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, HO<sub>2</sub>, NO<sub>3</sub>, N<sub>2</sub>O<sub>5</sub> 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 HO<sub>2</sub> by ~200% and overpredicts NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> by around 80 and 50%, respectively. Cycling between NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> is fast and thus we define the NO<sub>3x</sub> (NO<sub>3x</sub>=NO<sub>3</sub>+N<sub>2</sub>O<sub>5</sub>) family. Production of NO<sub>3x</sub> is overwhelmingly dominated by the reaction of NO<sub>2</sub> with O<sub>3</sub>, whereas its loss is dominated by aerosol uptake of N<sub>2</sub>O<sub>5</sub>, with NO<sub>3</sub>+VOCs (volatile organic compounds) and NO<sub>3</sub>+RO<sub>2</sub> playing smaller roles. The production of HO<sub>x</sub> and RO<sub>x</sub> radicals is mainly due to the reaction of NO<sub>3</sub> with VOCs. The loss of these radicals occurs through a combination of HO<sub>2</sub>+RO<sub>2</sub> reactions, heterogeneous processes and production of HNO<sub>3</sub> from OH+NO<sub>2</sub>, with radical propagation primarily achieved through reactions of NO<sub>3</sub> with peroxy radicals. Thus NO<sub>3</sub> at night plays a similar role to both OH and NO during the day in that it both initiates RO<sub>x</sub> radical production and acts to propagate the tropospheric oxidation chain. Model sensitivity to the N<sub>2</sub>O<sub>5</sub> aerosol uptake coefficient (γ<sub>N<sub>2</sub>O<sub>5</sub></sub>) is discussed and we find that a value of γ<sub>N<sub>2</sub>O<sub>5</sub></sub>=0.05 improves model simulations for NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>, but that these improvements are at the expense of model success for HO<sub>2</sub>. Improvements to model simulations for HO<sub>2</sub>, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.