ALBERT

Description: Total ozone column measurements can be made using Brewer spectrophotometers, which are calibrated periodically in intercomparison 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 Center (Canary Islands, Spain), and since 2011 the RBCC-E has transferred its calibration based on the Langley method using travelling standard(s) that are wholly and independently calibrated at Izaña. This work is focused on reporting the consistency of the measurements of the RBCC-E triad (Brewer instruments #157, #183 and #185) made at 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 takes a large number of measurements every day and, hence, it becomes necessary to calculate a representative value of all of them. This value was calculated from two different methods previously used to study the long-term behaviour of the world reference triad (Toronto triad) and Arosa triad. Applying their procedures to the data from the RBCC-E triad allows the comparison of the three instruments. In daily averages, applying the procedure used for the world reference triad, the RBCC-E triad presents a relative standard deviation equal to σ = 0.41 %, which is calculated as the mean of the individual values for each Brewer (σ157 = 0.362 %, σ183 = 0.453 % and σ185 = 0.428 %). Alternatively, using the procedure used to analyse the Arosa triad, the RBCC-E presents a relative standard deviation of about σ = 0.5 %. In monthly averages, the method used for the data from the world reference triad gives a relative standard deviation mean equal to σ = 0.3 % (σ157 = 0.33 %, σ183 = 0.34 % and σ185 = 0.23 %). However, the procedure of the Arosa triad gives monthly values of σ = 0.5 %. In this work, two ozone data sets are analysed: the first includes all the ozone measurements available, while the second only includes the simultaneous measurements of all three instruments. Furthermore, this paper also describes the Langley method used to determine the extraterrestrial constant (ETC) for the RBCC-E triad, the necessary first step toward accurate ozone calculation. Finally, the short-term or intraday consistency is also studied to identify the effect of the solar zenith angle on the precision of the RBCC-E triad.

Description: Non-ideal angular response of a spectroradiometer is a well-known error source of spectral UV measurements and for that reason instrument specific cosine error correction is applied. In this paper, the performance of the cosine error correction method of Brewer spectral UV measurements in use at the Finnish Meteorological Institute (FMI) is studied. Ideally, the correction depends on the actual sky radiation distribution, which can change even during one spectral scan due to rapid changes in cloudiness. The FMI method has been developed to take into account the changes in the ratio of direct to diffuse sky radiation and it derives a correction coefficient for each measured wavelength. Measurements of five Brewers were corrected for the cosine error and the results were compared to the reference travelling spectroradiometer (QASUME). Measurements were performed during the RBCC-E (Regional Brewer Calibration Center – Europe) X Campaign held at El Arenosillo, Huelva (37∘ N, 7∘ W), Spain, in 2015. In addition, results of site audits of FMI's Brewers in Sodankylä (67∘ N, 27∘ E) and Jokioinen (61∘ N, 24∘ E) during 2002–2014 were studied. The results show that the spectral cosine error correction varied between 4 and 14 %. After that the correction was applied to Brewer UV spectra the relative differences between the QASUME and the Brewer diminished even by 10 %. The study confirms that the method, originally developed for measurements at high latitudes, can be used at mid-latitudes as well. The method is applicable to other Brewers as far as the required input parameters, i.e. total ozone, aerosol information, albedo, instrument specific angular response and slit function are available.

Description: In this contribution we present the wavelength calibration of the travelling reference Brewer spectrometer of the Regional Brewer Calibration Center for Europe (RBCC-E) at PTB in Braunschweig, Germany. The wavelength calibration is needed for the calculation of the ozone absorption coefficients used by the Brewer ozone algorithm. In order to validate the standard procedure for determining Brewer's wavelength scale, a calibration has been performed by using a tunable laser source at PTB in the framework of the EMRP project ENV59 ATMOZ “Traceability for the total column ozone”. Here we compare these results to those of the standard procedure for the wavelength calibration of the Brewer instrument. Such a comparison allows validating the standard methodology used for measuring the ozone absorption coefficient with respect to several assumptions. The results of the laser-based calibrations reproduces those obtained by the standard operational methodology and shows that there is an underestimation of 0.8 % of the ozone absorption coefficients due to the use of the parametrized slit functions.

Description: In this contribution we present the wavelength calibration of the traveling reference Brewer spectrometer of the Regional Brewer Calibration Center for Europe (RBCC-E) at PTB in Braunschweig, Germany. The wavelength calibration is needed for the calculation of the ozone absorption coefficients used by the Brewer ozone algorithm. In order to validate the standard procedure for determining Brewer’s wavelength scale, a calibration has been performed by using a tuneable laser source at PTB in the framework of the EMRP project ENV59 ATMOZ Traceability for the total column ozone. Here we compare these results to those of the standard procedure for the wavelength calibration of the Brewer instrument. Such a comparison allows validating the standard methodology used for measuring the ozone absorption coefficient with respect to several assumptions. The results of the laser-based calibrations reproduces those obtained by the standard operational methodology and shows that there is a underestimation of 0.8 % due the use of the parametrized slit functions.

Description: 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.