Atmospheric CO 2 and CH 4 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.