The oxidation of primary emitted species such as volatile organic compounds (VOCs) acts as a source of ozone and therefore has detrimental effects on air quality and climate. In order to understand at what rate oxidation is occurring in the troposphere, i.e. the oxidation capacity, an understanding of the contributors to oxidation and possible markers of oxidation are imperative. Formic acid, a ubiquitous trace gas, which contributes significantly to the acidity of precipitation, could, because of its dominant source being suggested to be the ozonolysis of alkenes, be used as a marker for oxidation. A chemical ionisation mass spectrometer (CIMS) with a quadruple mass analyser utilising the I ionisation scheme was therefore deployed in London during both the summer and winter Clean Air for London (ClearfLO) Campaign to measure formic acid in order to asses this possibility. Simultaneous formic and nitric acid measurements with the CIMS, as well as supplementary gas phase data, in the winter ClearfLO campaign indicated the dominance of direct anthropogenic emissions of formic acid at this time and location. Global modelling simulations including a direct anthropogenic emission of formic acid suggested that this source is responsible for up to 30% of total formic acid in certain regions of the northern hemisphere, an area where this acid is most significantly underestimated. This ruled out the possibility of using formic acid as a marker for oxidation as a result of its vast range of sources. The mirrored summer campaign, also simultaneously detecting formic acid and nitric acid, showed that secondary photochemical production was, however, a dominant source in this time and location. The first UK measurements of ClNO2 are reported here – which is a region where modelling studies predict significant ClNO2 production all year round. Concentrations of ClNO2 up to 724 ppt and a mean of 207 ppt were measured in London during the summer ClearfLO campaign. Cl atoms from ClNO2 photolysis were shown to contribute up to 15%, 3% and 26% of the oxidation of alkanes, alkenes and alkynes, respectively over the full day. This will increase the tropospheric ozone forming potential at this time, particularly in the early morning. As well as very limited ClNO2 measurements in Europe, measurements from low NOx sites and airborne measurements are also highly understudied. Measurements of ClNO2 in the rural coastal site of Weybourne and an altitude profile from the FAAM Bae-146 Research aircraft were therefore undertaken. Concentrations of ClNO2 were significantly lower in Weybourne in comparison to the London study with peaks of 80 ppt with night-time average of 15 ppt being observed. The importance of Cl atoms to VOC oxidation at this site was much reduced in comparison to London owing to the higher OH concentrations and lower production of ClNO2 at this site.

Four cluster models for a copper(I)oxide (111) surface have been designed, of which three were studied with respect to their applicability in density functional calculations in the general gradient approximation. Formic acid adsorption on these systems was modelled and yielded four different adsorption structures, of which two were found to have a high adsorption energy. The energetically most favourable adsorption structure was further investigated with respect to its decomposition and a few reactions with adsorbed H and OH species using synchronous transit methods to estimate reaction barriers and single point energy calculations for the reaction energy.

Four cluster models for a copper(I)oxide (111) surface have been designed, of which three were studied with respect to their applicability in density functional calculations in the general gradient approximation. Formic acid adsorption on these systems was modelled and yielded four different adsorption structures, of which two were found to have a high adsorption energy. The energetically most favourable adsorption structure was further investigated with respect to its decomposition and a few reactions with adsorbed H and OH species using synchronous transit methods to estimate reaction barriers and single point energy calculations for the reaction energy.:1 Introduction 1.1 Preliminary Work 1.2 Known Reactions and Issues 1.3 Overview of Reactions and Species involved in Formic Acid Decomposition 2 Theoretical Background 2.1 The Schrödinger-Equation 2.2 Density Functional Theory 2.3 Exchange-Correlation Functionals 2.4 The Self-Consistent-Field Procedure 2.5 Geometry Optimization and Transition State Searches 2.6 Kinetics 3 Computational Details 3.1 Synchronous Transit Schemes 3.2 Transition State Searches using Eigenvector Following 4 Model System 5 Results and Discussion 5.1 Geometry of the Cu2O cluster structures 5.2 Adsorption of formic acid 5.3 Decomposition and Reaction Paths 5.3.1 Vibrational Analysis of the adsorbed Formic Acid Molecule 5.3.2 Reaction Modelling using Linear Synchronous Transit 5.3.3 Transition State Searches using Eigenvector Following 6 Summary and Outlook