Description:
Abstract. Total column ozone (TCO) is commonly measured by Brewer and Dobson spectroradiometers. Both types of instruments use solar irradiancemeasurements at four wavelengths in the ultraviolet radiation range toderive TCO. For the calibration and quality assurance of the measured TCOboth instrument types require periodic field comparisons with a referenceinstrument. This study presents traceable TCO retrievals from direct solar spectralirradiance measurements with the portable UV reference instrument QASUME.TCO is retrieved by a spectral fitting technique derived by a minimal leastsquare fit algorithm using spectral measurements in the wavelength rangebetween 305 and 345 nm. The retrieval is based on an atmospheric modelaccounting for different atmospheric parameters such as effective ozonetemperature, aerosol optical depth, Rayleigh scattering, SO2, groundair pressure, ozone absorption cross sections and top-of-the-atmosphere solarspectrum. Traceability is achieved by fully characterizing and calibratingthe QASUME spectroradiometer in the laboratory to SI standards(International System of Units). The TCO retrieval method from thisinstrument is independent from any reference instrument and does not require periodic in situ field calibration. The results show that TCO from QASUME can be retrieved with a relativestandard uncertainty of less than 0.8 % when accounting for uncertainties from the measurements and the retrieval model, such as different ozone absorption cross sections, different reference top-of-the-atmosphere solar spectra, uncertainties from effective ozone temperature or other atmosphericparameters. The long-term comparison of QASUME TCO with TCO derived from aBrewer and a Dobson in Davos, Switzerland, reveals that all threeinstruments are consistent within 1 % when using the ozone absorptioncross section from the University of Bremen. From the results and methodpresented here, other absolute SI calibrated cost-effective solarspectroradiometers, such as array spectroradiometers, may be applied fortraceable TCO monitoring.