Consistency of total column ozone measurements between the Brewer and Dobson spectroradiometers of the LKO Arosa and PMOD/WRC Davos Julian Gröbner1, Herbert Schill1, Luca Egli1, and René Stübi2 1 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, PMOD/WRC, Davos, Switzerland 2 MeteoSwiss, Payerne, Switzerland Introduction The world's longest continuous total column ozone time series was initiated in 1926 at the Lichtklimatisches Observatorium (LKO), at Arosa, in the Swiss Alps with Dobson D002 (Staehelin, et al., 2018). Currently, three Dobson (D051, D062, D101) and three Brewer (B040, B072, B156) spectroradiometers are operated continuously. Since 2010, the spectroradiometers have been successively relocated to the PMOD/WRC, located in the nearby valley of Davos (1590~m.a.s.l.), at 12 km horizontal distance from Arosa (1850~m.a.s.l.). The last two instruments, Brewer B040 and Dobson D062 have been transferred in February 2021, successfully completing the transfer of the measurements from LKO Arosa to PMOD/WRC, Davos.

Figure 3. View inside the MeteoSwiss with the Figure 1. Yearly (red), 11-year (burgundy and Gaussian-filtered Figure 2. The MeteoSwiss Brewer and Dobson automated Dobson spectroradiometer triad. 11-year (light green) averages of homogenized total column spectroradiometers in operation at PMOD/WRC. ozone at Arosa. Below are the timelines for the different instruments in operation at LKO Arosa and PMOD/WRC Davos. The problem The total column ozone measurements between Dobson and Brewer spectroradiometers show a significant seasonal variation of 1.6%, which is correlated to the effective ozone temperature. The effective ozone temperature is on average 225.2 K at PMOD/WRC, with a seasonal variation of amplitude 5.7 K.

Figure 5. Effective ozone temperature calculated from ozone sonde launches at MeteoSwiss station Payerne (blue curve), and from the ECMWF reanalysis (yellow curve).

Figure 4. Left figure: Relative differences of total column ozone between Brewer B156 and Dobson D101 using the operational ozone retrieval procedure with the ozone absorption cross-sections from Bass & Paur (BPOp). The yellow curve is a periodic fit to the data. The ozone absorption cross-sections Right figure: the same relative differences, shown versus effective ozone temperature. BPOp The nominal ozone absorption cross-sections of Bass&Paur at a temperature of 228 K (Brewer), and 227 K (Dobson) The study IGQ The quadratic polynomial temperature approximation of The total column ozone from Brewer and Dobson spectroradiometers were calculated using the following steps: Bass&Paur from the IGACO web-page (file bp.par).  Evaluating different ozone absorption cross-sections applying the effective ozone temperature at Arosa/Davos. DBM The dataset of Daumont, Brion, and Malicet.  Applying the measured line-spread functions of Dobson D101 (Šmid et al., 2020). IUP Dataset measured by the University of Bremen in 2013.  Rayleigh scattering coefficients for Dobson & Brewer using the formulation by Bodhaine et al., 1999. IUP_ATMOZ Dataset measured by the University of Bremen in 2017 in the frame of the project EMRP ATMOZ. The results ACS Dataset measured in the frame of the ESA project SEOM-IAS. The comparisons are based on simultaneous measurements within a 5 minute window, totalling more than 40000 measurements over the time period 1 January 2016 to 30 June 2020. The solar zenith angle varied between 23.3° References Gröbner, J., Schill, H., Egli, L., and Stübi, R.: Consistency of and 75.7° and the corresponding effective airmass for ozone between 1.1 and 3.9. total column ozone measurements between the Brewer and Dobson spectroradiometers of the LKO Arosa and Table: Average offset and seasonal amplitude in % of the relative differences between PMOD/WRC Davos, Atmos. Meas. Tech., 14, 3319–3331, Brewers B040, B072, B156, and B163 to Dobson D101 for the five investigated ozone https://doi.org/10.5194/amt-14-3319-2021, 2021. absorption cross-sections. The last row in the table (Br avg) represents the average of the Staehelin, et al.: Stratospheric ozone measurements at Arosa (Switzerland): history and scientific relevance, results of the four Brewers relative to D101. Atmos. Chem. Phys., 18, 6567–6584, https: //doi.org/10.5194/acp-18-6567-2018, 2018. Serdyuchenko, et al.: High spectral resolution ozone absorption crosssections – Part 2: Temperature dependence, Atmos. Meas. Tech. Discuss., 6, 6613–6643, doi:10.5194/amtd-6-6613-2013, 2013. Bohaine, B. A., et al.: On Rayleigh Optical Depth Calculations, J. Atmos. Ocean. tech., 16, 1854-1861, 1999. Šmíd, M., et al.: The design and development of a tuneable and portable radiation source for in situ spectrometer characterisation, Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2020-244, accepted, 2021. Gröbner, J.: The datasets used in this study can be downloaded from https://doi.org/10.5281/zenodo.4559802. The conclusions  The ozone absorption coefficient of the Brewer has a temperature dependence of 0.1% per 10K, while the Dobson has a nearly 10 times larger sensitivity to effective ozone temperature of 0.9% per 10 K.  Figure 6. Left panels: Relative differences of total column The highest consistency between Brewers and Dobson is obtained with the IUP ozone absorption cross-sections from ozone between Brewer B156 and Dobson D101. The yellow Serdyuchenko et al., 2013 and applying an effective ozone temperature correction. curves represent a seasonal fit to the data with a period of  The total column ozone dataset reprocessed using the ECMWF effective ozone temperatures gives nearly identical results to one year. the one obtained from the ozone sonde measurements at Payerne. Right panels: the same data shown with respect to the This research has been supported by the ESA project QA4EO (grant no. QA4EO/SER/SUB/09) and by the project INFO3RS funded by MeteoSwiss effective ozone temperature. (grant no. 123001926). The ozone measurement program at PMOD/WRC is financed by MeteoSwiss.