Measuring turbulent COsub2/sub fluxes with a closed-path gas analyzer in a marine environment

摘要

In this study, we introduce new observations of sea–air fluxes of carbon dioxide using the eddy covariance method. The measurements took place at the Ut? Atmospheric and Marine Research Station on the island of Ut? in the Baltic Sea in July–October?2017. The flux measurement system is based on a closed-path infrared gas analyzer (LI-7000, LI-COR) requiring only occasional maintenance, making the station capable of continuous monitoring. However, such infrared gas analyzers are prone to significant water vapor interference in a marine environment, where COsub2/sub fluxes are small. Two LI-7000 analyzers were run in parallel to test the effect of a sample air drier which dampens water vapor fluctuations and a virtual impactor, included to remove liquid sea spray, both of which were attached to the sample air tubing of one of the analyzers. The systems showed closely similar ( Rsup2/sup=0.99 ) sea–air COsub2/sub fluxes when the latent heat flux was low, which proved that neither the drier nor the virtual impactor perturbed the COsub2/sub flux measurement. However, the undried measurement had a positive bias that increased with increasing latent heat flux, suggesting water vapor interference. For both systems, cospectral densities between vertical wind speed and COsub2/sub molar fraction were distributed within the expected frequency range, with a moderate attenuation of high-frequency fluctuations. While the setup equipped with a drier and a virtual impactor generated a slightly higher flux loss, we opt for this alternative for its reduced water vapor cross-sensitivity and better protection against sea spray. The integral turbulence characteristics were found to agree with the universal stability dependence observed over land. Nonstationary conditions caused unphysical results, which resulted in a high percentage (65?%) of discarded measurements. After removing the nonstationary cases, the direction of the sea–air COsub2/sub fluxes was in good accordance with independently measured COsub2/sub partial pressure difference between the sea and the atmosphere. Atmospheric COsub2/sub concentration changes larger than 2 ppm during a 30 min averaging period were found to be associated with the nonstationarity of COsub2/sub fluxes.