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2. Technique
The instrument is a scanning spectrometer (named EVA) operating in the range of 430-450 nm for NO2 and 470-490 nm for O3 in zenith view mode at twilight using DOAS technique (Gil and Cacho 1992). A fully unattended version PC controlled through a serial line and temperature regulated was developed for this application. A set of 30 spectra, 200 samples each, are accumulated for each measurement in order to reduce the instrumental photomultiplier noise. The acquisition time for a single spectrum in each region takes 1s.
The spectrometers were compared each other after one year of measurements with the help of a NO2 cell containing a known amount of gas. The discrepancy among the three instruments was found to be below 4%. Instrumental random errors in vertical columns are 5x1014 molec.cm-2 for NO2 and 7x1016 molec.cm-2 (3 DU) for O3. In Belgrano, however, a higher random error results due to interference of an ionosonde antenna in operation up to the beginning of 2000.
Lower stratosphere temperature data for Belgrano, Marambio and Ushuaia are obtained from daily grids of NCAR/NCEP. Potential vorticity are obtained from NILU database.