We compare the atmospheric column-averaged dry-air mole fractions of carbon dioxide (XCO2) and methane (XCH4) measured with a pair of COllaborative Carbon Column Observing Network (COCCON) spectrometers at Kiruna and Sodankyla (boreal areas). We compare model data provided by the Copernicus Atmosphere Monitoring Service (CAMS) between 2017 and 2019 with XCH4 data from the recently launched Sentinel-5 Precursor (S5P) satellite between 2018 and 2019. In addition, measured and modeled gradients of XCO2 and XCH4 (Delta XCO2 and Delta XCH4) on regional scales are investigated. Both sites show a similar and very good correlation between COCCON retrievals and the modeled CAMS XCO2 data, while CAMS data are biased high with respect to COCCON by 3.72 ppm (+/- 1.80 ppm) in Kiruna and 3.46 ppm (+/- 1.73 ppm) in Sodankyla on average. For XCH4, CAMS values are higher than the COCCON observations by 0.33 ppb (+/- 11.93 ppb) in Kiruna and 7.39 ppb (+/- 10.92 ppb) in Sodankyla. In contrast, the S5P satellite generally measures lower atmospheric XCH4 than the COCCON spectrometers, with a mean difference of 9.69 ppb (+/- 20.51 ppb) in Kiruna and 3.36 ppb (+/- 17.05 ppb) in So-dankyla. We compare the gradients of XCO2 and XCH4 (Delta XCO2 and Delta XCH4) between Kiruna and Sodankyla derived from CAMS analysis and COCCON and S5P measurements to study the capability of detecting sources and sinks on regional scales. The correlations in Delta XCO2 and Delta XCH4 between the different datasets are generally smaller than the correlations in XCO2 and XCH4 between the datasets at either site. The Delta XCO2 values predicted by CAMS are generally higher than those observed with COCCON with a slope of 0.51. The Delta XCH4 values predicted by CAMS are mostly higher than those observed with COCCON with a slope of 0.65, covering a larger dataset than the comparison between S5P and COCCON. When comparing CAMS Delta XCH4 with COCCON Delta XCH4 only in S5P overpass days (slope = 0.53), the correlation is close to that between S5P and COCCON (slope = 0.51). CAMS, COCCON, and S5P predict gradients in reasonable agreement. However, the small number of observations coinciding with S5P limits our ability to verify the performance of this spaceborne sensor. We detect no significant impact of ground albedo and viewing zenith angle on the S5P results. Both sites show similar situations with the average ratios of XCH4 (S5P/COCCON) of 0.9949 +/- 0.0118 in Kiruna and 0.9953 +/- 0.0089 in Sodankyla. Overall, the results indicate that the COCCON instruments have the capability of measuring greenhouse gas (GHG) gradients on regional scales, and observations performed with the portable spectrometers can contribute to inferring sources and sinks and to validating spaceborne greenhouse gas sensors. To our knowledge, this is the first published study using COCCON spectrometers for the validation of XCH4 measurements collected by S5P.
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