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. We compare the atmospheric column-averaged dry-air mole fractions of carbondioxide ( XCO2 ) and methane ( XCH4 ) measured with a pair ofCOllaborative Carbon Column Observing Network (COCCON) spectrometers atKiruna and Sodankyla (boreal areas). We compare model data provided bythe 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 gradientsof XCO2 and XCH4 ( Δ XCO2 and Δ XCH4 ) onregional scales are investigated. Both sites show a similar and very goodcorrelation 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 onaverage. For XCH4 , CAMS values are higher than the COCCON observationsby 0.33 ppb ( ±11.93  ppb) in Kiruna and 7.39 ppb ( ±10.92  ppb)in Sodankyla. In contrast, the S5P satellite generally measures loweratmospheric XCH4 than the COCCON spectrometers, with a mean differenceof 9.69 ppb ( ±20.51  ppb) in Kiruna and 3.36 ppb ( ±17.05  ppb)in Sodankyla. We compare the gradients of XCO2 and XCH4 ( Δ XCO2 and Δ XCH4 ) between Kiruna andSodankyla derived from CAMS analysis and COCCON and S5P measurements tostudy the capability of detecting sources and sinks on regional scales. Thecorrelations in Δ XCO2 and Δ XCH4 between thedifferent datasets are generally smaller than the correlations in XCO2 and XCH4 between the datasets at either site. The Δ XCO2 valuespredicted by CAMS are generally higher than those observed with COCCON witha slope of 0.51. The Δ XCH4 values predicted by CAMS are mostly higherthan those observed with COCCON with a slope of 0.65, covering a largerdataset than the comparison between S5P and COCCON. When comparing CAMS Δ XCH4 with COCCON Δ 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 reasonableagreement. However, the small number of observations coinciding with S5Plimits our ability to verify the performance of this spaceborne sensor. Wedetect no significant impact of ground albedo and viewing zenith angle onthe 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 COCCONinstruments have the capability of measuring greenhouse gas (GHG) gradientson regional scales, and observations performed with the portablespectrometers can contribute to inferring sources and sinks and tovalidating spaceborne greenhouse gas sensors. To our knowledge, this is thefirst published study using COCCON spectrometers for the validation of XCH4 measurements collected by S5P.

atmospheric measurement techniques

Intercomparison of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; abundances on regional scales in boreal areas using Copernicus Atmosphere Monitoring Service (CAMS) analysis, COllaborative Carbon Column Observing Network (COCCON) spectrometers, and Sentinel-5 Precursor satellite observations