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Title: Measuring aerosol nanoparticles by ultraviolet photoionisation
Author: Nishida, Robert Takeo
ISNI:       0000 0004 7661 1924
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Aerosol particulate matter adversely affects the climate, environment and human health. Mechanistic studies have indicated that ultrafine aerosol nanoparticles, those under 100 nm in diameter, may have significant health impacts due to their relatively high number concentration, surface area and potential for deep penetration into the human lung. However, epidemiological evidence remains limited due to the lack of measurement networks that monitor local concentrations of ultrafine particles. Direct ultraviolet (UV) photoionisation electrically charges aerosol nanoparticles for subsequent detection by a mechanism distinct from the ion-particle collisions of conventional methods. The aim of this work is to evaluate photoionisation theory in order to understand and interpret measurements from a low-cost aerosol particle sensor. To accomplish this, theoretical equations are analysed, modelled and compared with experimental results for validation. The photoelectric yield of aerosol particles is explored in terms of particle size, concentration, material, and morphology giving insight into the interaction of light and particles. This thesis introduces the first analysis of photoionisation, recombination, convection/diffusion and transport of particles in an electric field using analytical, numerical, and computational fluid dynamics (CFD) techniques. Characteristic times and dimensionless parameters are defined to determine regimes under which the measurement system is dominated by each of the charging or transport mechanisms. The level of modelling detail required for accurate prediction of aerosol charging and capture methods is demonstrated over a range of conditions. In a continuous flow of aerosol particles, an electric field is applied to capture charge as it is photoemitted from particles and before the emitted charge and particles can recombine. This method yields a novel current measurement directly representative of photoemission. The CFD model agrees well with electrical current measurements demonstrating that the physics of the problem is suitably represented. It is demonstrated that photoemission is linearly proportional to total (mobility) surface area for a large range of sizes and concentrations of particles of self-similar material and morphology, with agglomerated silver particles having 5$\times$ yield of agglomerated carbon from a propane flame. It is shown for the first time that agglomerated particles have a significantly higher photoelectric yield (2.6$\times$) than sintered, close-packed spheres of the same mobility diameter and material, directly contradicting two of the three previous relevant studies. Close-packed spheres have less material exposed to both the photon flux and the particle's surroundings than an agglomerate of the same particle mobility diameter, thereby reducing photoelectric activity. The photoelectrically active area is defined explicitly in this work to reflect the effect of a particle's morphology; the revised definition produces good agreement with experimental results.
Supervisor: Hochgreb, Simone Sponsor: Alphasense Ltd ; Cambridge Trust ; Natural Sciences and Engineering Research Council of Canada (NSERC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Aerosols ; Photoelectric Charging ; Instrumentation