THE RELATIVE MERITS OF OPEN-PATH AND CLOSED-PATH ANALYZERS FOR MEASUREMENT OF EDDY FLUXES

Theoretical and practical aspects of measuring eddy fluxes of trace ga ses using open- and closed-path analysers are presented. Trace gas flu xes measured with an open-path analyser require the concurrent measure ment of sensible and latent heat fluxes to correct for density fluctua tions in trace gas concentration caused by these fluxes. A closed-path analyser eliminates the corrections due to sensible heat flux, but no t for water vapour, provided temperature fluctuations are completely r emoved without significantly reducing fluctuations in the trace gas mi xing ratio. Theory for the design of heat exchangers and for the atten uation of concentration fluctuations during air now through tubes is u sed to provide design criteria for closed-path systems. Spectral trans fer functions are used to estimate flux losses caused by now through t he sampling tube and gas analyser. Other factors considered include cr oss-sensitivity of infrared COP analysers to water vapour, and deterio ration of system performance caused by contaminants on the walls of sa mpling tubes, Of two open-path, infrared CO2 analysers tested, one sho wed a strong interaction between CO2 and water vapour, while the other showed little sensitivity to the presence of water vapour, other than caused by dilution. A commercial closed-path CO2 analyser also showed little cross-sensitivity to water vapour. Compared to results for a c lean sampling tube, the spectral bandwidth for water vapour fluctuatio ns decreased significantly after several weeks of sampling. No such de terioration in bandwidth was observed for CO2. These findings are attr ibuted to differential adsorption/desorption of water vapour by dust o r salt on the tubing walls. Rain and dust must be removed from open-pa th analysers to obtain satisfactory measurements, Careful. system desi gn and maintenance is required for both open- and closed-path systems to ensure satisfactory long-term measurement of trace gas fluxes. With these precautions, both approaches will provide satisfactory flux mea surements.