Abstract
Eddy-correlation measurements of the oceanic \(\hbox {CO}_2\) flux are useful for the development and validation of air–sea gas exchange models and for analysis of the marine carbon cycle. Results from more than a decade of published work and from two recent field programs illustrate the principal interferences from water vapour and motion, demonstrating experimental approaches for improving measurement precision and accuracy. Water vapour cross-sensitivity is the greatest source of error for \(\hbox {CO}_2\) flux measurements using infrared gas analyzers, often leading to a ten-fold bias in the measured \(\hbox {CO}_2\) flux. Much of this error is not related to optical contamination, as previously supposed. While various correction schemes have been demonstrated, the use of an air dryer and closed-path analyzer is the most effective way to eliminate this interference. This approach also obviates density corrections described by Webb et al. (Q J R Meteorol 106:85–100, 1980). Signal lag and frequency response are a concern with closed-path systems, but periodic gas pulses at the inlet tip provide for precise determination of lag time and frequency attenuation. Flux attenuation corrections are shown to be \(<\)5 % for a cavity ring-down analyzer (CRDS) and dryer with a 60-m inlet line. The estimated flux detection limit for the CRDS analyzer and dryer is a factor of ten better than for IRGAs sampling moist air. While ship-motion interference is apparent with all analyzers tested in this study, decorrelation or regression methods are effective in removing most of this bias from IRGA measurements and may also be applicable to the CRDS.
Highlights
Eddy correlation (EC) is a well established surface flux technique in carbon cycle research
There is some indication of turbulent variance following a −5/3 power law at low frequencies in the mean TORERO spectrum that may be due to turbulent diffusion of horizontal atmospheric concentration gradients, but signal variance is largely dominated by sensor noise
These results are not definitive with respect to the cause of excessive water vapour crosstalk in the IRGAs, but the interference is apparent in both LI7500 and LI7200 models
Summary
Eddy correlation (EC) is a well established surface flux technique in carbon cycle research. In a study comparing the effects of quadratic and cubic representations, Takahashi et al (2002) found a 70 % enhancement in both annual CO2 uptake and wind-speed sensitivity for the cubic kc model An uncertainty this large lends urgency to the task of identifying and quantifying the factors controlling air–sea gas exchange. The flux error may be specified as a function of variance in both vertical wind (w) and scalar (CO2) measurements, where CO2 variance is composed of an atmospheric vertical turbulent flux component (σc2a ) and an “other noise variance” component (σc2n , arising from analyzer noise, water vapour crosstalk and other interference), and where Ts is the sampling time in sec (see Fairall et al 2000 and Blomquist et al 2010), δ Fc =.
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