A compact array spectroradiometer that enables precise and robust
measurements of solar UV spectral direct irradiance is presented. We
show that this instrument can retrieve total ozone column (TOC)
accurately. The internal stray light, which is often the limiting
factor for measurements in the UV spectral range and increases the
uncertainty for TOC analysis, is physically reduced so that no other
stray-light reduction methods, such as mathematical corrections, are
necessary. The instrument has been extensively characterised at the
Physikalisch-Technische Bundesanstalt (PTB) in Germany. During an
international total ozone measurement intercomparison at the
Izaña Atmospheric Observatory in Tenerife, the high-quality applicability of
the instrument was verified with measurements of the direct solar
irradiance and subsequent TOC evaluations based on the spectral data
measured between 12 and 30 September 2016. The results showed
deviations of the TOC of less than 1.5 % from most other
instruments in most situations and not exceeding 3 %
from established TOC measurement systems such as Dobson or Brewer.
Introduction
Many applications in the ultraviolet spectral range cannot be
addressed with array spectroradiometers since they are often limited
by internal stray-light effects (Egli et al., 2016). An example is an
accurate measurement of solar irradiance in the UV-B spectral
range. The intense radiation of the sun in the visible (VIS) and
infrared (IR) generates stray light within the spectrometer, which
often dominates over the less intense solar UV-B radiation. However,
an accurate measurement of solar spectrum (Seckmeyer et al., 2001,
2010) is the basis for an accurate evaluation of many derived
quantities such as total ozone column (TOC; Dobson, 1931; Mayer and
Seckmeyer, 1998). Hence, these measurements are often performed with
double-monochromator-based systems (Hülsen et al., 2016), offering
high stray-light reduction capabilities. Operating such instruments is
often time- and cost-intensive and requires controlled ambient
conditions and highly experienced personnel.
To overcome such limitations, Gigahertz-Optik GmbH developed the
BTS2048-UV-S series array spectroradiometer. The system conjoins
compact instrument design, a physical-filter-based stray-light
correction and versatile radiometric applicability. One of the newly
developed devices has been adapted specifically for direct solar
irradiance measurements and was extensively characterised at the
Physikalisch-Technische Bundesanstalt (PTB) in Germany. It
then took part in the ATMOZ intercomparison campaign in Izaña,
Tenerife, in 2016 in the framework of the European Metrology Research
Programme (EMRP) project ENV59 ATMOZ –
a total ozone measurement intercomparison organised by the
Izaña Atmospheric Research Center of the Spanish Meteorological
Agency (AEMET) and the World Radiation Center (PMOD-WRC), where new
instruments and techniques developed within the project were compared
to well-established Dobson and Brewer methods.
Schematical setup of the BTS2048-UV-S and photo of the
instrument. (1) Incoming optical radiation, (2) direct entrance port
with cosine diffuser, (3) filter wheel, (4) sensor system,
(5) electrical connectors, (6) microprocessor for data processing
and communication.
Instrument design
The BTS2048-UV-S series array spectroradiometers developed and
manufactured by Gigahertz-Optik GmbH are based on the well-known
Czerny–Turner (Shafer et al., 1964) spectrometer design. The
spectrometer uses a temperature-controlled (8 ∘C)
back-thinned Hamamatsu CCD detector with 2048 pixels and an electronic
shutter integrated in a compact optical bench with 16 bit
analogue–digital converter (ADC) resolution. Integration times from
2 µs up to 60 s provide a high dynamic range of
the instrument in the spectral range from 200 to 430 nm. The
detector unit is complemented with a silicon carbide (SiC) photodiode to enable fast
time-resolved radiometric measurements.
