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Title: Studies of heterogeneous transformations of atmospheric particles
Author: Wamsley, Ruth
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2010
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The complexity of the processes whereby organic species are degraded in the atmosphere prevents many of the individual species (intermediates or products) from being unambiguously identified. Laboratory work necessarily focuses therefore on studies of idealised proxies with the aim of increasing general understanding of the physical and chemical processes which occur on particles and the types of species which they produce. Studies of the ubiquitous proxy oleic acid have resulted in the development of complex reaction schemes describing the various products and intermediates. These schemes include a diverse range of reactions and rates, thus highlighting the complications associated even with a relatively simple system. This thesis describes novel experimental studies designed to increase understanding of heterogeneous ozonolysis reactions of organic species in the aerosol phase using infrared spectroscopy as the principal analytical method. Reactions have been studied in solution (supported by off-line mass spectrometry), in thin films and in aerosols. The sensitivity of the infrared technique has also enabled the kinetics of reactions in thin films and aerosols to be followed. These methods were applied to both single- and mixed-component systems. Product studies successfully identified a number of primary and secondary species in the ozonised systems, with the secondary products formed from association reactions of the Criegee Intermediates with other species present (including self-reaction). In the mixed organic system these products were found to have originated from both a single reactant and from cross reactions between moieties from two different reactants. At low relative humidity, the ozonolysis reaction rates were monitored through the loss of the reactant species by infrared spectroscopy in the thin film and aerosol phase to give reactive uptake coefficients (gamma). At high relative humidity, the formation of products was followed. For the single-component thin films, the values obtained were gamma = 7.8 x 10-5 for stilbene and gamma = 2.0 x 10-7 for anthracene. In thin mixed films of oleic acid and stilbene, segregation occurred which prevented the effect of mixing upon the rate to be measured. A reactive uptake of gamma = 6.8 x 10-5 was obtained, identical to that of pure oleic acid. In the particle phase, the functional form of the reaction kinetics was found to be dependent on the type of particle. Pure stilbene and mixed oleic/stilbene aerosols were highly reactive and it proved necessary to treat reactive uptake coefficients under both diffusion-limited and surface-only reaction scenarios. For stilbene, the values obtained were gamma = 1.5 x 10-3 and gamma = 5.3 x 10-3 respectively. Spectral limitations in the mixed system meant that only the reaction of stilbene could be followed, giving gamma = 4.4 x 10-3 and gamma = 10.0 x 10-3 respectively. The enhanced rate in the mixture was attributed to secondary reactions. Anthracene and oleic acid coated particles were treated using a Langmuir-Hinshelwood mechanism from which the parameters KO3 (ozone partitioning coefficient) and kImax (maximum pseudo-first-order rate coefficient) could be extracted. For anthracene ozonolysis KO3 = 1.4 x 10-16 cm3 molecule-1 and kImax = 3.5x10-2 s-1. For oleic acid coated onto ammonium sulfate aerosols, values obtained were KO3 = 2.35 x 10-15 cm3 molecule-1 and kImax = 0.56 s-1 at low RH% and KO3 = 1.71 x 10-16 cm3 molecule-1 and kImax =0.33 s-1 at high RH%. The reduction in reactivity with increased RH% is principally attributed to the effect of surface polarity on ozone absorption.
Supervisor: Percival, Carl ; n, Andrew Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Heterogeneous transformations ; Aerosols ; Proxies ; Ozonolysis ; Infrared Spectroscopy ; ism ; Ozonolysis products ; Criegee mechanism ; Kinetics ; Reactive uptake