Improvements in infrared gas analyzers for measuring atmospheric gases on moving platforms

Accurate observations of atmospheric composition and exchange of greenhouse gases between the ecosystems and the atmosphere are critical for constraining climate models. Infrared gas analyzers (IRGA) using either broad band non-dispersive or narrow band tunable laser technologies are widely used for this purpose. Typically, such analyzers are installed on stationary meteorological towers over land; but an increasing number of systems are being deployed on mobile platforms and buoys to extend the spatial coverage and include measurements over water.  One technological challenge is that the motion of the platform influences the gas concentration measurements. Empirical correction methods have been proposed, but their universality is limited because the source of these sensor-related effects and their underlining mechanisms have not been understood. In this study we identified the dominant source of the error: orientation-dependent temperature stabilization of the thermoelectrically cooled infrared detector. To further investigate this hypothesis and gain insights to a solution, a new prototype closed-path IRGA with an improved infrared detector was developed. In the study, we compared the performance of the prototype to standard models of commercially available IRGA measuring CO2 and H2O.  Tilt experiments with side-by-side mounted IRGAs were first conducted on a controlled laboratory platform with independent pitch and roll axes. Over the ±30° range of angular position, the orientation-correlated errors were reduced by a factor of 4 to 10 on CO2 and a factor of 2 to 8 on H2O. Subsequent testing was performed duplicating realistic buoy motion in a deep-water tank with typical at-sea combined pitch and roll motion. In these tests, improvements in the measurement errors were similar to the laboratory experiments. Implications for the correction of past field measurements and insights for further sensor optimization and system improvements are discussed.