Radical chemistry at night: comparisons between observed and modelled HO_x, NO₃ and N₂O₅ during the RONOCO project

Abstract. The RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) aircraft campaign during July 2010 and January 2011 made observations of OH, HO₂, NO₃, N₂O₅ and a number of supporting measurements at night over the UK, and reflects the first simultaneous airborne measurements of these species. We compare the observed concentrations of these short-lived species with those calculated by a box model constrained by the concentrations of the longer lived species using a detailed chemical scheme. OH concentrations were below the limit of detection, consistent with model predictions. The model systematically underpredicts HO₂ by ~200% and overpredicts NO₃ and N₂O₅ by around 80 and 50%, respectively. Cycling between NO₃ and N₂O₅ is fast and thus we define the NO_3x (NO_3x=NO₃+N₂O₅) family. Production of NO_3x is overwhelmingly dominated by the reaction of NO₂ with O₃, whereas its loss is dominated by aerosol uptake of N₂O₅, with NO₃+VOCs (volatile organic compounds) and NO₃+RO₂ playing smaller roles. The production of HO_x and RO_x radicals is mainly due to the reaction of NO₃ with VOCs. The loss of these radicals occurs through a combination of HO₂+RO₂ reactions, heterogeneous processes and production of HNO₃ from OH+NO₂, with radical propagation primarily achieved through reactions of NO₃ with peroxy radicals. Thus NO₃ at night plays a similar role to both OH and NO during the day in that it both initiates RO_x radical production and acts to propagate the tropospheric oxidation chain. Model sensitivity to the N₂O₅ aerosol uptake coefficient (γ_N2O5) is discussed and we find that a value of γ_N2O5=0.05 improves model simulations for NO₃ and N₂O₅, but that these improvements are at the expense of model success for HO₂. Improvements to model simulations for HO₂, NO₃ and N₂O₅ can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.