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.
Nowadays the primary ground-based instruments used to report total ozone
column (TOC) are Dobson and Brewer spectrophotometers. Based on the
irradiances measured by these instruments in the ultraviolet (UV) spectral
range and on well-defined retrieval procedures, TOC values are derived. The
Brewer spectrometer
Slits and wavelengths used in the Brewer operative algorithms. The
table provides the mean and the standard deviation of the central wavelengths
and the full-width-half-maximum (FWHM) in nanometres of the slits of the
“average” Brewer spectrophotometer determined during RBCC-E campaigns
The calculation of the ozone absorption coefficients from the calibration directly
in the ozone mode. 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. Here 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 cross-sections without the need of assumption about the slit
functions and the dispersion relations used in the normal calibration
procedure (See Sect. The calculation of the dispersion relation based on regularly spaced reference
spectral lines, provided by the tunable laser instead of the irregularly
distributed emission lines of the Hg, Cd and Zn spectral lamps.
During the experiment we performed three measurements:
The standard method for the dispersion measurements using spectral lamps described in Sect. 2. Direct dispersion measurements (laser scanning). While the Brewer spectrophotometer is measuring in the ozone mode, the laser wavelength is swept Dispersion measurements using the tunable laser (Brewer scanning).
While the laser is emitting at a fixed wavelength in the range from 290 to
365 nm with an increment of 5 nm, the Brewer instrument scans
Emission lines of the discharge lamps used for Brewer calibration.
Results of Brewer measurements in the ozone mode obtained while the laser wavelength was changed every 0.04 nm. The central wavelength and the FWHM calculated are displayed in red using the same methodology of the dispersion test.
The Brewer instrument measures the irradiance of direct sunlight at six
nominal wavelengths (
This widely eliminates absorption features which depend, in local approximation, linearly on the wavelength, like for example the contribution from aerosols.
We can divide the calibration in three steps including instrumental
calibration, wavelength calibration, and ETC transfer:
The instrumental calibration includes all the parameters that affect the
double ratios ( Wavelength calibration is needed to determine the ozone absorption
coefficient. The so-called “dispersion test” is used to obtain the
particular wavelengths for the instrument and the slits, or instrumental
functions, of each spectrophotometer. Note that the precise wavelengths of
every Brewer spectrophotometer are slightly different from instrument to
instrument. Finally, the ETC transfer is performed by comparison with the reference
Brewer instrument or, in the case of the reference instruments, by the
Langley method.
The Brewer wavelength calibration follows the operative procedure
The ozone absorption coefficient were defined as follows
where
The Brewer operative method uses the following assumptions:
Only “ideal” slits are used; the slit functions are parametrized as trapezoids,
i.e. isosceles triangles truncated at 0.87 height. Stray light is not considered, i.e. zero slit function values are
assumed outside the triangle. The FWHM of the triangle is considered different for every slit.
It is calculated from the dispersion test, determined in micrometre steps
and then converted to wavelengths using the dispersion relation
(Fig. The ozone cross sections are expressed by the Bass & Paur absorption
coefficient data set. Solar spectrum is considered constant on the slit.
Under these assumptions, the ozone effective absorption is essentially
obtained the same way as in the approximation method of
The Brewer spectrophotometer is
constructed based either on a single or a double monochromator of modified
Ebert–Fastie type, generally referred to as single or double Brewer,
respectively. The first monochromator disperses the incoming radiation onto
six exit slits. In the case of the double-monochromator Brewer, the six exit
slits (intermediate slits) of the first monochromator are the entrance slits
to a second monochromator that is used in subtractive mode. The wavelength is
selected by choosing one of the six exit slits (ozone mode) or rotating the
grating (scanning mode). The rotation of the grating is managed by a drive
mechanism consisting of a motor-driven micrometre linked to an arm that
rotates the grating. The smallest wavelength increment corresponding to one
stepper motor step varies steadily from approximately 8.0 to 7.0 pm
Slit function of the slit 3 (320 nm) measured in the scanning mode:
the standard method uses the normalized values only between 0.2 and 0.8
(points with crosses). The figure shows the signals recorded during the scans
(orange diamonds) and the non-linearity-corrected values (blue circles) in
counts s
Pulsed OPO-based setup at PTB that was used for measuring the slit functions of the Brewer spectrophotometer.
The dispersion relation, which provides the relation between the micrometre
steps and the monochromator set wavelengths, is determined by scanning the
emission lines as described in Sect. 1. The line scans are carried out with
an increment of 10 motor steps (0.7 Å). From the results, the central
position and the FWHM of the slit function are calculated in motor steps
assuming an isosceles triangle. Both sides of the peak are fitted to a
straight line taking only the function values above 20 and below 80 % of
the normalized peak. The central point is calculated by the intersection
point and the FWHM is the width of the triangle (Fig.
