Radical chemistry at night: comparisons between observed and modelled HOsubx/sub, NOsub3/sub and Nsub2/subOsub5/sub during the RONOCO project

摘要

The RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of theAtmOsphere) aircraft campaign during July 2010 and January 2011 madeobservations of OH, HO₂, NO₃, N₂O₅ and a number ofsupporting measurements at night over the UK, and reflects the firstsimultaneous airborne measurements of these species. We compare the observedconcentrations of these short-lived species with those calculated by a boxmodel constrained by the concentrations of the longer lived species using adetailed chemical scheme. OH concentrations were below the limit ofdetection, consistent with model predictions. The model systematicallyunderpredicts HO₂ by ~200% and overpredicts NO₃and N₂O₅ by around 80 and 50%, respectively.Cycling between NO₃ and N₂O₅ is fast and thus we define theNO_3x (NO_3x=NO₃+N₂O₅) family. Productionof NO_3x is overwhelmingly dominated by the reaction of NO₂ withO₃, 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 thereaction of NO₃ with VOCs. The loss of these radicals occurs through acombination of HO₂+RO₂ reactions, heterogeneous processes andproduction of HNO₃ from OH+NO₂, with radical propagationprimarily achieved through reactions of NO₃ with peroxy radicals. ThusNO₃ at night plays a similar role to both OH and NO during the day inthat it both initiates RO_x radical production and acts to propagate thetropospheric oxidation chain. Model sensitivity to the N₂O₅aerosol uptake coefficient (γ_N2O5) is discussed and we findthat a value of γ_N2O5=0.05 improves model simulations forNO₃ and N₂O₅, but that these improvements are at the expenseof model success for HO₂. Improvements to model simulations forHO₂, NO₃ and N₂O₅ can be realised simultaneously oninclusion of additional unsaturated volatile organic compounds, however thenature of these compounds is extremely uncertain.