To enable stray-light-corrected measurements, a miniaturised filter
wheel with up to six different optical filters is integrated into the
optical path between the entrance optic and spectrometer unit
(Fig. 1). A set of selected optical filters, such as
bandpass- and edge-type filters, can be used to preselect the
radiation entering the sensor system. In the device one long-pass
filter, a bandpass filter (298 to 390 nm) and four
interference filters (centre wavelength: 254, 285, 300 and
400 nm) are integrated, which sufficiently block in the
whole remaining spectral responsivity range of the detector. Hence,
several sub-measurements with different filters in the optical path
can be performed. The combination of these sub-measurements allows for
optimised stray-light-reduced spectra. In addition, multiple different
combinations of filters, integration times and sub-measurements
optimise the measurement scenario to any kind of specific radiometric
application. For instance, a so-called out-of-range (OoR) stray-light
correction method has been implemented, where an additional
measurement with a long-pass filter is performed to quantify
contribution of the out-of-range stray light to the measured signal,
which can then be subtracted. In order to perform reliable solar UV
measurements, a specific measurement scenario, the so-called solar
bandpass correction method, was used. This measurement scenario is
based on a series of measurements with several narrow-bandpass filters
which complement each other (Shaw and Goodman, 2008). Hence, the
overall measurement time to get a full spectral measurement is the sum
of all integration times of the sub-measurements. Typically this
measurement time is in the range of a few seconds, depending on the
light source to measure. This active stray-light correction process
is a straightforward alternative to mathematical stray-light
correction methods that have been established for array
spectroradiometers (Zong et al., 2006; Nevas et al., 2012). In some
cases, e.g. when silicon (Si) detectors are used solely in the UV
spectral range, the introduced technology can reduce the stray light
more efficiently. The mathematical stray-light correction methods are
based on a precise characterisation of the optical imaging performance
with so-called line spread functions (LSFs). These functions should be
determined at each detection wavelength of the spectroradiometer. This
so-called in-range stray light can be corrected to improve the
measurement threshold by about 2 orders of magnitude (Zong et al.,
2006). However, this correction method is not able to correct for
so-called out-of-range stray light that is generated in spectral
intervals outside the spectroradiometer range where the detector used
is still sensitive. If, for instance, a Si-detector-based
spectroradiometer is designed for the spectral range from 200 to
400 nm, the in-range stray light can only be corrected for
this spectral range. However, the Si detector itself is
radiation-sensitive up to 1100 nm, and stray light originating
from this out-of-range spectral region cannot be characterised with
LSFs. This limitation might be resolved by measuring the responsivity
of the spectroradiometer to the radiation at OoR wavelength using
a calibrated detector or an additional spectroradiometer with extended
wavelength range, for Si ideally up to 1100 nm (Nevas et al.,
2014). However, such a correction method requires knowledge about the
OoR spectrum not only during instrument calibration, which generally
is readily available from the standard lamp calibration data, but also
for the radiant sources under investigation, e.g. the direct solar
irradiance, which may not be available for practical reasons. Hence,
the presented optical-filter-based stray-light correction technology
of the BTS2048-UV-S series offers an attractive alternative,
especially in the UV range.
Characterisation
At PTB, several instrument parameters have been characterised to
verify the quality and measurement capability of the BTS2048-UV-S. The
wavelength calibration was performed with the help of wavelength-tuneable
laser systems and checked using mercury pen lamps. The
uncertainty for the wavelength calibration was found to be better than
0.1 nm. For measurements of solar irradiance however, the
wavelength scale was additionally adapted with standard deviations (SDs) better than
0.02 nm to the solar Fraunhofer lines using the MatShic
algorithm (Egli et al., 2014). The spectral bandpass was determined
using LSFs measured with tuneable laser systems
(Nevas et al., 2014). The bandpass function appears to be nearly
symmetrical below 360 nm with an average bandwidth of
0.6 nm, full width at half-maximum (FWHM) (see Fig. 2).
Bandpass function around centroid λc for
different wavelengths with bandwidth (FWHM, dashed lines)
indicated.
