects the radiative balance of the planet directly by scattering and absorbing electromagnetic radiation, and indirectly as cloud condensation nuclei. One of the largest uncertainties in climate modelling remains the magnitude of the negative forcing by atmospheric aerosol, which can be reduced through full characterization of the aerosol particle's light scattering properties over atmospherically relevant wavelengths. The optical properties of atmospheric aerosol vary with particle size, morphology, and composition, and are modified through atmospheric chemical reactions and the presence of neighbouring particles. The thesis presented here includes a series of papers that aim to further the understanding of the chemistry and scattering properties of atmospheric aerosol over tropospheric wavelengths, utilizing optical trapping, Mie and Raman spectroscopy and T-Matrix computational modelling. To meet these aims, a new ultraviolet-optimised optical system was designed, constructed, and optimized to measure the Mie scattering of single optically trapped aerosol particles at near-ultraviolet to visible wavelengths of 0.32-0.48 μm.
The first two papers use the spectroscopic system to determine the radius and wavelength-resolved refractive index dispersions of unreacted and atmospherically aged organic aerosol, including organic extracts of tropospheric aerosol samples from remote, forestry, and urban environments to precisions of 0.001 μm and 0.002 respectively. Secondly, the optical properties of squalene, a model organic aerosol, were monitored during in-air oxidation and ozonolysis reactions. The presence of ozone is demonstrated to result in faster particle solidification, higher volume loss, and formation of smaller particles.
A further two papers focus on the near-field and multiple scattering effects that arise in systems with neighbouring aerosol particles. Firstly, the backscattering of light by two separate spheres in multiple orientations is computationally modelled for a range of particle separations using the Multiple Sphere T Matrix (MSTM) code, and major changes in the scattering of light as a function of wavelength by the particles are predicted for endfire orientations (incident light along interparticle axis). Secondly, the ultraviolet-optimised optical system is used to experimentally investigate structural changes in the scattering for two neighbouring aerosol particles in an endfire incidence, with the results modelled using the MSTM code. Modelled scattering spectra show significant shifts with decreasing interparticle separation, however the modelled shifts are not observed in the experimental data. The final paper monitors the physical, chemical and optical changes of an optically trapped silica precursor aerosol undergoing acid and base catalysed polymerisation to form silica in situ. Mie spectroscopy demonstrates the formation of a dense, expanding silica core as the reaction progresses, and multiple, optically-trapped, partially-reacted aerosol particles are collided, deposited, and imaged to show that particle viscosity increases with reaction time. The combination of ultraviolet and multiple-particle studies in this thesis has overall increased the understanding of the ultraviolet light scattering of atmospheric aerosol.
The first two papers use the spectroscopic system to determine the radius and wavelength-resolved refractive index dispersions of unreacted and atmospherically aged organic aerosol, including organic extracts of tropospheric aerosol samples from remote, forestry, and urban environments to precisions of 0.001 μm and 0.002 respectively. Secondly, the optical properties of squalene, a model organic aerosol, were monitored during in-air oxidation and ozonolysis reactions. The presence of ozone is demonstrated to result in faster particle solidification, higher volume loss, and formation of smaller particles.
A further two papers focus on the near-field and multiple scattering effects that arise in systems with neighbouring aerosol particles. Firstly, the backscattering of light by two separate spheres in multiple orientations is computationally modelled for a range of particle separations using the Multiple Sphere T Matrix (MSTM) code, and major changes in the scattering of light as a function of wavelength by the particles are predicted for endfire orientations (incident light along interparticle axis). Secondly, the ultraviolet-optimised optical system is used to experimentally investigate structural changes in the scattering for two neighbouring aerosol particles in an endfire incidence, with the results modelled using the MSTM code. Modelled scattering spectra show significant shifts with decreasing interparticle separation, however the modelled shifts are not observed in the experimental data. The final paper monitors the physical, chemical and optical changes of an optically trapped silica precursor aerosol undergoing acid and base catalysed polymerisation to form silica in situ. Mie spectroscopy demonstrates the formation of a dense, expanding silica core as the reaction progresses, and multiple, optically-trapped, partially-reacted aerosol particles are collided, deposited, and imaged to show that particle viscosity increases with reaction time. The combination of ultraviolet and multiple-particle studies in this thesis has overall increased the understanding of the ultraviolet light scattering of atmospheric aerosol.
| Original language | English |
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| Qualification | Ph.D. |
| Awarding Institution |
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| Supervisors/Advisors |
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| Thesis sponsors | |
| Award date | 1 Feb 2023 |
| Publication status | Unpublished - 2023 |
Keywords
- Atmospheric Aerosol
- Optical Trapping
- Ultraviolet Light
- Squalene Ozonolysis
- Refractive Index
- Mie Scattering
- T-Matrix
- Near-Field
- MSTM
- Sol-Gel
- Raman Spectroscopy
- Radiative Forcing
- Climate Change
