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.