The cubic approximation method of
The stability of the wavelength calibration during Brewer operations is
checked by measuring the internal Hg lamp. In most of the Brewers, the
302
Log–log plot of the normalized ratio of measured Brewer counts to
the monitor photodiode signal, which is proportional to the laser power,
plotted as a function of the Brewer counts s
Brewer measurements in the ozone mode while the laser wavelength is changed every 0.04 nm. The orange curve corresponds to the dark counts obtained from the measurements of slit 1.
For the characterization of the bandpass functions of the Brewer instrument,
an upgraded PLACOS setup
In contrast with the standard calibration procedure, where the Brewer
instrument scans the lines of the spectral lamps, in this experiment the
Brewer measures in ozone mode. Here, the Brewer grating is fixed at the ozone
position while the coupled laser beam is measured using the seven slits (slit
#1 is used to obtain the dark signal values). During these measurements
the wavelength of the OPO system is scanned with 0.04
The Brewer detector system, which is based on a PMT, responds non-linearly to pulsed sources. For the measurements of pulsed sources, the PMT manual advises changing the electronics configuration. As the main objective was to validate the operational wavelength calibration of the Brewer, we decided to keep the instrument configuration equivalent to that during the field operations. The non-linearity problem was solved by determining the respective correction function. For this purpose, the power of the laser beam was varied while simultaneously measuring signals of the PMT and the linear monitor photodiode.
Plot of the parametrized (thick lines, left axis) and the measured
slit functions (dots, left axis), as well as the different ozone
cross-sections in
Ozone absorption coefficients in atm cm
The ratios of the measured Brewer counts to the recorded monitor photodiode
signals are shown in Fig.
Differences between the central wavelengths
Ozone absorption coefficient, in atm cm
We observed that the recorder dark signal values (measurements performed with the blocked slit #1) were highest immediately after exposing the PMT to the laser light (see Fig. 5). The dark signal of the PMT was then gradually fast decreasing with time after the excitation, which may cause the signal values obtained for slit #1 (measured immediately after slit #0) be higher than for the other slits measured afterwards.
The Brewer algorithms assume trapezoidal slits functions cut at 0.87 of the
height (Fig.
Among the available data sets there are versions of Bass and Paur (1985)
cross-sections denoted as Brewer operational (Brw), IGACO quadratic
coefficient (B&P), the cross-sections of Daumont Brion Malicet (DBM)
(
Using the measured slit functions, the calculated effective ozone cross
sections are
Residuals of the quadratic (filled circles, solid lines) and cubic
(dashed lines) fits. The colour indicates the six Brewer slits with the laser
wavelengths in equally spaced grid every 5
The experiment allows us to validate the Brewer standard methodology used to perform the wavelength calibration. For this purpose, we compare laser-based wavelength calibration results to those yielded by the standard operative method based on scanning the spectral lamps in case of both the quadratic and the cubic fit to the dispersion relation.
Figure
The differences between the ozone absorption coefficients calculated from the
scanning and the direct measurements of the slit functions are summarized for
the six measurements in Table
Using the measured slit functions instead of the parametrized ones
increases the ozone absorption coefficients and consequently the calculated
ozone values by 0.8 %. The quadratic dispersion relation fit used in the standard Brewer algorithm
is not suitable outside the ozone spectral range 310–320 The comparison of the results of the three experiments shows a maximum
difference of 0.3 % if the cubic fit is used to approximate the
dispersion relation of the Brewer instrument. The respective difference
between the ozone absorption coefficient that is obtained from the direct
measurements of the tunable laser in the ozone mode and from the operative
discharge lamp method is only of 0.1 %, if both use the parametrized or
measured slit. This confirms the standard procedure used for the RBCC-E
calibrations. This work validates the current wavelength calibration method of the
Brewer network and shows that in general there is no need for the
characterization of the network Brewer instruments with a tuneable source.
However, the limitations of the quadratic dispersion approximation used over
extended spectral range of Brewers MK-III and MK-IV are evident. Thus, we
suggest updating the operating Brewer software for this model instruments
with the recent version, which already includes the cubic dispersion.
The data used for the present study can be made available after personal communication with the authors of the paper.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Quadrennial Ozone Symposium 2016 – Status and trends of atmospheric ozone (ACP/AMT inter-journal SI)”. It is a result of the Quadrennial Ozone Symposium 2016, Edinburgh, United Kingdom, 4–9 September 2016.
This work has been supported by the European Metrology Research Programme (EMRP) within the joint research project ENV59 “Traceability for atmospheric total column ozone” (ATMOZ). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Edited by: Alkiviadis Bais Reviewed by: Lionel Doppler and Julian Gröbner