A New Open-Path Methane Analyzer and Eddy Covariance Methane Flux Measurement.

Methane is one of the most important greenhouse gases. The estimated total positive radiative forcing on global warming from CH₄ is about 30% of that from CO₂. Current atmospheric methane concentration and flux measurements are limited in time and space due to instrumental limitations. This hinders our understanding of those processes affecting atmospheric CH₄ abundance and their impact on climatic change. Open-path analyzers offer a number of advantages for measuring methane fluxes using the Eddy Covariance technique over agricultural fields, various natural ecosystems, and other contexts (e.g. landfills, animal husbandry etc.). Advantages include undisturbed in-situ flux measurement, no frequency response errors from tube attenuation, and the capability of remote deployment with solar panel power supply systems.  In this paper we present a new open-path methane analyzer and its field performance making flux measurements. The instrument is built using TDLS (Tunable Diode Laser Spectroscopy) technology in a multipass optical cell configuration. Field maintenance is minimized by a self-cleaning mechanism to remove contamination on the lower mirror.  The RMS noise is around 2 ppb ∙Hz^-1/2.   Eddy Covariance measurements of methane flux using this analyzer for the period between 2006 and 2009 in four ecosystems with contrasting weather and moisture conditions are presented. Four ecosystems include (1) fallow agricultural field in Nebraska, (2) sawgrass wetland in the Florida Everglades, (3) coastal wetlands in Arctic tundra, and (4) pacific mangroves in Mexico. Data were processed using EdiRe software following standard FLUXNET protocols, including stationarity test, frequency response correction, and Webb-Pearman-Leuning term.