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We present cross-validation of remote sensing measurements ofmethane profiles in the Canadian high Arctic. Accurate and precisemeasurements of methane are essential to understand quantitativelyits role in the climate system and in global change. Here, we showa cross-validation between three data sets: two from spaceborneinstruments and one from a ground-based instrument. All are Fouriertransform spectrometers (FTSs). We consider the Canadian SCISATAtmospheric Chemistry Experiment (ACE)-FTS, a solar occultationinfrared spectrometer operating since 2004, and the thermal infraredband 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 andterrestrial infrared wavelengths, since 2009. The ground-basedinstrument is a Bruker 125HR Fourier transform infrared (FTIR)spectrometer, measuring mid-infrared solar absorption spectra at thePolar Environment Atmospheric Research Laboratory (PEARL) Ridge Laboratoryat Eureka, Nunavut (80° N, 86° W)since 2006. For each pair of instruments, measurements arecollocated within 500 km and 24 h. Anadditional collocation criterion based on potential vorticity values was foundnot to significantly affect differences between measurements.Profiles are regridded to a common vertical grid for each comparisonset. To account for differing vertical resolutions, ACE-FTSmeasurements are smoothed to the resolution of either PEARL-FTS orTANSO-FTS, and PEARL-FTS measurements are smoothed to the TANSO-FTSresolution. Differences for each pair are examined in terms ofprofile and partial columns. During the period considered, thenumber of collocations for each pair is large enough to obtaina good sample size (from several hundred to tens of thousandsdepending on pair and configuration). Considering full profiles,the degrees of freedom for signal (DOFS) are between 0.2 and 0.7 forTANSO-FTS and between 1.5 and 3 for PEARL-FTS, while ACE-FTS hasconsiderably more information (roughly 1 DOFSper altitude level). We take partial columns between roughly 5and 30 km for the ACE-FTS–PEARL-FTS comparison, andbetween 5 and 10 km for the other pairs. The DOFS for thepartial columns are between 1.2 and 2 for PEARL-FTS collocated withACE-FTS, between 0.1 and 0.5 for PEARL-FTS collocated with TANSO-FTSor for TANSO-FTS collocated with either other instrument, while ACE-FTShas much higher information content. For all pairs, the partialcolumn differences are within ±3 × 1022 molecules cm−2. Expressed asmedian ± median absolute deviation (expressed in partial columnunits or as a percentage),these differences are 0.11 ± 9.60  × 1020 molecules cm−2 (0.012 ± 1.018 %)for TANSO-FTS–PEARL-FTS, −2.6 ± 2.6 × 1021  cm−2 (−1.6 ± 1.6 %) forACE-FTS–PEARL-FTS, and 7.4 ± 6.0 × 1020 molecules cm−2 (0.78 ± 0.64 %) forTANSO-FTS–ACE-FTS. The differences for ACE-FTS–PEARL-FTS andTANSO-FTS–PEARL-FTS partial columns decrease significantly asa function of PEARL partial columns, whereas the range of partialcolumn values for TANSO-FTS–ACE-FTS collocations is too small todraw any conclusion on its dependence on ACE-FTS partial columns

atmospheric measurement techniques

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