Development and laboratory characterization of a sampling system for airborne measurements of peroxy radicals using chemical amplification

Abstract

Hydroxyl- and organylperoxy radicals, HO2 and RO2 (R - organic chain), are key intermediates playing a crucial role in the photo-oxidation processes of volatile organic compounds in the troposphere and in the formation and depletion of the tropospheric ozone. This doctoral thesis presents the outcome of laboratory studies carried out to optimize the sampling conditions and the inlet for airborne measurements of peroxy radicals with the PeRCA (Peroxy Radical Chemical Amplification) technique. The first aspect of the investigation is related to the sampling efficiency of these radicals which partly depends on the flow resistance encountered by the sampling gas through the set-up components. Based on the experimental data, the relative resistances of the components were quantified. The results show that the insufficient amount of air sampled through the reactor is not related primarily to the resistance of the sampling orifice of the inlet but mainly to the resistance of the components behind the reactor. The second focus concerns a study of heterogeneous losses of peroxy radicals on the instrument surfaces to reduce the losses. It was observed that there was a reduction of the radical losses with decrease of the area to volume ratio of the sampling surface and with decrease of the radical retention time within the surface. In contrast to other literature values, the determined radical loss coefficients for HO2 and CH3O2 are alike. The determined radical removal on surfaces coated with different materials: quartz and Teflon is similar. Furthermore, as PeRCA measures the total sum of radicals ([RO_2^*= HO_2 ΣRO_2]) the capabilities of the technique for the separate measurement of HO2 and RO2 radicals were investigated. This is crucial for the study of the processes affecting the oxidative capacity of the atmosphere. The proposed speciation is based on the partial removal of RO2 by the chemical reaction with NO. As the removal efficiency is not equal for all RO2, the quantitative measurements of HO2 and RO2 concentrations are possible for the radical mixture characterized by similar removal efficiency or with one dominant organic radical. Such conditions are expected in the upper troposphere where CH3O2 is likely the main RO2. Thereby, CH3O2 can be discriminated from HO2 through employment of varying NO mixing ratios in the PeRCA reactor which is a potential improvement for the deployment of PeRCA for the airborne measurements. Based on the findings of this study, recommendations for the optimal set-up and the measurement conditions on board of the HALO aircraft for determination of abundance of peroxy radicals are provided.