Stability of the Regional Brewer Calibration Center for Europe Triad during the period 2005–2016

Abstract. Total ozone column can be measured using Brewer sprectrophotometers which are calibrated periodically in inter- comparison campaigns with respect to a reference instrument. In 2003 the Regional Brewer Calibration Centre for Europe (RBCC-E) was established at the Izaña Atmospheric Research Centre (Canary Islands, Spain) and from 2011 it has transferred its own calibration mainly to other European Brewers using the Brewer #185 as reference instrument. The RBCC-E organizes regular inter-comparisons which are held annually alternating between Arosa (Switzerland) and El Arenosillo (Spain). This work is focused on showing the stability of the measurements of the RBCC-E Triad (Brewers #157, #183 and #185) made in the Izaña Atmospheric Observatory during the period 2005–2016. In order to study the long-term precision of the RBCC-E Triad, it must be taken into account that each Brewer performs a large number of measurements every day and, hence, it be- comes necessary to calculate a representative value of all of them. This value was calculated from methods previously used to study the long-term behaviour of the World Reference and Arosa Triads. Applying their procedures in our triad allows us to compare the three instruments. In this way, the difference between the values calculated for each Brewer and the triad mean was analyzed. In daily averages, applying the procedure used for the World Triad Reference, the RBCC-E Triad presents a relative standard deviation mean equal to 0.41 % (σ157 = 0.362 %, σ183 = 0.453 % and σ185 = 0.428 %). In opposite, using the procedure of the Arosa Triad, the RBCC-E presents a relative standard deviation around at σ = 0.5 %. In monthly averages, the method of the World Triad Reference give a relative standard deviation of 0.33 %, 0.34 % and 0.23 % for Brewers #157, #183 and #185, respectively (0.3 % in mean). Whereas, the procedure of the Arosa Triad gives a monthly values 0.3 %. In this work, two ozone datasets are analyzed: the first included all the ozone measurements available while the second only includes the simultaneous measurements of all three instruments. Furthermore, in this paper we also describe the Langley method used in the RBCC-E Triad to calculate the Extra-terrestrial constant (ETC), which is the necessary first step to ozone retrieval. Finally, the short-term, or intraday, stability is also studied to identify the effect of the solar zenith angle on the accuracy of the RBCC-E Triad.