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Cross-validation of methane retrievals from ACE-FTS, GOSAT, and ground-based solar absorption measurements, at Eureka, Nunavut.

We present cross-validation of remote sensing observations of methane profiles at Eureka, Nunavut (80°N, 86°W).  Methane is the third most important greenhouse gas on Earth, and second only to carbon dioxide in its contribution to anthropogenic global warming.  Accurate and precise observations of methane are essential to understand quantitatively its role in the climate system and in global change.  The Arctic is a particular region of concern, as melting permafrost and disappearing sea ice might lead to accelerated release of methane into the atmosphere. Pan-Arctic observations require spaceborne instruments, in particular in remote regions where surface measurements are sparse and expensive to carry out.  Remote sensing of methane is an underconstrained problem, and specific validation under Arctic circumstances is required.  Here, we show a cross-validation between two spaceborne instruments and ground-based measurements, all Fourier Transform Spectrometers (FTSs). We consider the Canadian SCISAT ACE-FTS, a solar occultation spectrometer operating since 2004, and the Japanese GOSAT TANSO-FTS, a nadir-pointing FTS operating at solar and terrestrial infrared wavelengths, since 2009. The ground-based instrument is a Bruker Fourier Transform Infrared (FTIR) spectrometer, measuring mid-infrared solar absorption spectra at the Polar Environmental and Atmospheric Research Laboratory (PEARL) Ridge Lab at Eureka, Nunavut (80°N, 86°W, 610m ASL) since 2006. Measurements are collocated considering temporal, spatial, and geophysical criteria and regridded to a common vertical grid.  We perform smoothing on the higher-resolution instrument results to account for different vertical resolutions.  Based on error estimates for each retrieval, we calculate the random error covariance matrix for the comparison ensemble.  Then, profiles and partial columns of differences for each pair of instruments are examined.  Any bias between instruments, or any accuracy that is worse than expected, needs to be understood prior to using the data. The results of the study will serve as a guideline on how to use the vertically resolved methane products from ACE and GOSAT within the High Arctic region.