Three years of semicontinuous greenhouse gas measurements at the Puy de Dôme station (central France)

Abstract

Three years of greenhouse gas measurements, obtained using a gas chromatograph (GC) system located at the Puy de Dôme station at 1465 m a.s.l. in central France, are presented. The GC system was installed in 2010 at Puy de Dôme and was designed for automatic and accurate semicontinuous measurements of atmospheric carbon dioxide, methane, nitrous oxide and sulfur hexafluoride mole fractions. We present in detail the instrumental setup and the calibration strategy, which together allow the GC to reach repeatabilities of 0.1 μmol mol1, 1.2 nmol mol1, 0.3 nmol mol1 and 0.06 pmol mol1 for CO2, CH4, N2O and SF6, respectively. The analysis of the 3-year atmospheric time series revealed how the planetary boundary layer height drives the mole fractions observed at a mountain site such as Puy de Dôme where air masses alternate between the planetary boundary layer and the free troposphere. Accurate long-lived greenhouse gas measurements collocated with 222Rn measurements as an atmospheric tracer allowed us to determine the CO2, CH4 and N2O emissions in the catchment area of the station. The derived CO2 surface flux revealed a clear seasonal cycle, with net uptake by plant assimilation in the spring and net emission caused by the biosphere and burning of fossil fuel during the remainder of the year. We calculated a mean annual CO2 flux of 1310 ± 680 t CO2 km2. The derived CH4 and N2O emissions in the station catchment area were 7.0 ± 4.0 t CH4 km2 yr1 and 1.8 ± 1.0 t N2O km2 yr1, respectively. Our derived annual CH4 flux is in agreement with the national French inventory, whereas our derived N2O flux is 5 times larger than the same inventory.