An Approach to Minimizing Artifacts Caused by Cross-Sensitivity in the Determination of Air–Sea $$\text{ CO }_{2}$$ Flux Using the Eddy-Covariance Technique

Abstract

The air–sea $$\text{ CO }_{2}$$ flux was measured from a research vessel in the North Yellow Sea in October 2007 using an open-path eddy-covariance technique. In 11 out of 64 samples, the normalized spectra of scalars ( $$\text{ CO }_{2}$$ , water vapour, and temperature) showed similarities. However, in the remaining samples, the normalized $$\text{ CO }_{2}$$ spectra were observed to be greater than those of water vapour and temperature at low frequencies. In this paper, the noise due to cross-sensitivity was identified through a combination of intercomparisons among the normalized spectra of three scalars and additional analyses. Upon examination, the cross-sensitivity noise appeared to be mainly present at frequencies $${<}0.8\,\text{ Hz }$$ . Our analysis also suggested that the high-frequency fluctuations of $$\text{ CO }_{2}$$ concentration (frequency $${>}0.8\,\text{ Hz }$$ ) was probably less affected by the cross-sensitivity. To circumvent the cross-sensitivity issue, the cospectrum in the high-frequency range 0.8–1.5 Hz, instead of the whole range, was used to estimate the $$\text{ CO }_{2}$$ flux by taking the contribution of the high frequency to the $$\text{ CO }_{2}$$ flux to be the same as the contribution to the water vapour flux. The estimated air–sea $$\text{ CO }_{2}$$ flux in the North Yellow Sea was $$-0.039\,\pm \,0.048\,\text{ mg } \text{ m }^{-2}\,\text{ s }^{-1},$$ a value comparable to the estimates using the inertial dissipation method and Edson’s method (Edson et al., J Geophys Res 116:C00F10, 2011).