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3. Data.

All year around data are available for separated twilights in Ushuaia and Marambio. At Belgrano only two periods between mid February to the end of April and from beginning of August to the end of October measurements are possible due to the requirement of sza close to 90º. Vertical columns are obtained by using the same Air Mass Factor (AMF) for all the year. As the AMF is sensitive to the vertical profile of the absorbent species, an error of up to 9% is introduced in this procedure (Sarkissian et al., 1995). Data between sza 88º and 92º are averaged to produce a single data per twilight. No chemical correction has been included for NO₂. The instrument is optimised for NO₂ measurements. Ozone is retrieved from the transition peaking at 482 nm, in the 470-490 nm range. The model used in this study is the SLIMCAT 3D chemical transport model, described in detail by Chipperfield (1999).