Atmospheric CO₂ and CH₄ measurements at Schauinsland station, Germany: A comparative study of diurnal and seasonal variations at 12 and 35 m intake height

Abstract. At the atmospheric monitoring station Schauinsland (Black Forest, Germany), high-precision long-term measurements of atmospheric CO₂ and CH₄ have been conducted since 1972 and 1991, respectively. An additional air intake at 35 m a.g.l. (above ground level) has been installed alongside the existing intake at 12 m a.g.l in September 2021. To ensure consistency and continuity of the historic CO₂ and CH₄ time series, a three-year comparison between the two intake heights has been carried out. This revealed systematic differences in CO₂ mole fractions between the two levels: During summer daytime diurnal mean CO₂ mole fractions at 35 m are up to 0.5 ppm higher, while night-time values are up to 1.1 ppm lower, compared to 12 m, resulting in smaller diurnal amplitudes at 35 m. Seasonal mean CO₂ mole fractions are lowered by up to 0.36 ppm at 35 m relative to 12 m in summer, whereas no significant differences are observed in winter. The diurnal and seasonal mean CH₄ mole fraction differences between the two intake heights are negligible. However, highly frequent CH₄ spikes originating from nearby grazing cows, which are present only during the summer months, increase hourly mean mole fractions at 12 m by up to 38 ppb, while no significant influence is measured at 35 m. The REBS (Robust Extraction of the Baseline Signal) algorithm is applied to the minutely CH₄ summer data at 12 m and proved to be an effective approach for identification of data affected by very local influences.