Air samples are generally collected at all ICOS class-1 stations at weekly interval using automated samplers into glass flasks and transferred to the ICOS-CAL ☛ for analysis. This provides a continuous quality control for continuously measured greenhouse gas concentrations of of CO2, CH4, N2O as well as CO. Flask sampling also aims at collecting supplementary information on the sample origin allowing to differentiate different source processes.
The ICOS-CAL is furthermore equipped with instruments for the quantification of additional tracers in air that are not accessible with the instrumentation at the ICOS stations. This includes other trace gases H2, CO and SF6 as well as stable isotopes of CO2 δ13C and δ18O (FCL-Jena), Δ14C in CO2 (CRL-Heidelberg) as well as O2 and Ar in air at a very high precision (FCL-Jena).

Flask handling experience

Glass flasks for storage and transport of air samples are commonly used since many years. For many years extensive experiences in flask handling and the influence of flask properties on the sample composition (e.g. by the flask sealing) were collected at the MPI-BGC that resulted in the following procedure. Before the first usage the flasks need to run through a conditioning procedure including pumping to 10-5 mbar and heating to 80°C for three days in order to get rid of water and other contaminants on the inner flask walls. Oxygen in water molecules (H2O) exchanges with oxygen in CO2 molecules and distorts the isotopic ratio measurements of δ18O.

Absence of humidity in air samples

Air samples need to be dried. After accidental contact with humid air the flasks need to be re-conditioned. Before shipping flasks to the stations the flasks are filled with a transport air at the same pressure (1.6 bar) as the preset sampling pressure. The transport air with ambient levels of the tracers that are analyzed assures that the deposition of molecules at the flask walls is already equilibrated and will not influence the measured concentrations. The flask overpressure is measured upon arrival at the laboratory to detect any pressure loss. This guarantees air samples were not changed since they were sampled. After completion of all analyses and before filling in the transport gas every flasks is leak checked to identify defective flasks and avoid sample loss.