IRGA GAS SAMPLING SYSTEM DIMENSIONING: LABORATORY AND FIELD EXPERIMENTS

The gas sampling system (GSS), which carries air from the sampling point to the IRGA, is an essential component of the eddy covariance system. It has to meet several constraints, among which minimizing high frequency attenuation of concentration measurement and keeping pressure drop in the measurement cell in an acceptable range. Rain cup, filters, tubes and pumps are key elements of this system and need proper dimensioning.
The elaboration of the ICOS protocol for IRGA required such dimensioning and optimization.
Laboratory and field measurements were carried out with this aim.
In the laboratory, a dynamic calibration bench was developed to investigate experimentally the pressure drop and the concentration fluctuation attenuation caused by different filters. In the field, three LI-7200 equipped with different GSS were installed and run at the Dorinne Terrestrial Observatory (ICOS-Belgium).
Main experiment conclusions were that:
• The shape and size of the rain cup has a critical impact on cut off frequencies
• The filter porosity and size has a critical impact on pressure drop
• Filter heating is necessary in order to avoid condensation and filter blocking
These experiments led to the definition of the GSS functioning range that is finally proposed in the ICOS IRGA protocol.