On the Quality of the Total Ozone Dobson and Brewer Measurements: Results of the ESA-CALVAL Campaigns
Redondas, Alberto¹; Köhler, Ulf²; Kyrö, Esko³; Evans, Robert D.^4
1Izaña Atmpshpheric Research Centre (AEMET), SPAIN; ²Deutscher Wetterdienst, Hohenpeissenber, GERMANY; ³Finnish Meteorological Institute, Sodankylä, FINLAND; ⁴NOAA/ESRL, UNITED STATES

The European branch of the Global Ozone Monitoring Network operates two Regional Calibration Centers for the two different types of spectrophotometers in use. The RDCC-E for Dobsons has been located at the Meteorological Observatory Hohenpeissenberg (Germany) since 1999, the RBCC-E for Brewers at the Izana Observatory on Tenerife (Spain) since 2003. The ESA project, the CEOS Intercalibration of Ground-Based Spectrometers and Lidars , was initiated in 2009 to support these investigations and to improve the ground-based data base for trend analyses and validation of data obtained by satellite borne instruments. In the frame of this project several campaigns were organized at various locations under extremely different atmospheric conditions allowing the characterization of the properties of the Dobson and Brewer instrument and their differences.

Three types of campaigns were conducted. Absolute calibration campaigns at Izaña (2) focused on the calibration check of reference Brewer and Dobson through the Langley method. Nordic calibration campaigns (2) focused on the study of the stray light effect at high ozone and zenith angle conditions and routine calibration campaigns were carried out at Arosa (2) and Huelva (1) with the main objective to transfer the calibration of the regional Center to the network instruments.

Since 2011 the RBCC transfer during these campaigns their own calibration obtained from Langley absolute calibration due to the restrictions on the maintenance of the WRT. The travelling reference stability is checked before and after every campaign, with the triad. These instruments are continuously monitored and demonstrate a long term precision of .25% between them. The status of the Brewer network revealed at calibration campaigns shows that all of the operative instruments are in the +/-2% range, 80% are within 1% range and 2/3 show a perfect agreement of +/- .5% after two years calibration period.Improvements to the calibration procedure were introduced which allow the classifications of the quality of the Brewer instruments based on their linearity. The stray light effect was characterized during the campaigns and this data used to develop an instrumental model capable of corrections based on the characteristics of the instrument.

The comparison with the standard Dobson with its current calibration level (traced back to the World Primary Standard D083) measures about 1% lower ozone than the standard Brewer with somewhat larger but explainable differences at low sun, high ozone and high turbidity. A key outcome of the project is that the Izaña site was found to be adequate for absolute calibrations of both types of spectrophotometers using the Langley Plot method. The resulting calibrations are comparable to those commonly performed at Mauna Loa using the World Primary Standard. The application of the Langley calibration to Dobson and Brewer together with the analysis of different absorption coefficients derived from the ACSO ("Absorption Cross Sections of ozone",http://igaco-o3.fmi.fi/ACSO/ws_2013.html ) initiative give encouraging results solving the Brewer-Dobson discrepancies when the Unversity of Bremen cross sections are used.