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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-2020-316
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-316
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  04 Sep 2020

04 Sep 2020

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This preprint is currently under review for the journal AMT.

Characterisation and potential for reducing optical resonances in FTIR spectrometers of the Network for the Detection of Atmospheric Composition Change (NDACC)

Thomas Blumenstock1, Frank Hase1, Axel Keens2, Denis Czurlok2, Orfeo Colebatch3, Omaira Garcia4, David W. T. Griffith5, Michel Grutter6, James W. Hannigan7, Pauli Heikkinen8, Pascal Jeseck9, Nicholas Jones5, Rigel Kivi8, Erik Lutsch3, Maria Makarova10, Hamud Kh. Imhasin10, Johan Mellqvist11, Isamu Morino12, Tomoo Nagahama13, Justus Notholt14, Ivan Ortega7, Mathias Palm14, Uwe Raffalski15, Markus Rettinger16, John Robinson17, Matthias Schneider1, Christian Servais18, Dan Smale17, Wolfgang Stremme6, Kimberly Strong3, Ralf Sussmann16, Yao Té9, and Voltaire A. Velazco5 Thomas Blumenstock et al.
  • 1Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe, Germany
  • 2Bruker Optics GmbH, Ettlingen, Germany
  • 3Department of Physics, University of Toronto, Toronto, Canada
  • 4Izaña Atmospheric Research Centre (IARC), Meteorological State Agency of Spain (AEMET), Tenerife, Spain
  • 5Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia
  • 6Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México (UNAM), Mexico City, México
  • 7National Center for Atmospheric Research (NCAR), Boulder, CO, USA
  • 8Finnish Meteorological Institute (FMI), Sodankylä, Finland
  • 9Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA-IPSL), Sorbonne Université, CNRS, Observatoire de Paris, PSL Université, 75005 Paris, France
  • 10Saint Petersburg State University, Atmospheric Physics Department, St. Petersburg, Russia
  • 11Department of Earth and Space Science, Chalmers University of Technology, Göteborg, Sweden
  • 12National Institute for Environmental Studies (NIES), Tsukuba, Ibaraki 305-8506, Japan
  • 13Institute for Space-Earth Environmental Research (ISEE), Nagoya University, Nagoya, Japan
  • 14Institute of Environmental Physics, University of Bremen, Bremen, Germany
  • 15Swedish Institute of Space Physics (IRF), Kiruna, Sweden
  • 16Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany
  • 17National Institute of Water and Atmospheric Research Ltd (NIWA), Lauder, New Zealand
  • 18Institut d’Astrophysique et de Géophysique, Université de Liège, Liège, Belgium

Abstract. Although optical components in Fourier transform infrared (FTIR) spectrometers are preferably wedged, in practice, infrared spectra typically suffer from the effects of optical resonances (“channeling”) affecting the retrieval of weakly absorbing gases. This study investigates the level of channeling of each FTIR spectrometer within the Network for the Detection of Atmospheric Composition Change (NDACC). Dedicated spectra were recorded by more than twenty NDACC FTIR spectrometers using a laboratory mid-infrared source and two detectors. In the InSb detector domain (1900–5000 cm−1), we find that the amplitude of the most pronounced channeling frequency amounts to 0.1 to 2.0 ‰ of the spectral background level, with a mean of (0.68 ± 0.48) ‰ and a median of 0.60 ‰. In the HgCdTe detector domain (700–1300 cm−1), we find even stronger effects, with the largest amplitude ranging from 0.3 to 21 ‰ with a mean of (2.45 ± 4.50) ‰ and a median of 1.2 ‰. For both detectors, the leading channeling frequencies are 0.9 and 0.11 or 0.23 cm−1 in most spectrometers. These observed spectral frequencies correspond to the optical thickness of the air gap in between the beam splitter and compensator plate (0.9 cm−1) and the beam splitter substrate itself (0.11 and 0.23 cm−1). Since the air gap is a significant source of channeling and the corresponding amplitude differs strongly between spectrometers, we propose new beam splitters with the wedge of the air gap increased to at least 0.8°. We tested the insertion of spacers in a beam splitter’s air gap to demonstrate that increasing the wedge of the air gap decreases the 0.9 cm−1 channeling amplitude significantly. This study shows the potential for reducing channeling in the FTIR spectrometers operated by the NDACC, thereby increasing the quality of recorded spectra across the network.

Thomas Blumenstock et al.

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Thomas Blumenstock et al.

Thomas Blumenstock et al.

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Short summary
This study investigates the level of channeling (optical resonances) of each FTIR spectrometer within the Network for the Detection of Atmospheric Composition Change (NDACC). Since the air gap of the beam splitter is a significant source of channeling, we propose new beam splitters with an increased wedge of the air gap. This study shows the potential for reducing channeling in the FTIR spectrometers operated by the NDACC, thereby increasing the quality of recorded spectra across the network.
This study investigates the level of channeling (optical resonances) of each FTIR spectrometer...
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