Nighttime measurements of HO_x during the RONOCO project and analysis of the sources of HO₂

Abstract

Abstract. Measurements of the radical species OH and HO2 were made using the Fluorescence Assay by Gas Expansion (FAGE) technique during a series of nighttime 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 nighttime 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 nighttime flight on 20 July 2010, when the highest concentrations of NO3 and O3 were also recorded. Analysis of the rates of reaction of OH, O3, and the NO3 radical with measured alkenes indicates that the summer nighttime troposphere can be as important for the processing of VOCs as the winter daytime troposphere. 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 nighttime production of HO2 during summer, and reactions of O3 dominated nighttime HO2 production during winter.