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.
Highlights
The first measurements of the total ozone column (TOC) were started in the 1920s
Beside this positive effect gained from the data with higher metrological quality that is needed for trend analyses and satellite validation, it will be possible to explain uncommon behaviours of field Dobsons during calibration services, especially when a newly developed transportable device TuPS from Czech Metrology Institute (CMI) proves its capability to provide similar results as the stationary setups in the laboratories of National Metrology Institutes
The Physikalisch-Technische Bundesanstalt (PTB) used for the slit function measurements was a spectrally tuneable laser facility operating in a nanosecond-pulsed mode
Summary
The first measurements of the TOC were started in the 1920s. Such observations became possible after the development of the Dobson spectrophotometer by G. Whereas the ETCs of field Dobsons are specific for each instrument and can be determined by regular inter-comparisons with absolutely calibrated reference Dobsons, the absorption coefficients are assumed to be the same for all Dobsons This assumption is based on the idea, that the optical alignments of individual Dobsons match the specifications in G.M.B. Dobson’s manuals (Dobson, 1957a, b; Dobson and Normand, 1962). All these error sources are not so large and crucial, that the overall accuracy of TOC observations with well aligned and calibrated Dobsons are affected too much (see Basher, 1982) In contrast to this simplification of using nominal absorption coefficients for all Dobsons, the more modern Brewer spectrophotometer, developed and introduced into the global network in the late 1970s and early 1980s, uses EACs, which are specific for each individual instrument and can be determined during the basic calibration procedure (Kerr et al, 1985).
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