The linearity of the BTS2048-UV-S was tested using both the
integration time and the irradiance variation methods (Pulli
et al. 2017). For the integration time method, the spectral irradiance
of the incident radiation is kept constant while the instrument's
integration time is varied over a wide range. Although the measurement
signal in raw counts then varies with the integration time, the
normalised count rates per second should remain constant for all
measurements. The irradiance variation is performed at constant
integration times of the instrument, while the irradiance is linearly
reduced between each measurement. Here the ratio of measurement signal
(count rate) and spectral irradiance should remain constant for all
measurements. By applying a mathematical correction for non-linearity,
the spectrometer showed linearity with a deviation smaller than
1 % over the full dynamic range for the characterised measurement
mode. For spectral measurements the instrument saturation level is
kept below 80 % to operate the instrument in an optimum between
saturation and linearity. By using the solar bandpass-filter method,
with adapted spectral integration times for each sub-measurement, the
dynamic range of the instrument could be extended. Whenever the
non-linearity exceeds 1 % (at very low signals), a non-linearity
correction is automatically applied.
The instrument has been radiometrically calibrated using 250 W
halogen lamps and 30 W deuterium lamps as transfer standards
to perform spectral irradiance measurements traceable to the PTB
(Sperfeld et al., 2010). In the overlapping spectral region of the two
different lamp types between 280 and 360 nm the calibration
matches well within the achieved measurement uncertainties. This
demonstrates the good linearity and stray-light suppression capability
of the instrument. Although these standard lamps required a comparably
long integration time, due to a high dynamic range of the instrument
(typically 5×10-5 to 5×104W(m2nm)-1 at 300 nm) it was possible
to perform reliable measurements with very short integration times on
high-power UV sources, such as medium-pressure mercury lamps, without
the need for an additional attenuation.
The standard lamps used allow recalibration of the instrument in the
laboratory and in the field. The resulting expanded measurement
uncertainty (k=2) was estimated as 2.5 % in the short wavelength
range. The uncertainty contributions are listed in Table 1 (see also
Vaskuri et al., 2018).
Measurement uncertainties for the spectral irradiance measurement.
During the measurement campaign, described below, the radiometric
calibration of the BTS2048-UV-S could be verified with a SD of less
than 1 % compared to the calibrations in the laboratory before and
after the campaign. During the campaign, the instrument was removed
from the tracker at night to perform calibration measurements on
a portable optical bench.
(a) The BTS2048-UV-S-WP modified for direct solar
measurements mounted on a solar tracker at the Izaña Atmospheric Observatory
in Tenerife. (b) Field-of-view (FOV) measurement in x and
y direction for the entrance optics tube according to Blanc
et al. (2014). αs=1∘ represents the
slope angle, αl=1.8∘ the limiting angle
and α=1.4∘ the half opening angle. The full opening
angle results in 2×α=2.8∘.
Performance evaluation
Direct spectral irradiance (blue and red line) measured by
the BTS and the double-monochromator-based QASUME instrument in
Izaña on 20 September, at 10:45 UTC and 18:15 UTC. The ratio (grey
line for single data and green line for moving average) of
measurements (right axis) shows satisfactory agreement with average
deviations of less than 2.5 % between 300 and 420 nm for
low solar zenith angles at around noontime. The unaveraged ratio (grey line) can be explained by remaining small
differences in the optical bandwidth in combination with small
wavelength shifts. Due to non-averaging of the BTS data and limited
integration time (measurement every 8 s), noise of the 16 bit ADC
in the spectral region below the UV-B solar edge can be observed.
BTS/QASUME ratio of the wavebands used for
the calculation of TOC values. The measurements were performed on
20 September 2016. The corresponding air mass of the direct
irradiance and the solar zenith angle on the corresponding day are
also shown (right axis). For solar zenith angles of more than
74∘ or an air mass above 3.6, deviations rise to 5 %.
Direct irradiance spectra of the lookup table modelled with
libRadtran. Shown are all spectra for an SZA of 48∘, ranging
from 250 (purple) to 320 DU (red) TOC. The wavebands used for the
TOC retrieval are marked blue and orange.
