A CO2 and H2O gas analyzer with reduced error due to platform motion

Abstract

One long-standing technical problem affecting the accuracy of eddy correlation air-sea CO2 flux estimates has been motion contamination of the CO2 mixing ratio measurement. This sensor-related problem is well known but its source remains unresolved. This report details an attempt to identify and reduce motion-induced error and to improve the infrared gas analyzer (IRGA) design. The key finding is that a large fraction of the motion sensitivity is associated with the detection approach common to most closed- and open-path IRGA employed today for CO2 and H2O measurements. A new prototype sensor was developed to both investigate and remedy the issue. Results in laboratory and deep water tank tests show marked improvement. The prototype shows a factor of 4-10 reduction in CO2 error under typical at-sea buoy pitch and roll tilts in comparison to an off-the-shelf IRGA system. A similar noise reduction factor of 2-8 is observed in water vapor measurements. The range of platform tilt motion testing also helps to document motion-induced error characteristics of standard analyzers. Study implications are discussed including findings relevant to past field measurements and the promise for improved future flux measurements using similarly modified IRGA on moving ocean observing and aircraft platforms.