The relative merits of open- and closed-path analysers for measurement of eddy fluxes

Theoretical and practical aspects of measuring eddy fluxes of trace gases using open-and closed-path analysers are presented. Trace gas fluxes measured with an open-path analyser require the concurrent measurement of sensible and latent heat fluxes to correct for density fluctuations in trace gas concentration caused by these fluxes. A closed-path analyser eliminates the corrections due to sensible heat flux, but not for water vapour, provided temperature fluctuations are completely removed without significantly reducing fluctuations in the trace gas mixing ratio. Theory for the design of heat exchangers and for the attenuation of concentration fluctuations during air flow through tubes is used to provide design criteria for closed-path systems. Spectral transfer functions are used to estimate flux losses caused by flow through the sampling tube and gas analyser. Other factors considered include cross-sensitivity of infrared CO₂ analysers to water vapour, and deterioration of system performance caused by contaminants on the walls of sampling tubes. Of two open-path, infrared CO₂ analysers tested, one showed 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 clean sampling tube, the spectral bandwidth for water vapour fluctuations decreased significantly after several weeks of sampling. No such deterioration in bandwidth was observed for CO₂. These findings are attributed to differential adsorption/desorption of water vapour by dust or salt on the tubing walls. Rain and dust must be removed from open-path analysers to obtain satisfactory measurements. Careful system design 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 measurements.