Intercomparison of spectroradiometric measurements
For outdoor measurements of direct solar irradiance, a weatherproof
(BTS2048-UV-S-WP) version of the BTS2048-UV-S was constructed. The
instrument was integrated in a weatherproof housing which is
temperature-controlled (ambient temperature range from -25 to
+50 ∘C) and waterproof. During the 3-week measurement
campaign in September 2016 at the Izaña Atmospheric Observatory (altitude:
2.373 m; coordinates: 28∘18′32 N, 16∘30′58 W), the typical temperature
variation within the housing was below 0.1 ∘C (measured close
to the spectrometer unit, with temperature set to 38 ∘C). There was
cloudless sky during the measurements from which data have been used for
this intercomparison. The housing was equipped with an entrance optic
tube to limit the field of view to 2.8∘ (full opening angle;
see Fig. 3). This tube is based on a baffle design to prevent stray
light hitting the diffusor. Mounted on a solar tracker (EKO STR-32G)
with a pointing accuracy of <0.01∘, the instrument
measured direct solar irradiance. Solar measurements were performed
using the solar bandpass correction method. Here, several
narrow-bandpass filters are used in the spectral range between 280 and
420 nm. The single sub-measurements for every filter at
varying integration times are subtracted by their assigned dark
measurements with closed shutter and combined in their overlap region
to one measurement of the full spectral range. This allows the steep slope of
the solar spectrum to be measured below 300 nm with a high
dynamic range (see Fig. 4). A full spectrum was recorded every
8 s. The duration of this measurement interval is mainly given by
filter movement and dark-signal measurements, whereas the integration
time for the measurements with different bandpass filters was
optimised and varied with different solar zenith angles (SZAs). These
settings allowed measurements with noise-equivalent irradiance in the
range of 10-4W(m2nm)-1 (see Fig. 4).
The measurements were compared to the results of the
double-monochromator-based Quality Assurance of Solar Spectral Ultraviolet
Irradiance Measurements carried out in Europe (QASUME) instrument, which is extensively
characterised for global irradiance (Gröbner and Sperfeld,
2005, Gröbner et al., 2005;
Hülsen et al., 2016). For this intercomparison the QASUME
instrument was equipped with a collimator-based entrance optic for
direct solar irradiance measurements with a maximum field of view of
2.5∘ (full opening angle; Gröbner et al., 2017). To be
able to compare two sets of data, the measured spectra of both
instruments had to be synchronised in time and adapted in
bandwidth. The QASUME system operates in sequential mode, measuring
step by step from lower to higher wavelength. The recording of a full
spectrum from 290 to 500 nm in steps of 0.25 nm takes
about 16 min. Every measurement at a single wavelength is marked with
a time stamp so that the corresponding measurement and the wavelength
of the BTS2048-UV-S-WP (from now on called BTS) could be
synchronised. As both spectroradiometer systems possess different
bandwidth, the resulting spectra were convolved with a standard
1 nm triangular bandpass function. This data evaluation
results in deviations between BTS and QASUME lower than
±2.5 % averaged from 300 to 420 nm (see Fig. 4).
The diurnal variation between QASUME and BTS of different waveband
ratios shows deviations of less than ±2 %. The ratios rise
for SZAs larger than 74∘ or an air mass larger than
3.6 (Fig. 5). At higher SZAs or air mass the signal-to-noise ratio
decreases, especially at low wavelengths, due to the increasing
atmospheric absorption path of the incident irradiance.
Intercomparison of TOC values
Example of the atmospheric correction applied to a TOC
calculation of BTS data on 20 September 2016, 10:06 UTC. The
waveband ranging from 330 to 355 nm was used to derive the
slope of the blue fitting line. The green line is the ratio of a BTS
measurement and a libRadtran calculation after subtracting the blue
fitting line.
