Optical characterisation of three reference Dobsons in the ATMOZ Project – verification of G. M. B. Dobson's original specifications

Abstract

Abstract. Three reference Dobsons (regional standard Dobsons No. 064, Germany and No. 074, Czech Republic as well as the world standard No. 083, USA) were optically characterized at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig in 2015 and at the Czech Metrology Institute (CMI) in Prague in 2016 within the EMRP ENV 059 project “Traceability for atmospheric total column ozone”. Slit functions and the related parameters of the instruments were measured and compared with G. M. B. Dobson's specifications in his handbook. All Dobsons show a predominantly good match of the slit functions and the peak (centroid) wavelengths with deviations between −0.11 and +0.12 nm and differences of the full width half maximum (FWHM) between 0.13 and 0.37 nm compared to the nominal values at the shorter wavelengths. Slightly larger deviations of the FWHMs from the nominal Dobson data, up to 1.22 nm, can be seen at the longer wavelengths, especially for the slit function of the long D-wavelength. However, differences between the effective absorption coefficients (EACs) for ozone derived using Dobson's nominal values of the optical parameters on one hand and these measured values on the other hand are not too large in the case of both “old” Bass–Paur (BP) and “new” IUP-ozone (Institut für Umweltphysik, University of Bremen) absorption cross sections. Their inclusion in the calculation of the total ozone column (TOC) leads to improvements of significantly less than ±1 % at the AD-wavelengths between −1 and −2 % at the CD-wavelengths pairs in the BP-scale. The effect on the TOC in the IUP-scale is somewhat larger at the AD-wavelengths, up to +1 % (D074), and smaller at the CD-wavelengths pair, from −0.44 to −1.5 %. Beside this positive effect gained from the data with higher metrological quality that is needed for trend analyses and satellite validation, it will be also possible to explain uncommon behaviours of field Dobsons during calibration services, especially when a newly developed transportable device TuPS (tuneable portable radiation source) from CMI proves its capability to provide similar results as the stationary setups in the laboratories of National Metrology Institutes. Then, the field Dobsons can be optically characterized as well during regular calibration campaigns. A corresponding publication will be prepared using the results of TuPS-based measurements of more than 10 Dobsons in field campaigns in 2017.

The effect of instrumental stray light on Brewer and Dobson total ozone measurements

“…The slit functions of the world standard Dobson no. 83 were experimentally measured by Komhyr et al (1993) and recently verified by Köhler et al (2018) using a tuneable light source. However, the published slit functions are restricted to the core band-passes.…”

“…Several wavelength pairs are used by the Dobson algorithm for calculating total column ozone. In order to minimize the effect of aerosol and other absorbers, two wavelength pairs are used such as AD, AC or CD where the A pair is 305.5-325.4 nm, C is 311.5-332.4 nm and D is 317.6-339.8 nm (Evans and Komhyr, 2008;Komhyr et al, 1993). For example, the total ozone using AD wavelength pair observations is retrieved using the following expression:…”

“…Other double wavelength pairs such as CD can be used for the ozone calculation by modifying Eq. (3) accordingly (Komhyr et al, 1993). The basic measurement principle for the Brewer instrument is the same as the Dobson.…”

The Design and Development of a Tunable and Portable Radiation Source for in Situ Spectrometer Characterisation

“…The values were set to 0,1 nm Full Width at Half Maximum (FWHM) for bandwidth and 0,05 nm OR better in the uncertainty of setting the central wavelength of emitted radiation. Optimal level of total radiant flux emitted from TuPS was found experimentally during the laboratory-based characterization of Dobson 074 (Köhler, et al, 2018) as a value that ranges from 10 nW to 1000 nW for an input spectral bandwidth of 0.1 nm. The optical layout was designed using the optical simulation tool Zemax with the aim to design a tuning machine that uses exclusively commercially available off the shelf opto-mechanical components and that provides the high spectral accuracy, the narrow bandwidth and the necessary optical power required by the Dobson.…”

“…Currently tuneable monochromatic sources which could be used for characterisation of Dobsons and Brewers are complex and cumbersome systems that are only found in a few laboratories world-wide and cannot be used for in field calibrations as requested by this global spectrometers network. Such laboratory-based characterisations were performed in CMI and PTB (Köhler, et al, 2018) having requested typically a couple of days' time for each spectrometer plus additional time necessary for shipping of often heavy and large DUTs from their permanent in-filed installation down to the metrology laboratory.…”