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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Mar 2019

Research article | 20 Mar 2019

Review status
This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Atmospheric Measurement Techniques (AMT).

Recent improvements of Long-Path DOAS measurements: impact on accuracy and stability of short-term and automated long-term observations

Jan-Marcus Nasse1, Philipp G. Eger1,a, Denis Pöhler1, Stefan Schmitt1, Udo Frieß1, and Ulrich Platt1 Jan-Marcus Nasse et al.
  • 1Institute of Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany
  • anow at: Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

Abstract. Over the last decades, Differential Optical Absorption Spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer-Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy.

Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light emitting diodes (LEDs) as light sources. Here we present the application of a Laser Driven Light Source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition the lifetime of LDLS is about an order of magnitude higher than of typical Xe-arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5–10 · 10−5 RMS (in units of optical density). This represents a reduction of detection limits of typical trace gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance automated LP-DOAS systems as demonstrated here by more than two years of continuous observations in Antarctica.

Jan-Marcus Nasse et al.
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Status: final response (author comments only)
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Jan-Marcus Nasse et al.
Jan-Marcus Nasse et al.
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Publications Copernicus
Short summary
We present several changes to the setup of Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instruments including the application of a laser-driven light source, a modified coupling of the measurement signal between components, improved stray-light suppression, and better signal homogenisation measures. These changes reduce detection limits of typical trace gas species by a factor of 3–4 compared to previous setups and enabled automated long-term observations in Antarctica.
We present several changes to the setup of Long-Path Differential Optical Absorption...