Eddy covariance measurements with a new fast-response, enclosed-path analyzer: Spectral characteristics and cross-system comparisons
A new class of enclosed path gas analyzers suitable for eddy covariance applications combines the advantages of traditional closed-path systems (small density corrections, good performance in poor weather) and open-path systems (good spectral response, low power requirements), and permits estimates of instantaneous gas mixing ratio. Here, the extent to which these advantages are realized in field deployment is assessed, with a focus on the suitability of such an analyzer (the EC155, manufactured by Campbell Scientific) for long-term flux measurements in a new flux monitoring site in the southern Appalachians (NC, USA). The scalar-vertical velocity co-spectra for CO2 fluxes measured with the EC155 were similar to those measured with a co-located open-path system. When humidity was high, attenuation of the EC155 water vapor fluxes for non-dimensional frequencies greater than ∼2 was noted, though results from an ogive analysis suggest that eddies operating on these time scales contributed <2% of the total turbulent flux in this tall forest ecosystem. Inertial sub-range decay of the vertical velocity-scalar co-spectra generally conformed to a -7/3 power law during near-neutral atmospheric stability conditions, supporting the use of an analytical spectral correction approach to the raw measured fluxes. The EC155 fluxes computed directly from instantaneous mixing ratio agreed with will those calculated from mass–density concentration measurements, provided density terms for temperature, water vapor, and pressure were applied. Biases were observed when the EC155 flux records were compared to those measured with the open-path system. These differences were related to wind angle of attack and to an estimate of apparent fluxes related to instrument self-heating, and the biases were minimized after the application of a friction velocity filter. Finally, the EC155 considerably outperformed open-path analyzers during adverse weather conditions favorable to fog development, which occur frequently in the study site.