Night-time measurements of HO_x during the RONOCO project and analysis of the sources of HO₂

Abstract. Measurements of the radical species OH and HO₂ were made using the fluorescence assay by gas expansion (FAGE) technique during a series of night-time and daytime flights over the UK in summer 2010 and winter 2011. OH was not detected above the instrument's 1σ limit of detection during any of the night-time flights or during the winter daytime flights, placing upper limits on [OH] of 1.8 × 10⁶ molecule cm⁻³ and 6.4 × 10⁵ molecule cm⁻³ for the summer and winter flights, respectively. HO₂ reached a maximum concentration of 3.2 × 10⁸ molecule cm⁻³ (13.6 pptv) during a night-time flight on 20 July 2010, when the highest concentrations of NO₃ and O₃ were also recorded. An analysis of the rates of reaction of OH, O₃, and the NO₃ radical with measured alkenes indicates that the summer night-time troposphere can be as important for the processing of volatile organic compounds (VOCs) as the winter daytime troposphere. An analysis of the instantaneous rate of production of HO₂ from the reactions of O₃ and NO₃ with alkenes has shown that, on average, reactions of NO₃ dominated the night-time production of HO₂ during summer and reactions of O₃ dominated the night-time HO₂ production during winter.