The hydroxyl radical, OH, and the hydroperoxy radical, HO2 (known collectively as HOx), play a key role in tropospheric chemistry and are intricately related to chemical cycles that control the concentration of greenhouse gases and have important implications for air quality. Through accurate measurements of these two important radicals, and thorough investigation of the chemical mechanisms that control their formation and removal, we can develop a better understanding of atmosphere. Simulation chambers offer the unique ability to study these processes under atmospherically relevant conditions, using a wide variety of instrumentation to probe many different species. The Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) is a stainless steel chamber based at the University of Leeds and was previously designed to operate over a range of temperatures and pressures. HIRAC was implemented to validate important oxidation mechanisms of volatile organic compounds, furthering mechanism databases, such as the Master Chemical Mechanism (MCM). This thesis concentrates on the continued development of a dedicated HOx radical detection instrument, based on laser induced fluorescence spectroscopy at low pressure (fluorescence assay by gas expansion (FAGE)), for use in an atmospheric simulation chamber. In the field, FAGE instruments are designed to operate on board aircraft, which subject the instrument to a range of external operating pressures. Thorough characterisation and calibration of the FAGE instrument was performed using traditional methods, accounting for several factors known to affect instrument sensitivity. This calibration procedure was successfully validated using two newly developed calibration methods for OH and HO2, which take advantage of the HIRAC chamber and its ability to operate over a range of temperatures and pressures. After thorough calibration, the instrument was implemented in the investigation of direct OH radical production from the reaction of HO2 with acetylperoxy radicals in the HIRAC chamber. Reactions of RO2 radicals with HO2 have previously been thought to be a radical sink in atmospherically pristine environments (i.e., low NOx). However, more recently, higher than anticipated concentrations of OH have been observed in areas where biogenic loadings are high. Recycling of OH from reactions of RO2 with HO2 could provide part of the current mechanism shortfall. Acetyl peroxy radicals are of particular importance as they are formed directly from the oxidation of MVK, a major product of isoprene oxidation. Reported here is the first study sensitive to products from all three branching pathways of the reaction.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:617280 |
| Date | January 2014 |
| Creators | Winiberg, Frank Alexander Frederick |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.whiterose.ac.uk/6812/ |
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