The cavity ring-down spectrometer (CRDS) is used to measure carbon dioxide and methane.
The CRDS needs much less frequent calibration than chromatography based methods (GC-MD and Medusa) due to its inherent measurement method. CO2 and CH4 are calibrated in partnership with the Integrated Carbon Observing System, ICOS.
Standards used on the CRDS are of natrual composition and were calibrated at the World calibration centre for CO2 and CH4 at Empa in Zurich.
Four calibration tanks with a range of CO2 and CH4 concentrations are measured monthly to assess for non-linear drift. A target tank is measured daily to assess and account for linear drift.
Calibrations on the Picarro are run every time the instrument is restarted (for instance if there was a power cut or the instrument had to be shut-down for transport) and every 30 days. This method was adopted as it was the protocol recommended by the European carbon gases measurement community IMECC.
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