Knowledge of volatile organic compounds (VOCs) and their mixing ratios present in air on a regional and global scale is critical for understanding of air quality and climate. Measurement of individual non-methane hydrocarbons (NMHCs) present in ambient air presents a significant challenge, and the separation of hydrocarbon structures from measurement techniques such as infrared spectroscopy is rendered impossible by the number of species of NMHCs, on the order of 10,000 - 100,000, directly emitted or produced by oxidative chemistry in the atmosphere. Whilst a majority of NMHCs cannot be directly measured, the oxidation products, oxygenated VOCs (OVOCs), are more accessible to detection techniques. The qualification and quantification of VOC species within the troposphere may be resolved through the increased mechanistic understanding of glyoxal (CHOCHO) and formaldehyde (HCHO) production from a range of precursor VOCs. The ratio of glyoxal to formaldehyde (RGF) produced through oxidation varies dependent on the VOC precursor, and yield measurements of these products are therefore paramount to deciphering global tropospheric RGF measurements and the nature of their origin.
This thesis has a focus on chamber studies of glyoxal yields from the OH-initiated oxidation of biogenic VOCs (BVOCs), specifically acetaldehyde and isoprene, in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC). HIRAC is equipped with several selective detection techniques, including a Proton Transfer Reaction-Time of Flight-Mass Spectrometer (PTR-ToF-MS), a glyoxal laser-induced-phosphorescence (GLYOX-LIP) instrument, and a Fluorescence Assay by Gas Expansion (FAGE) instrument. Simultaneous measurement with these instruments during oxidation reactions allowed detection of hydroxyl (OH) radicals, glyoxal, and BVOCs of interest, including acetaldehyde and isoprene, for the direct measurement of glyoxal yields.
Sources of OH were investigated and compared for chamber studies involving glyoxal, with a focus on GLYOX-LIP interference and first-order loss rates of glyoxal. Photolysis of either hydrogen peroxide or ozone were determined to be the most effective methods of OH generation in NOx-free reactions (NOx = NO and NO2), whilst methyl nitrite (CH3ONO) was distinguished as an effective OH precursor for studies involving NOx. FAGE measurements of OH were investigated through the comparison of two FAGE calibration techniques; water vapour photolysis, calculating [OH] from water vapour concentrations in a flow of air; and VOC decay, calculating [OH] from PTR-ToF-MS detection of cyclohexane removal in HIRAC. Upon adjustment of the relative sampling positions for FAGE and PTR-ToF-MS, both calibration techniques showed excellent agreement in the OH calibration factor (COH) for FAGE (to within 4 %). However, further investigation of COH measured through isoprene decay revealed a large discrepancy in measured COH between both calibration techniques. COH determined through water vapour photolysis was at least 7 times higher, however consistent ratios between measurements could not be determined. FAGE detection of OH during isoprene oxidation therefore relied upon COH determined through the isoprene decay.
Yields of glyoxal (YGLY) were measured both from the oxidation of acetaldehyde (YGLY = 0.12 ± 0.03 %) and isoprene (YGLY = 0.52 ± 0.06 %). The low glyoxal yield through acetaldehyde oxidation has implications for readdressing a missing source of glyoxal in the marine boundary layer (MBL), and discrepancies between observed and modelled glyoxal mixing ratios in the MBL remain unaccounted for. Meanwhile, to the author’s knowledge, this work represents the first direct measurement of glyoxal yields from isoprene oxidation in the absence of NOx through selective GLYOX-LIP detection. Measurements in this study combined with future work considering the dependence of glyoxal yield on NOx mixing ratios will help reduce uncertainties in the global budget of glyoxal production through isoprene emissions in the troposphere.
