Kinetic and Mechanistic Studies in Atmospheric Chemistry Using Photoionization Mass Spectrometry

The aim of this thesis is to provide updated, accurate kinetic (and where possible mechanistic) information regarding the oxidation pathways of various volatile organic compounds (VOCs) and trace gas species within the troposphere. The vast majority of the experiments discussed were conducted using a pulsed laser photolysis, photoionization mass spectrometry (PLP-PIMS) set-up, which operated at low pressures (p = 2 Torr). PLP-PIMS is a time-resolved technique can be used to directly monitor multiple species simultaneously and with time; details of this method are found in Chapter II. Initially, one shortcoming of the PLP-PIMS technique was that it could not detect OH radicals (due to the ionization potential of OH). However, in Chapter III a 1 + 1’ multiphoton ionization (MPI) scheme was implemented, making OH detection possible. In Chapter IV, kinetic and mechanistic information regarding the oxidation of acetaldehyde, CH3CHO, by OH is reported. From this study a rate coefficient of kOH = (1.6 ± 0.2) × 10-11 cm3 molecule-1 s-1 was measured, this is in good agreement with other literature values. Additionally, it was observed that a small yield of methyl radicals (YCH3 ≈ 15%) was also produced. Methyl radicals were an unexpected product from this reaction and it is hypothesised that they are formed from the dissociation of nascent, chemically activated acetyl radicals. Notably, the yield of methyl radicals observed is significantly lower than would be expected from a statistical distribution of the energy between the products. Therefore, a post-reaction complex is proposed to allow for the redistribution of energy. In Chapters VI, reactions of the C1 and C2 Criegee intermediates (CH2OO and CH3CHOO, respectively) with SO2 and NO2 are reported. For the C1 Criegee intermediate (CI), rate coefficients of kC1SO2 = (3.8 ± 0.3) × 10-11 cm3 molecule-1 s-1 and kC1NO2 = (1.2 ± 0.3) × 10-12 cm3 molecule-1 s-1 are reported for the reactions with SO2 and NO2 respectively. For the C2 CI, rate coefficients of kC2SO2 = (1.8 ± 0.3) × 10-11 cm3 molecule-1 s-1 and kC2NO2 = (7.0 ± 1.6) × 10-13 cm3 molecule-1 s-1 were measured. Moreover, in the C2 CI + SO2 reaction, acetaldehyde was positively identified as a product from this reaction at low pressures. Finally, in Chapter VII the reactivity of Criegee intermediates towards different organic species was investigated. From these experiments it was observed that the C1 CI had significant rate coefficients with carboxylic acids, and rate coefficients of kC1FA = (3.7 ± 0.6) × 10-11 cm3 molecule-1 s-1 and kC1AA = (7.1±1.1) × 10-11 cm3 molecule-1 s-1 measured for the reactions with formic and acetic acid, respectively. However, the reactivity of CIs towards alkenes is thought to be considerably slower with upper limits of kC1ethene < 2 × 10-14 cm3 molecule-1 s-1 and kC1isoprene < 3 × 10-13 cm3 molecule-1 s-1 measured for the reactions of the C1 CI with ethene and isoprene.