Measurement of the OH chemical lifetime, concentrations of speciated RO2 radicals, and a new method for the field detection of OH radicals

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The concentrations of short-lived reactive intermediates in the atmosphere, for example OH, HO2 and peroxy radicals (RO2), are determined by local in situ chemistry and not by transport processes. The hydroxyl radical (OH) is the natural cleanser in the atmosphere, and removes the majority of trace gases, whereas peroxy radicals react with nitric oxide to generate ground-level ozone. Therefore, these radicals are ideal target species with which to validate the accuracy of chemical mechanisms within atmospheric models. It is very important that the chemistry is accurate, as these mechanisms are at the heart of chemistry-transport/global circulation/regional air quality models that are used to predict the future composition of our atmosphere, and hence any associated changes in climate or air quality as a result of future changes in biogenic and anthropogenic emissions. Comparison of radical measurements made in the field with model calculations provides a commonly-used framework to examine whether the models are able to adequately describe the underlying chemistry for a range of clean and polluted environments. In this proposal we will develop new instruments to provide field measurements for comparison with model calculations. We will make measurements of the chemical lifetime of OH, which tells us the rate at which OH is removed from the atmosphere through reaction with all of its sinks. Comparison with the OH lifetime calculated by models tells us whether the sinks of OH are adequately represented in the model. If there is disagreement this could explain why models are not able to accurately reproduce measured OH concentrations. Peroxy radicals react with nitrogen oxide in the atmosphere leading to ozone formation, yet current field instruments are normally only able to make a measurement of their total concentration. We will develop a new detection method that is closely related to the FAGE technique that we have successfully used to measure OH and HO2 radicals, in order to achieve some speciation for the measurement of peroxy radicals. The proposal also aims to develop a new method for the detection of OH in the field, based on a reaction of OH with the sodium dimer producing an excited sodium atom that emits light. The method is relatively low-cost and lightweight, and offers potential for OH measurements from a wide range of locations with long-term capability. The apparatus developed in this proposal will be housed in a new, instrumented vehicle and deployed during field campaigns.