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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 5 | Copyright
Atmos. Meas. Tech., 9, 1961-1980, 2016
https://doi.org/10.5194/amt-9-1961-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 03 May 2016

Research article | 03 May 2016

Methane cross-validation between three Fourier transform spectrometers: SCISAT ACE-FTS, GOSAT TANSO-FTS, and ground-based FTS measurements in the Canadian high Arctic

Gerrit Holl1,a, Kaley A. Walker1, Stephanie Conway1, Naoko Saitoh2, Chris D. Boone3, Kimberly Strong1, and James R. Drummond4 Gerrit Holl et al.
  • 1Department of Physics, University of Toronto, Toronto, ON, Canada
  • 2Center for Environmental Remote Sensing, Chiba University, Chiba, Japan
  • 3Department of Chemistry, University of Waterloo, Waterloo, ON, Canada
  • 4Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS, Canada
  • anow at: Department of Meteorology, University of Reading, Reading, UK

Abstract. We present cross-validation of remote sensing measurements of methane profiles in the Canadian high Arctic. Accurate and precise measurements of methane are essential to understand quantitatively its role in the climate system and in global change. Here, we show a cross-validation between three data sets: two from spaceborne instruments and one from a ground-based instrument. All are Fourier transform spectrometers (FTSs). We consider the Canadian SCISAT Atmospheric Chemistry Experiment (ACE)-FTS, a solar occultation infrared spectrometer operating since 2004, and the thermal infrared band of the Japanese Greenhouse Gases Observing Satellite (GOSAT) Thermal And Near infrared Sensor for carbon Observation (TANSO)-FTS, a nadir/off-nadir scanning FTS instrument operating at solar and terrestrial infrared wavelengths, since 2009. The ground-based instrument is a Bruker 125HR Fourier transform infrared (FTIR) spectrometer, measuring mid-infrared solar absorption spectra at the Polar Environment Atmospheric Research Laboratory (PEARL) Ridge Laboratory at Eureka, Nunavut (80°N, 86°W) since 2006. For each pair of instruments, measurements are collocated within 500km and 24h. An additional collocation criterion based on potential vorticity values was found not to significantly affect differences between measurements. Profiles are regridded to a common vertical grid for each comparison set. To account for differing vertical resolutions, ACE-FTS measurements are smoothed to the resolution of either PEARL-FTS or TANSO-FTS, and PEARL-FTS measurements are smoothed to the TANSO-FTS resolution. Differences for each pair are examined in terms of profile and partial columns. During the period considered, the number of collocations for each pair is large enough to obtain a good sample size (from several hundred to tens of thousands depending on pair and configuration). Considering full profiles, the degrees of freedom for signal (DOFS) are between 0.2 and 0.7 for TANSO-FTS and between 1.5 and 3 for PEARL-FTS, while ACE-FTS has considerably more information (roughly 1DOFS per altitude level). We take partial columns between roughly 5 and 30km for the ACE-FTS–PEARL-FTS comparison, and between 5 and 10km for the other pairs. The DOFS for the partial columns are between 1.2 and 2 for PEARL-FTS collocated with ACE-FTS, between 0.1 and 0.5 for PEARL-FTS collocated with TANSO-FTS or for TANSO-FTS collocated with either other instrument, while ACE-FTS has much higher information content. For all pairs, the partial column differences are within ±3 × 1022molecules cm−2. Expressed as median±median absolute deviation (expressed in partial column units or as a percentage), these differences are 0.11±9.60 × 1020molecules cm−2 (0.012±1.018%) for TANSO-FTS–PEARL-FTS, −2.6 ± 2.6 × 1021 cm−2 (−1.6 ± 1.6%) for ACE-FTS–PEARL-FTS, and 7.4 ± 6.0 × 1020molecules cm−2 (0.78 ± 0.64%) for TANSO-FTS–ACE-FTS. The differences for ACE-FTS–PEARL-FTS and TANSO-FTS–PEARL-FTS partial columns decrease significantly as a function of PEARL partial columns, whereas the range of partial column values for TANSO-FTS–ACE-FTS collocations is too small to draw any conclusion on its dependence on ACE-FTS partial columns.

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Methane is a powerful greenhouse gas, and we need to measure it globally with satellite instruments. We compare measurements from two satellites with measurements from the ground in Eureka, Nunavut, Canada to assess their different strengths and weaknesses. The differences between measurements are discussed and assessed considering the details of each measurement technique and processing. Recommendations are provided for utilization of these data sets for monitoring methane in the high Arctic.
Methane is a powerful greenhouse gas, and we need to measure it globally with satellite...
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