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.
