Carbon dioxide (CO2) and methane (CH4) measurements are made using a commercially available Cavity Ring-Down Spectrometer (CRDS). The Picarro G2301 is the a reliable, field tested instrument used for making high-frequency CO2 and CH4 measurements.
The G2301 is a wavelength-scanned CRDS with a 15-20 km optical path achieved with multiple reflections inside a 35 cm3 optical cell. It is well suited for automated minimum-maintenance operation at a remote site. The CRDS also measures water vapour (H2O), this is necessary as H2O effects the measurement of CO2 and CH4. Corrections are applied to CO2 and CH4 data to account for this H2O interference. A Picarro CRDS is operating at Mace Head (funded by the LSCE), measurements of CH4 from this instrument compare well with CH4 measurements from the AGAGE Flame Ionisation Detector (part of the GC-MD at Mace Head) in 2009, after applying water vapour corrections to CRDS data. The sampling scheme consists of an air inlet connected to a primary pump to ensure sufficient flushing of the inlet line. Filters are used to prevent airborne particles from entering the analyser. Calibration is achieved using cylinders with precisely known concentrations spanning the concentration range expected to be measured in ambient air. Automatic sampling is achieved using a multi-position valve controlled by the G2301.
Calibration of all instrumentation is rigorous and conducted with high-frequency. A different calibration scheme is used for the Picarro than for the other instruments as the Picarro is making almost continuous measurements and the other instruments take periodic air samples.
The Picarro needs much less frequent calibration due to its inherent measurement method. Standards used for this instrument are filled natural air and calibrated at the World Calibration Centre for CO2 and CH4 at Empa in Zurich. These four calibration cylinders provide a wide range of concentrations over which to run the instrument and enable correction of data in the case of any potential non-linear drift. A Target gas is run daily to assess and correct for any potential 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 ICOS.