Title

Characterization and potential for reducing optical resonances in Fourier transform infrared spectrometers of the Network for the Detection of Atmospheric Composition Change (NDACC)

Authors

Thomas Blumenstock, Karlsruhe Institute of Technology
Frank Hase, Karlsruhe Institute of Technology
Axel Keens, Bruker Optics Inc.
Denis Czurlok, Bruker Optics Inc.
Orfeo Colebatch, University of Toronto
Omaira Garcia, Meteorological State Agency of Spain (AEMET)
David W.T. Griffith, Faculty of Science, Medicine and Health
Michel Grutter, Universidad Nacional Autónoma de México
James W. Hannigan, National Center for Atmospheric Research
Pauli Heikkinen, Finnish Meteorological Institute
Pascal Jeseck, Sorbonne Universite
Nicholas Jones, Faculty of Science, Medicine and Health
Rigel Kivi, Finnish Meteorological Institute
Erik Lutsch, University of Toronto
Maria Makarova, Saint Petersburg State University
Hamud K. Imhasin, Saint Petersburg State University
Johan Mellqvist, Chalmers University of Technology
Isamu Morino, National Institute for Environmental Studies of Japan
Tomoo Nagahama, Nagoya University
Justus Notholt, Universität Bremen
Ivan Ortega, National Center for Atmospheric Research
Mathias Palm, Universität Bremen
Uwe Raffalski, Instiutet för rymdfysik
Markus Rettinger, Karlsruhe Institute of Technology, Campus North
John Robinson, National Institute of Water and Atmospheric Research, New Zealand
Matthias Schneider, Karlsruhe Institute of Technology
Christian Servais, Universite de Liege
Dan Smale, National Institute of Water and Atmospheric Research, New Zealand
Wolfgang Stremme, Universidad Nacional Autónoma de México
Kimberly Strong, University of Toronto
Ralf Sussmann, Karlsruhe Institute of Technology, Campus North

Publication Name

Atmospheric Measurement Techniques

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 20 NDACC FTIR spectrometers using a laboratory mid-infrared source and two detectors. In the indium antimonide (InSb) detector domain (1900-5000 cm-1), we found 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 mercury cadmium telluride (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. The observed spectral frequencies of 0.11 and 0.23 cm-1 correspond to the optical thickness of the beam splitter substrate. The 0.9 cm-1 channeling is caused by the air gap in between the beam splitter and compensator plate. 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. A wedge of the air gap of 0.8 reduces the channeling amplitude by about 50 %, while a wedge of about 2 removes the 0.9 cm-1 channeling completely. 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.

Open Access Status

This publication may be available as open access

Volume

14

Issue

2

First Page

1239

Last Page

1252

Funding Number

IN107417

Funding Sponsor

National Science Foundation

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Link to publisher version (DOI)

http://dx.doi.org/10.5194/amt-14-1239-2021