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Stratospheric nitrogen dioxide in the Antarctic

Several UV–visible spectrometers have been developed at the ISAC‐CNR Institute. Differential Optical Absorption Spectroscopy (DOAS) methodology is applied to their measurements to monitor the amounts of stratospheric trace gases: mainly ozone (O3) and nitrogen dioxide (NO2) which is involved in the ozone cycle. Observations of the scattered zenith‐sky light were performed with one of these instruments installed at the Terra Nova Bay station (TNB), Antarctica. GASCOD (Gas Analyzer Spectrometer Correlating Optical Differences) is described briefly and a method for data analysis and validation of the results introduced. Some aspects of the DOAS technique are presented: the algorithm allowing the best spectral alignment between spectra obtained with GASCOD and a high‐resolution wavelength calibrated spectrum, is explained. Simple considerations allow for calculation of the NO2 concentration in the background spectrum used in DOAS analysis. For the period of activity of the GASCOD at TNB (1996–2003), the results of NO2 vertical column density (VCD) at twilight show a maximum in the summer and a minimum in the winter. Three years of measurements (2001–2003) are analysed in terms of stratospheric temperature and potential vorticity to obtain information about stratospheric warming that occurred in 2002 over Antarctica. The correlation between NO2 atmospheric content and stratospheric temperature is highlighted. The diurnal variations of NO2, which are controlled by photochemistry, show an unusual behaviour at high latitudes. Analysis of the a.m./p.m. ratios—the sunrise NO2 VC (a.m.) over the sunset VC (p.m.)—during different seasons and at various Solar Zenith Angles (SZA) is presented and discussed.