Implementation of a chemical background method (OH-CHEM) for measurements of OH using the Leeds FAGE instrument: Characterisation and observations from a coastal location

Abstract:

The removal of pollutants and greenhouse gases in the troposphere is dominated by reactions with the hydroxyl radical (OH), which is closely coupled to the hydroperoxy radical (HO₂). Comparisons of the levels of OH and HO₂ observed during field campaigns to the results of detailed chemical box models serve as a vital tool to assess our understanding of the underlying chemical mechanisms involved in tropospheric oxidation. Recent measurements of OH and HO₂ radicals are significantly higher than those predicted by models for some instruments measuring in certain environments, especially those influenced by high emissions of biogenic compounds such as isoprene, prompting intense laboratory research to account for such discrepancies. While current chemical mechanisms are likely incomplete, it is also possible that, at least in part, these elevated radical observations have been influenced by instrumental biases from interfering species.

Recent studies have suggested that fluorescence assay by gas expansion (FAGE) instruments may be susceptible to an unknown interference in the measurement of OH. This hypothesis can be tested through the implementation of an alternative method to determine the OH background signal, whereby OH is removed by the addition of a chemical scavenger prior to sampling by FAGE. The Leeds FAGE instrument was modified to facilitate this method by the construction of an inlet pre-injector (IPI), where OH is removed through reaction with propane. The modified Leeds FAGE instrument was deployed at a coastal location in southeast England during summer 2015 as part of the ICOZA (Integrated Chemistry of OZone in the Atmosphere) project. Measurements of OH made using both background methods will be presented, alongside results from laboratory characterisation experiments and details of the IPI design.