Test of the night-time polar stratospheric NO2 decay using wintertime SAOZ measurements and chemical data assimilation

[1] Chemistry Transport Models (CTMs) tend to underestimate very severely NO2 concentrations in the polar lower stratosphere during the winter suggesting that either the NOx sink is overestimated or a source of NOx is missing in models. We study the night-time decay of polar NO2 because it is controlled by the reaction between NO2 and O3 which is thought to be the main NOx sink at high latitudes winter. The model-calculated night-time decay of polar NO2 is tested using solar occultation measurement of NO2 and O3 taken within the Northern polar vortex by the ‘‘Système d’Analyse parObservation Zénithal’’ (SAOZ) instrument on board of a long duration balloon in February 2000. A trajectory analysis is performed in order to find air parcels which have been sampled at sunset and at sunrise by the SAOZ instrument and have stayed in the dark between the measurements. Sunset (or sunrise) SAOZ measurements are then assimilated in a trajectory photochemical model in a variational mode and compared to the corresponding sunrise (or sunset) SAOZ measurements which are called validation measurements. The results are used to assess the ability of the model to reproduce the observed night-time evolution of NO2. Overall, there is a good agreement between analyzed NO2 and validation measurements indicating that the nighttime chemistry of NO2 appears to be properly described by the model. The results do not hint at all at the existence of a NOx source, and certainly not a source strong enough to counteract the NO2 sink (NO2 + O3 reaction).