The role of chemical activation in the formation and loss of atmospheric carbonyl species

The Earth’s atmosphere is often compared to a low temperature combustion system in which solar energy drives radical oxidation of trace gases. The OH radical is the key daytime radical oxidant and reacts rapidly with the majority of the anthropogenic and biogenic volatile organic compounds (VOCs) released to the atmosphere. Over the past two decades, field campaigns in remote regions, characterised by high concentrations of hydrocarbons, such as isoprene, but relatively low concentrations of NOx (NO + NO2) have highlighted significant discrepancies between measured and modelled concentrations of OH; with modelled OH concentrations underestimating the measured daytime values by up to an order of magnitude. Consequently, a number of experimental and theoretical studies have sought novel OH generating reactions that are currently not implemented into atmospheric models. One such suggestion is that under low NOx conditions (sub 100 pptv), certain peroxy radical species, formed following the addition of O2 to radicals produced through OH initiated VOC oxidation, might undergo unimolecular dissociation reactions that regenerate OH. In this thesis, a number of OH initiated oxidation systems have been studied which produce radical intermediates that recycle OH in the presence of O2. These systems have been investigated experimentally by monitoring the OH directly using laser flash photolysis coupled with laser induced fluorescence (LFP – LIF). By monitoring the OH kinetics directly, it is possible to quantify the yield of OH recycled in the presence of O2 as a function of pressure, temperature, and O2 concentration from the ratio of rate coefficients measured in the presence and absence of O2; this OH cycling methodology was used extensively in the work presented here. The first experimental work presented in this thesis focused on the OH initiated oxidation of a series of alkynes (acetylene, propyne, and 2-butyne). These reactions proceed initially via OH addition across the alkyne triple bond, to generate an adduct that exists in two energetically distinct conformations. These adducts react rapidly with O2 to generate a bicarbonyl species and recycle OH, or an organic acid and acyl radical as first generation products; with product branching ratios dictated by the stereochemistry of the adduct at the point of reaction with O2. The nascent adduct forms following the OH + alkyne reaction with excess energy. It is widely accepted that at pressures relevant to the troposphere, any excess energy in reaction products is dissipated through inelastic collisions prior to the onset of secondary bimolecular chemistry. However, experimental and theoretical work presented here suggests that under atmospheric conditions, a significant fraction of the total product yield associated with the OH + alkyne/O2 systems, form before the internal quantum states of the adducts have fully relaxed. The product branching observed for the OH + alkyne/O2 system is said to be influenced by chemical activation, whereby the exothermicity of an initial reaction is utilised by the products to undergo secondary reactions not accessible to the thermalised products. Attention then turns to OH oxidation reactions that proceed via a hydrogen-atom abstraction channel. Abstraction reactions are often considered to deposit the majority of the available reaction exothermicity into the newly formed bond, particularly if the reaction involved has an early transition state. Experimental evidence presented here suggests that some atmospherically relevant carbonyl reactions, that are considered to proceed via direct hydrogen-atom abstraction, partition a significant fraction of the reaction exothermicity into the radical fragment. The OH + acetaldehyde, CH3CHO, reaction is considered an archetypal abstraction reaction. The acetyl, CH3CO, produced is known to react with O2 at low pressures to generate OH, with a unity yield at zero pressure. However, the pressure dependent OH yields observed for the OH + CH3CHO/O2 system suggest that ~15% of the CH3CO produced through the OH + CH3CHO reaction dissociates promptly to CH3 + CO. CH3CO fragmentation requires more than 50% of the total exothermicity of the OH + CH3CHO reaction to be channelled into the CH3CO. The second hydrogen-abstraction channel considered here is the OH + glyoxal, (HCO)2, reaction that results in production of the HC(O)CO radical. HC(O)CO chemistry is governed by a competition between unimolecular dissociation, and bimolecular association with O2. Recent calculations have suggested that the HC(O)CO + O2 reaction proceeds directly to OH + CO + CO2. This channel has been verified here through experiment, with OH yields associated with the OH + (HCO)2/O2 reaction quantified for the first time as a function of pressure (5 – 80 Torr), temperature (212 – 295 K), and O2 concentration. The OH yields increase with O2 concentration under all experimental conditions, as the bimolecular HC(O)CO + O2 reaction increasingly competes with unimolecular HC(O)CO decomposition, but converge on a limiting yield under high O2 conditions, suggesting that a fraction of the HC(O)CO produced following the OH + (HCO)2 reaction dissociates promptly to HCO + CO. In the final experimental section of this thesis a laser system was developed to detect HCO via LIF. Attempts were made to monitor both prompt and growth HCO signal following the Cl + (HCO)2 reaction, and quantify the rate of thermal HC(O)CO decomposition as a function of pressure at low temperatures (212 K). However, rapid HCO removal was observed at the low experimental temperatures required. Further experimental evidence suggested that HCO reacts rapidly with (HCO)2 and other aldehydes at 212 K. Quantitative studies focused on the reaction of HCO with formaldehyde, HCHO, and acetaldehyde, CH3CHO, with rate coefficients of (3.44 ± 0.15) and (1.24 ± 0.05) × 10-11 cm3 molecule-1 s-1 measured, respectively.