Long-term observations of atmospheric CO₂ and CH₄ trends and comparison of two measurement systems at Pallas-Sammaltunturi station in Northern Finland

Abstract. Accurate and precise observations of atmospheric greenhouse gas mixing ratios are crucial for understanding the carbon cycle. However, challenges can arise when comparing data between different observation sites, due to different measurement routines and data formats used. To combat these challenges, different research infrastructures have been established in order to harmonize measurement routines and data processing and to make the data from different stations readily available. One of the few stations in the boreal region that observes atmospheric greenhouse gas mixing ratios is the Pallas station, located atop Sammaltunturi fell in Finnish Lapland. The station's location above the arctic circle, far away from large settlements, makes it ideal for measurement of background mixing ratios. The station hosts instrumentation for two different research infrastructures, Integrated Carbon Observation System (ICOS) and Global Atmosphere Watch (GAW), with completely separate gas analyzers, calibration standards and sampling systems. We present the long-term time series of the mixing ratios of CO₂ and CH₄ and their evolution measured at the station, as well as a long-term comparison of the two instruments during the period when both have been installed. We find that the average difference in the hourly values for CO₂ is <0.01 ppm and 0.47 ppb for CH₄. The trends and growth rated calculated for both instruments agree well. For a more detailed comparison, the ICOS and GAW systems were simultaneously audited by ICOS Mobile Laboratory and the World Calibration Centre (WCC) of GAW, respectively. The audit results show good agreement between the different systems, with the differences ranging from -0.06 ppm to 0.02 ppm for CO₂ and from -0.24 ppb to 0.30 ppb for CH₄. No significant dependence on mixing ratio values was found for the differences between the systems. However, for one of the analyzers we found a clear influence of sample drying, especially for CH₄. We also compared the long time series with the marine boundary layer reference values in the Northern Hemisphere. For CO₂, the values measured at Pallas are on average 1.9 ppm higher than the MBL for Northern Hemisphere, and 54 ppb higher for CH₄. The difference is larger during summer for CO₂, but not significantly for CH₄.