Corrections on CH 4 Fluxes Measured in a Rice Paddy by Eddy Covariance Method with an Open-path Wavelength Modulation Spectroscopy

Abstract

CH4 is a trace gas and one of the key greenhouse gases, which requires continuous and systematic monitoring. The application of eddy covariance technique for CH4 flux measurement requires a fast- response, laser-based spectroscopy. The eddy covariance measurements have been used to monitor CO2 fluxes and their data processing procedures have been standardized and well documented. However, such processes for CH4 fluxes are still lacking. In this note, we report the first measurement of CH4 flux in a rice paddy by employing the eddy covariance technique with a recently commercialized wavelength modulation spectroscopy. CH4 fluxes were measured for five consecutive days before and after the rice transplanting at the Gimje flux monitoring site in 2012. The commercially available EddyProTM program was used to process these data, following the KoFlux protocol for data-processing. In this process, we quantified and documented the effects of three key corrections: (1) frequency response correction, (2) air density correction, and (3) spectroscopic correction. The effects of these corrections were different between daytime and nighttime, and their magnitudes were greater with larger CH4 fluxes. Overall, the magnitude of CH4 flux increased on average by 20-25% after the corrections. The National Center for AgroMeteorology (www.ncam.kr) will soon release an updated KoFlux program to public users, which includes the spectroscopic correction and the gap-filling of CH4 flux. Key