A visible spectrometer dedicated to ozone and NO2 monitoring for the next few years has been installed at the French Antarctica base of Dumont d'Urville at 66°S, 140°E. The first three months of data in Austral summer and fall allow the observation of the NO2 seasonal decay together with its diurnal variation. The data are found incompatible with the currently accepted NOX photochemistry. A conversion of nitrogen compounds into another storage species is required. The reaction which probably involves N2O5 should be active at temperature as high as - 45°C.