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

Stone, D.; Evans, M. J.; Walker, H.; Ingham, T.; Vaughan, S.; Ouyang, B.; Kennedy, O. J.; McLeod, M. W.; Jones, R. L.; Hopkins, J.; Punjabi, S.; Lidster, R.; Hamilton, J. F.; Lee, J. D.; Lewis, A. C.; Carpenter, L. J.; Forster, G.; Oram, D. E.; Reeves, C. E.; Bauguitte, S.; Morgan, W.; Coe, H.; Aruffo, E.; Dari-Salisburgo, C.; Giammaria, F.; Di Carlo, P.; Heard, D. E.

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.

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Stone, D. / Evans, M. J. / Walker, H. / et al: Radical chemistry at night: comparisons between observed and modelled HOx, NO3 and N2O5 during the RONOCO project. 2014. Copernicus Publications.

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