1. Quadrennial Ozone Symposium
Edinburgh, Scotland
5. – 9. September 2016
Optical Characterization of Three Reference Dobsons in the ATMOZ Project– Verification of Prof. Dobson’s Original Specifications
U. Köhler (1), S. Nevas (2), G. McConville (3), R. Evans (3), M. Smid (4), M. Stanek (5), A. Redondas (6), and F. Schönenborn (1)
(1) Deutscher Wetterdienst, Met. Obs. Hohenpeissenberg, Hohenpeissenberg, Germany (2) Physikalisch-Technische Bundesanstalt, Braunschweig ,Germany, (3) NOAA ESRL, Boulder, USA, (4) Czech Metrology Institute, Prague, Czech Republic, (5) Czech Hydrometeorological Institute, Hradec Kralove, Czech Republic, (6) AEMET, Izaña, Tenerife/Spain
Summary
Three standard Dobsons (D064 Germany, D074 Czech Republic, D083 USA were radiometrically characterized at PTB (Physikalisch- Technische Bundesanstalt in Braunschweig) and CMI (Czech Metrology Institute in Prague).
Wavelengths and bandpass characterizations were measured and compared with Dobson’s specification in his handbook
A good match of the bandpass functions between D083, D064 and D074 could be observed.
The agreement of the peak (centroid) wavelengths was also good
The measured values deviate a bit from the nominal Dobson data
The differences of the derived absorptions coefficients to the Dobson’s nominal ones are not very large and the effects on the calculation on the total ozone column TOC are minor.
Due to the lack of time, scan of the bandpass functions into the stray light regions was not done.
Figure 1. Schematic representation of bandpass function and wavelength measurements of Dobson spectrophotometers at the PLACOS setup of PTB Braunschweig
Setup for measuring Dobsons at PTB Braunschweig
Experimental set-up of D074 calibration at CMI Prague
Double grating subtractive
1200 G/mm
Wavelength scale validated at u = nm
FWHM of measuring beam 0,1 nm
Output beam at F= #/10
Low photon flux reference SSDS detector system used as a monitor detector
Custom made light tight box constructed to suppress the optical background
Flipping mirror used to align the beam with the calibrated #074 Dobson spectrophotometer
f/10
Figure 2. Schematic representation of bandpass function and wavelength measurements of Dobson No. 074 spectrophotometer at the CMI Prahue
Measurements in the laboratory: wavelength setting and bandpass
Three campaigns in two different laboratories were carried out:
D083 (world standard, NOAA, Boulder, USA) and D064 (European standard, DWD, Hohenpeissenberg, Germany) were characterised at the Physikalisch-Technische Bundesanstalt PTB in Braunschweig from May 28 – 29 and 18 – 21, 2015, calibration done on laser-based system (figure 1).
D074 as the second European standard (CHMI, Hradec Kralove, Czech Republic) was characterised at the Czech Metrology Institute CMI in Prague from March 21 – 24, 2016, calibration done on monochromator-based system (figure 2).
Despite some problems with signal processing and non-linear behaviour of the Dobson electronics, especially during the first campaign with the Hohenpeissenberg Dobson No. 064, all laboratory measurements provided good data, which enabled to calculate the instrumentally specific wavelength settings and bandbass/slit functions of the various wavelengths for each Dobson instrument (see table 1 and figure 3 for all instruments and figure 4 as an example for D064 in all wavelengths).
The differences of the wavelengths peaks to the specified values are between
-0.04nm (D083 at A-S2) and +0.12nm (D064 at D-S2). Most of the peaks‘ differences are, however, clearly less than 0.1nm. The FWHM and the shape of the slitfunctions at the short wavelengths are very similar to the Dobson specifications, whereas the longer wavelengths show larger deviations from the ideal curve
Determination of effective ozone absorption coefficients
The convolution of the these individual slit functions at the observed wavelengths with the currently valid Bass/Paur ozone cross sections provides effective ozone absorption coefficients (see table 2). The comparison with the nominal absorption coefficients, given by Prof. G.M.B. Dobson, confirms, that the effects on the total ozone calculation are between -0.1% and +0.6% in the mostly used wavelengths pair AD. The effects on the CD TOC are approximately -1 to -2%. This result can explain the principal difference between the AD- and CD-TOC.
Table 1. Measured wavelength settings and FWHM (Full Width Half Maximum) for all Dobson and wavelengths
Measured slitfunctions of all Dobsons
Figure 3. Slitfunctions of D064, D074, D083 and nominal Dobson curve for short A-wavelangth
Figure 4. Slitfunctions of D064 (blue) for all wavelengths compared with the nominal Dobson curves (green)
Conclusion
The results of these investigations of the optical properties of standard Dobsons, the upcoming introduction of the proposed new ozone cross sections and the recommended use of real effective temperature of the ozone layer, resulting in new individual Effective Absorption Coefficients (EACs) for each standard and field instrument, will help to enhance the quality of the Dobson TOC-measurements in the global ozone monitoring network significantly.
Table 2. Effective Absorption Coefficients derived from laboratory investigations for all Dobson in comparison with specification and older results (D083 in 2010)
Meteorological Observatory Hohenpeissenberg
The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union