Advances in OH reactivity instruments for airborne field measurements

Abstract. Hydroxyl radical OH reactivity, which is the inverse lifetime of the OH radical, provides information on the burden of air pollutants, since almost all air pollutants react with OH. OH reactivity measurements from field experiments can help to identify gaps in the measurement of individual reactants and serve as a proxy for the potential formation of secondary pollutants, including ozone and particles. However, OH reactivity is not regularly measured specifically on airborne platforms due to the technical complexity of the instruments and/or the need for careful instrumental characterisation to apply accurate correction factors to account for secondary chemistry in the instruments. The method used in this work, based on the time-resolved measurement of OH radicals produced by laser flash photolysis in a flow tube, does not require corrections as secondary chemistry in the instrument is negligible for typical atmospheric conditions. However, the detection of OH radicals by laser-induced fluorescence is challenging. In this work, an OH reactivity instrument has been further developed specifically for airborne measurements. The laser system used to detect the OH radicals has been simplified compared to previous setups, thereby significantly reducing the need for user interaction. The improved sensitivity allows measurements to be made with high time resolution on the order of seconds with a measurements precision of 0.3 s⁻¹. The OH reactivity measurements were validated by using a propane gas standard, which allowed the determination of the reaction rate constant of the OH reaction with propane. The values are in excellent agreement with literature recommendations within a range of 4 to 8 %. Deviations are well within the combined uncertainties. The accuracy of the OH reactivity measurements is mainly limited by the determination of the instrumental zero, which has a typical maximum uncertainty of 0.5 s⁻¹. The high sensitivity of the improved instrument facilitates the data acquisition on board an aircraft as demonstrated by its deployment during the AEROMMA campaign in 2023.