Calibrations of Brewer spectrophotometers for total ozone observations can be performed either by intercomparisons towards a reference instrument (the most frequent method) or by absolute calibration using Langley plot method (feasible only at specific locations). The world Brewer calibration scale is defined by the Triad of Brewer instruments maintained by MSC (Meteorological Service of Canada) in Toronto. Each Brewer of the Triad is calibrated absolutely at the Mauna Loa Observatory, Hawaii (MLO). Instruments in the world network are routinely calibrated by comparisons with travelling references, outside of Canada mostly by the Brewer 017 operated by the International Ozone Service - a Canadian commercial company. Brewer 017 is regularly checked towards the reference Triad.

In November 2003 the WMO/GAW Regional Brewer Calibration Centre for RA-VI region (RBCC-E) has been establish at the Observatory Izaña of INM, Canary Islands (IZO). RBCC-E owns a full calibration and reference-maintenace equipment composed by three Brewer spectroradiometers (The IZO Triad): a Regional Primary Reference (Brewer 157), a Regional Secondary Reference (Brewer 185) and a Regional Travelling Reference (Brewer 183) which can be transported for calibration campaigns outside IZO. Besides, an Optics Laboratory is available for indoors calibrations as well as equipment tests.

IZO is located in subtropical region (28ºN) on the top of Izaña Mountain (2370 m.a.s.l.) with pristine skies and low ozone variability. This location allows routine absolute calibrations of the references in similar conditions to the MLO site. The IZO Triad is linked to the MSC Triad by yearly calibrations towards the Canadian travelling reference Brewer 017. Though the IZO Triad is routinely calibrated by the Langley method this absolute calibration is not used to define a new scale. The MSC Triad is respected as the official bearer of the GAW Brewer international scale. However, the establishment of the IZO Triad allows implementation of a self-sufficient European Brewer calibration system that respects the world scale but works as an independent GAW infrastructure. This is an essential step for creation of a coordinated European Brewer Network which is needed both for present and future quality control and quality assurance of ground total ozone observations, and for validation of satellite instruments operated under European space missions, among others. The function of RBCC-E also allows development and testing of new measurement techniques for the ground Brewer network like zenith, polarimetric, UV or aerosol optical depth measurements at IZO.

RBCC-E is located on the top of Izaña mountain (2367 m above the sea level), in Tenerife Island (28°N,16°E), the largest of the Canaries. The excellent quality of Canarian skies (which are protected by a special law) is ideal to perform Atmospheric Science. Several other institutions such as Instituto Astrofísico de Canarias (IAC) and Observatorio Atmosférico de Izaña (OAI) have already set their HQs in Tenerife.

1. To implement a technology for routine absolute calibrations of the European Brewer regional reference instruments at Izana that is fully compatible with absolute calibrations of the world reference Triad at MLO.
2. The Regional Primary Reference B157 to be used for calibration campaigns organized just at IZO and for a check of calibration stability of the Regional Travelling Reference B183 spectrophotometer used for continental campaigns.

#Regular comparisons of the Regional Brewer Primary Reference B157 with the Regional Dobson Reference D074 to monitor relation between both calibration scales in the Region. #To study the sources of errors of the absolute calibrations and to determine the accuracy of total ozone measurement achievable by this method due to atmospheric conditions or instrumental characteristics.

With reference to responses submitted after the RBSG-E meeting held at ElArenosillo/Mazagon, September 2003 the following specialists from Europe and MSC have been affiliated with particular working tasks defined at the meeting. Underlined are the experts which have expressed their preliminary interest to lead/coordinate groups on the working tasks.

*WT1 Establishment and maintenance of Regional Brewer Reference Spectrophotometers. E. Cuevas, A. Redondas, M. Stanek, T. Grajnar *WT3 Calibration histories of instruments. K. Vanicek *WT4 Relation between the Dobson and Brewer calibration scales. J. Staehelin, H. DeBacker, U. Koehler, J. Krzyscin, K. Vanicek *WT5 UV calibration of Brewer spectrophotometers. J. Groebner, A. Bais, T. Koskela *WT6 Measurements of UV Aerosol Optical Depth with Brewer spectrophotometers. A. Bais, H. DeBacker, J. Groebner, J. Jaroslawski