Abstract
Abstract. 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.
Highlights
Nowadays the primary ground-based instruments used to report total ozone column (TOC) are Dobson and Brewer spectrophotometers
The Brewer spectrometer (Brewer, 1973; Kerr et al, 1981; Kerr, 2010) was introduced in the 1980s as an automatic device measuring direct solar UV radiation and global UV irradiance. Both the Brewer and the Dobson instruments were considered by the World Meteorological Organization (WMO) in the framework of the Global Atmosphere Watch program (GAW) as the standard instruments for TOC monitoring
While the Brewer spectrophotometer is measuring in the ozone mode, the laser wavelength is swept ±2 nm with a step of 0.04 nm around the six Brewer operational wavelengths selected by the rotating slit mask for different grating positions (Fig. 1)
Summary
Nowadays the primary ground-based instruments used to report total ozone column (TOC) are Dobson and Brewer spectrophotometers. The normal determination of the ozone absorption coefficients involves scanning of the spectral lines in the scanning mode of the instrument so that the dispersion relation is required to convert the grating positions in micrometre steps to the respective wavelengths. We can determine the instrumental slit functions directly in the ozone mode by scanning them with the tunable laser and weight them with the ozone absorption crosssections without the need of assumption about the slit functions and the dispersion relations used in the normal calibration procedure While the Brewer spectrophotometer is measuring in the ozone mode, the laser wavelength is swept ±2 nm with a step of 0.04 nm around the six Brewer operational wavelengths selected by the rotating slit mask for different grating positions (Fig. 1). Cd (multiplet) 361.163 5 lamps at the end of spectral range of the Brewer spectrophotometer and due to the fact that the emission lines of the used lamps are not spaced
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