Radical chemistry at night: comparisons between observed and modelled HO _x, NO ₃ and N ₂O ₅ during the RONOCO project

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 ₂, NO ₃, N ₂O ₅ 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 ₂ by ~200% and overpredicts NO ₃ and N ₂O ₅ by around 80 and 50%, respectively. Cycling between NO ₃ and N ₂O ₅ is fast and thus we define the NO _3x (NO _3x=NO ₃+N ₂O ₅) family. Production of NO _3x is overwhelmingly dominated by the reaction of NO ₂ with O ₃, whereas its loss is dominated by aerosol uptake of N ₂O ₅, with NO ₃+VOCs (volatile organic compounds) and NO ₃+RO ₂ playing smaller roles. The production of HO _x and RO _x radicals is mainly due to the reaction of NO ₃ with VOCs. The loss of these radicals occurs through a combination of HO ₂+RO ₂ reactions, heterogeneous processes and production of HNO ₃ from OH+NO ₂, with radical propagation primarily achieved through reactions of NO ₃ with peroxy radicals. Thus NO ₃ 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 N ₂O ₅ aerosol uptake coefficient (γ _N2O5) is discussed and we find that a value of γ _N2O5=0.05 improves model simulations for NO ₃ and N ₂O ₅, but that these improvements are at the expense of model success for HO ₂. Improvements to model simulations for HO ₂, NO ₃ and N ₂O ₅ can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.