Total ozone column (TOC) derived by the direct solar
measurements of the BTS in comparison to other instruments. The
measurements have been conducted during the ATMOZ intercomparison
campaign on 20 September 2016. The TOC values stated in the legend
for each ground-based instrument have been derived by averaging the
values between 09:00 and 11:00 UTC (blue area). Grey dots symbolise
the BTS TOC measurements captured every 8 s; only integer values
are apparent due to the 1 DU resolution of the lookup table.
Comparison of the BTS-retrieved TOC values to other instrument
values measured during the ATMOZ campaign. The data have been
calculated based on measurements performed on 20 September 2016
between 09:00 and 11:00 UTC, shown in Fig. 8. For the comparison to
the OMI data the average of the BTS TOC values measured between 12:30
and 13:00 UTC has been calculated. The SD of the averaged TOC values
shows the dispersion of TOC values during that time period.
InstrumentTOC/DUTime interval/UTCDifference to BTS/%SD of values used/DUBTS267.509:00–11:00–1.0QASUME271.709:00–11:00+1.60.5IZO Brewer269.709:00–11:00+0.80.4NOAA Dobson265.509:00–11:00-0.80.6BTS27012:30–13:00–0.7Aura OMI27512:30–13:00+1.8–
The spectral data have also been used to calculate the total ozone
column based on a retrieval algorithm proposed by Masserot
et al. (2002). For this purpose, the ratios of two wavelength bands,
ranging from 305 to 310 nm and from 340 to 350 nm,
have been calculated. The ratio is directly related to the TOC since
the first band lies inside and the second one outside the ozone
absorption range. By comparing these ratios to a set of pre-calculated
model values stored in a lookup table, the most probable TOC value
present during the measurement can be determined. The model values
have been calculated with the libRadtran software package for
radiative transfer calculations (Emde et al., 2016). The lookup table is a data cube with three dimensions and
consists of roughly 6500 direct irradiance spectra with a wavelength
range of 280 to 420 nm for SZAs between 24 and 90∘ and
for TOC values between 250 and 350 DU (one Dobson unit (DU) is
equivalent to 0.4462 mmolm-2; Basher, 1982). For the
calculation of the ozone value of a specific measurement, all modelled
spectra at the SZA apparent during the time of the measurement are
first selected from the lookup table (see Fig. 6), and the ratios
between the BTS measurement and all selected spectra are
calculated. For the calculation of the lookup table, the following
values for the input parameters have been chosen: an albedo of 0.2,
a pressure of 773 hPa, an altitude of 2.36 km, an
atmospheric profile typical for mid-latitude summer (Anderson et al.,
1986) and the ozone cross section of Bass and Paur (1984). The
temperature and ozone profile of the chosen atmospheric profile lead
to an effective ozone temperature of 232.3 K. In contrast to
Masserot et al., direct irradiance instead of global irradiance has
been modelled as input for the lookup table to adapt the algorithm to
the measurements performed with the BTS. Despite the low aerosol
content in Izaña the aerosol default values of libRadtran have
been used. This crude modelling of aerosol parameters is intentional
in order to reflect the usually limited knowledge of the atmospheric
aerosol parameters. The chosen aerosol parameters will lead to
deviations between measured and modelled spectra over the whole
wavelength range due to differences between the actual atmospheric
condition and the assumptions made for the modelled spectra. This is
addressed by performing a linear fit to the ratio between 330 and
355 nm (see Fig. 7), where ozone absorption is negligible and
no “local” spectral features due to, e.g., strong absorption lines
in the solar spectrum are apparent. The derived linear fit is applied
to each ratio afterwards, effectively adjusting the ratios for
atmospheric scattering processes with low wavelength dependencies
(e.g. Mie scattering by aerosols or cloud droplets). The ratios are
then averaged in the two wavelength bands from 305 to 310 nm
and 340 to 350 nm. The resulting numbers are
divided by each other for each ratio. The ratio closest to 1
corresponds to the modelled spectrum with the most likely ozone value
apparent during the BTS measurement.
