Carbon dioxide measurements

Carbon dioxde is measured using a Non-dispersive Infrared Radiometer (NDIR), Li-COR model 7000.
Because water vapor obstructs the measurements by absorbing in the same wave-length used by the instrument, the sample air is first drawn through a M&C EC30 cooler to remove water vapor to a frost point of -30 ° C. The air then passes through a 7 µm filter, a VALCO injector valve and a BROOKS massflow controller before entering the radiometer.
The Radiometer is run in differential mode using a reference gas with a CO2 content near the measured concentrations. Roughly every 2 hours calibration of the radiometer is done using 3 different CO2 conentrations spanning the expected atmospheric conentration interval.

In addition flask samples are taken weekly and analyzed for CO₂, CH₄, CO, and 13CO₂, H₂, N₂O, SF₆ and ¹⁸O in CO₂ by NOAA/CMDL, Boulder, Colorado, USA (Thomas J. Conway).
For more info on this program, please visit the CCGG home page.