Determination of OH reactivity in the atmosphere by way of the Comparative Reactivity Method

Hydroxyl (OH) radicals play an important role in ‘cleansing’ the atmosphere of many pollutants such as, nitric oxides (NOx), methane (CH4) and various volatile organic compounds (VOCs). To measure the reactivity of OH, both the sinks and sources of OH need to be quantified, and currently the overall sinks of OH seem not to be fully constrained. In order to measure the total rate loss of OH in an ambient air sample, all OH reactive species must be considered and their concentrations and reaction rate coefficients with OH known. Using the method pioneered by Sinha and Williams at the Max Plank Institute Mainz, the Comparative Reactivity Method (CRM) directly quantifies total OH reactivity in ambient air without the need to consider the concentrations of individual species within the sample that can react with OH. The CRM measures the concentration of a reactive species that is present only in low concentrations in ambient air, in this case pyrrole, flowing through a reaction vessel and detected using Proton Transfer Reaction- Time of Flight- Mass Spectrometry (PTR-ToF-MS). The research presented in this thesis will aim to build, develop, calibrate and validate a suitable instrument the measures total OH reactivity using the CRM technique, with the final aim of employing this technique in a number field campaigns.