(Invited Presentation) Recent Advances in Air-Sea Gas Flux Measurements

Recent developments in analytical instrumentation and data analysis have made possible open-ocean direct measurements of carbon monoxide evasion and improved the sensitivity of carbon dioxide air-sea exchange measurements.

We conducted a field trial in December, 2011 of ship-based air-sea flux measurements of carbon monoxide (CO) by direct eddy correlation using an infrared-laser trace gas analyzer. The analyzer utilizes Off-Axis Integrated-Cavity-Output Spectroscopy (OAICOS) to achieve high selectivity for CO, rapid response (10Hz) and low noise. Over a two-day sea trial, peak daytime seawater CO concentrations were 1.5nM and wind speeds were consistently 10–12ms-1. A clear diel cycle in CO flux with an early afternoon maximum was observed. An analysis of flux error sources suggests air-sea CO flux measurements are best performed in regions remote from continental pollution sources. Horizontal fluxes add variance that can obscure the vertical flux signal for long-lived gases.

The recent availability of fast, high precision cavity-ringdown (CRDS) analyzers offered a prospect of significant improvements in both precision and detection limit for eddy correlation CO2 flux measurements. A CRDS flux system was deployed on the R/V Ka'imimoana during TORERO in January and February of 2012. As a test of flux precision, estimated air/sea pCO2 differences were derived from the observed flux and modeled gas transfer coefficients (COAREG ver. 3.1). The estimated delta pCO2 compares favorably with February mean pCO2 climatology for the 95W - 110W region of the eastern Central Pacific.

Initial results indicate the CRDS analyzer delivers significantly improved sensitivity for flux measurement. Delta pCO2 for a detection criterion of dF/F = 1 (100% relative error) appears to be 4-8 times lower than the best performance of LICOR open-path non-dispersive IR analyzers. The use of a decorrelation technique substantially reduces residual ship-motion artifacts in winds and fluxes.

This work is a part of the HiWinGS program (High Wind Gas Exchange Study). These methods will be deployed during a high-wind cruise in the Fall of 2013 alongside DMS flux measurements and detailed physical characterizations of bubbles, waves, turbulence, and other potential flux-controlling factors.