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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2017-74
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
27 Mar 2017
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Measurement Techniques (AMT) and is expected to appear here in due course.
Calibration and evaluation of CCD spectroradiometers for airborne measurements of spectral actinic flux densities
Birger Bohn1 and Insa Lohse1,2 1Institut für Energie- und Klimaforschung, IEK-8: Troposphäre, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
2Deutscher Wetterdienst, BTZ-Langen, 63225 Langen, Germany
Abstract. The properties and performance of CCD array spectroradiometers for the measurement of atmospheric spectral actinic flux densities and photolysis frequencies were investigated. These instruments are widely used in atmospheric research and are suitable for aircraft applications because of high time resolutions and high sensitivities in the UV range. The laboratory characterization included instrument-specific properties like wavelength accuracy, dark signals, dark noise and signal-to-noise ratios. Spectral sensitivities were derived from measurements with spectral irradiance standards. The calibration procedure is described in detail and a straightforward method to minimize the influence of stray light on spectral sensitivities is introduced. Detection limits around 1×1010cm−2 s−1 nm−1 were derived for spectral actinic flux densities in a 300 nm range (1 s integration time). As a prerequisite for the determination of stray light under field conditions, atmospheric cutoff wavelengths were defined using radiative transfer calculations as a function of solar zenith angles and ozone columns. The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromator reference spectroradiometer. Spectral actinic flux densities were compared as well as photolysis frequencies j(NO2) and j(O1D), representing UV-A and UV-B ranges, respectively. The spectra expectedly revealed an increased daytime level of residual noise below atmospheric cutoff wavelengths that is caused by stray light. The influence of instrument noise and stray light induced noise was found to be insignificant for j(NO2) and rather limited for j(O1D), resulting in estimated detection limits of 5×10−7 s−1 and 1×10−7 s−1, respectively. For j(O1D) the detection limit could be further reduced by setting spectral actinic flux densities below cutoff wavelengths to zero. The accuracies of photolysis frequencies were determined from linear regressions with reference instrument data. The agreement was typically within ±5 %. Optical receiver aspects were widely excluded in this work and will be treated in a separate paper in particular with regard to airborne applications. Overall, the investigated instruments are clearly suitable for high quality photolysis frequency measurements with high time resolution as required for airborne applications. An example of data from a flight on the research aircraft HALO is presented.

Citation: Bohn, B. and Lohse, I.: Calibration and evaluation of CCD spectroradiometers for airborne measurements of spectral actinic flux densities, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-74, in review, 2017.
Birger Bohn and Insa Lohse
Interactive discussionStatus: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version      Supplement - Supplement
 
RC1: 'Review', Anonymous Referee #1, 04 Apr 2017 Printer-friendly Version 
AC1: 'Reply to Referee #1', Birger Bohn, 04 Jul 2017 Printer-friendly Version Supplement 
 
RC2: 'Reviewof Bohn and Lohse, 2017', Anonymous Referee #2, 02 May 2017 Printer-friendly Version 
AC2: 'Reply to Referee #2', Birger Bohn, 04 Jul 2017 Printer-friendly Version Supplement 
Birger Bohn and Insa Lohse
Birger Bohn and Insa Lohse

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Short summary
CCD spectroradiometers are widely used for measurements of atmospheric photolysis frequencies. Their fast response makes them suitable for airborne applications despite the well-known stray light problem. In this work we describe simple and reliable procedures to minimize the stray light influence on calibrations and field measurements. Comparisons with a reference instrument confirm high accuracies and low detection limits of important photolysis frequencies.
CCD spectroradiometers are widely used for measurements of atmospheric photolysis frequencies....
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