High-precision measurements of the methane flux over a larch forest based on a hyperbolic relaxed eddy accumulation method using a laser spectrometer

摘要

The precision of current micrometeorological techniques has restricted their applications in measuring small methane (CH4) fluxes in forest ecosystems. In this study, we continuously measured CH4 fluxes using a state of the art laser-based gas analyzer and a hyperbolic relaxed eddy accumulation (HREA) method. The precision of the half-hourly fluxes was estimated at 2.93 nmol m~(-2) s~(-1). The HREA method also contained uncertainties associated with scalar dissimilarity of 1.26 nmol m~(-2) s~(-1). The precision was higher than those obtained in previous eddy covariance studies, because the HREA system did not require density fluctuation and high-frequency attenuation corrections. The observed CH_4 uptake decreased with increases in soil water content during the summer months. The estimated annual methane sink was 673 mg CH_4 m~(-2) yr~(-1) with uncertainties of ±231 mg CH_4 m~(-2) yr~(-1). This annual methane sink increased to 868 mg CH_4 m~(-2) yr~(-1) with the application of friction velocity (u-) filtering. The uncertainties in gap-filling, storage correction, and scalar dissimilarity were less important than the uncertainties in the u-filtering. Our technique using the HREA method is a suitable tool for measuring small CH_4 fluxes in forest ecosystems, especially in remote sites where frequent maintenance is not practical.