Long-term observations of atmospheric CO ₂ and CH ₄ trends and comparison of two measurement systems at Pallas-Sammaltunturi station in Northern Finland

Abstract. Accurate and precise observations of atmospheric greenhouse gas mole fractions are crucial for understanding the carbon cycle. However, challenges can arise when comparing data between different observation sites owing to the 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 mole fractions 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 the measurement of background mole fractions. The station hosts instrumentation for two different research infrastructures – Integrated Carbon Observation System (ICOS) and Global Atmosphere Watch (GAW) – with completely independent measurement instruments, calibration standards, and sampling systems. We present the long-term time series of the mole fractions of CO2 and CH4 and their evolution, measured at the Pallas 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 for CH4 is 0.47 ppb. 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 the ICOS Mobile Laboratory and the World Calibration Centre (WCC-Empa) of GAW, respectively. The audit results show good agreement between the different systems, with the differences ranging from −0.06 to 0.02 ppm for CO2 and from −0.24 to 0.30 ppb for CH4. No significant dependence on mole fraction values is found for the differences between the systems. However, for one of the instruments, we find a clear influence of sample drying, especially for CH4. We also compare the long time series with the marine boundary layer (MBL) reference values, derived by NOAA based on weekly air sample measurements in the Northern Hemisphere. For CO2, the values measured at Pallas are on average 1.9 ppm higher than the MBL for the Northern Hemisphere, and for CH4, they are 54 ppb higher. The difference is larger during summer for CO2 but not significantly for CH4.