Fast-response CO2 mixing-ratio measurement with an open-path gas analyzer for eddy-flux applications

Abstract:

Infra-red gas analyzers operate on the principle of light absorption and measure the density of the gas in the sensing path. To account for density fluctuations caused by barometric pressure, thermal expansion and contraction, and water-vapor dilution, flux calculations using CO₂ density measurements need to be corrected for sensible and latent heat transfer (also known as WPL corrections). In contrast, these corrections are not required if the flux calculation involves CO₂ mixing ratio relative to dry air. Historically, CO₂ mixing ratio measurements have been available only for analyzers with a closed-path where temperature fluctuations in the air sample are attenuated in the intake tubing to a level that they are adequately measured by a contact thermometer. Open-path gas analyzers are not able to make in situ CO₂ mixing-ratio measurements because of the unavailability of a reliable, accurate and fast-response air-temperature sensor in the optical path. A newly developed eddy-flux system integrates an aerodynamic open-path gas analyzer with a sonic anemometer where the sensing volumes of the two instruments coincide. Thus the system has the ability to provide temporally and spatially synchronized fast-response measurements of the 3D wind vector, sonically derived air temperature, CO₂ and water vapor densities. When these measurements are combined with a fast-response static pressure measurement an instantaneous in-situ CO₂ mixing ratio can be calculated on-line, eliminating the need for density corrections in post-processing. In this study fluxes computed from CO2 mixing-ratio are compared to WPL corrected fluxes using CO₂ density. Results from a field inter-comparison with an aspirated temperature probe suggest that accurate, fast response air temperature can be derived from humidity-corrected speed of sound measurements. Biases due to heat exchange with the analyzer surface are evaluated by comparing atmospheric sensible heat flux measurements with a traditional sonic anemometer. The results suggest that measuring air temperature, CO₂ mixing ratio and vertical wind fluctuations in the same open-air sample volume reduces the uncertainty in flux calculations by eliminating the density and sensor spatial separation corrections.