In Fig. 8, the results of the TOC calculations based on spectra of the
direct solar irradiance measured on 20 September 2016 are
shown. In addition, measurements performed on the same day with other
instruments are displayed, namely with the WRC QASUME
spectroradiometer, the world primary standard Dobson ozone
spectrophotometer D083 from the World Dobson Calibration Center (WDCC)
at NOAA, the regional primary reference Brewer spectrophotometer B157
of the Izaña Atmospheric Observatory and satellite measurements from the Aura
Ozone Monitoring Instrument (OMI). Additionally, to directly compare the
BTS-retrieved TOC to the other instrument data, the mean TOC values
have been calculated during the time between 09:00 and 11:00 UTC. For
the comparison with the Aura OMI data,
the BTS data have been averaged ±15 min around the flyover time
of the Aura satellite, resulting in a BTS TOC of 270 DU. The results
are illustrated in Table 2.
The systematic differences between BTS and QASUME TOC values, even if
the spectra of both instruments agree well as shown in Fig. 4, arise
from different model approaches which are used for the TOC
determination. In addition, the modelled TOC values of BTS and QASUME
are based on slightly different input parameters for the atmospheric
conditions. This is the case since we have chosen our parameters
without knowledge of the QASUME parameters to ensure an unbiased
comparison.
During the other measurement days of the campaign, where direct
irradiance measurements were performed with the BTS spectroradiometer,
the deviation to the other instruments did not exceed 3 % between
09:00 and 17:00 UTC. At air masses larger than 4 during sunrise and
sunset, the signal-to-noise ratio decreases in the shortwave region of
the spectrum and, therefore, the TOC estimations becomes noisier. In
addition, at lower irradiance levels the detection threshold of the
instrument increasingly affects the wavelength band from 305 to
310 nm, which leads to higher systematic uncertainties for the
calculation. An initial analysis of the TOC determination uncertainty of
the BTS device, which is in the range of 5 DU, was carried out by
Vaskuri et al. (2017) based on a Monte Carlo method.
Discussion and conclusion
The BTS2048-UV-S series spectroradiometer is a versatile measurement
system for spectroradiometric measurements in the UV spectral
range. Its compact design, the fast sensor system and the hardware-based
stray-light correction achieved with several optical filters may
enable a wide range of radiometric applications.
After adapting the BTS2048-UV-S with a weatherproof housing for
direct solar irradiance measurements (BTS2048-UV-S-WP) and an
extensive device characterisation, the array spectroradiometer proved
its capability in the challenging measurements of solar irradiance for
atmospheric research.
Absolute direct solar irradiance measurements by the BTS2048-UV-S-WP
showed deviations from the double-monochromator-based QASUME lower
than ±2.5 % averaged over the spectral range from 300 to
420 nm. In the spectral range below the UV-B solar edge the
deviation rises mostly due to slight differences in the wavelength
calibration and insufficient signal-to-noise ratio since no averaging
of the BTS data or longer integration time was possible for the
8 s measurement interval. TOC values derived from
BTS2048-UV-S-WP data show agreement comparable to those obtained by
Dobson and Brewer reference instruments. Based on the results and the
experience gained during the measurement campaign, the design and the
radiometric sensitivity of the BTS measurement system could be further
improved by a factor of 4. This will very likely improve the
performance of the system especially at higher SZAs and shall be
tested in future measurement campaigns.
Data availability
The data sets of the direct spectral irradiance measurements of the
BTS2048-UV-S-WP at the Izaña Atmospheric Observatory on 20–22 September 2016 during the ATMOZ intercomparison campaign can be
obtained at 10.6084/m9.figshare.6170345.v1
(Sperfeld et al., 2018).
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the European Metrology Research
Programme (EMRP) within the joint research project EMRP ENV59 ATMOZ
“Traceability for atmospheric total column ozone”. The EMRP is
jointly funded by the EMRP participating countries within EURAMET
and the European Union. The authors thank Julian Gröbner from
PMOD for providing the QASUME data sets.
Edited by: Pawan K. Bhartia
Reviewed by: two anonymous referees
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