Methane is one of the most important greenhouse gases, with a warming potential of about 23 times that of carbon dioxide over a 100-year cycle (Houghton et al., 2001). Open-path analyzers offer a number of advantages for directly measuring methane fluxes using the Eddy Covariance technique. Advantages include undisturbed in-situ flux measurements, spatial integration using the Eddy Covariance approach, zero frequency response errors due to tube attenuation, confident water and thermal density terms from co-located fast measurements of water and sonic temperature, and remote deployment due to lower power demands in the absence of a pump.
In this presentation we introduce and test in the field the prototype open-path methane analyzer. The instrument is based on a VCSEL (vertical-cavity surface-emitting laser) technology, and employs an open Herriott cell to measure levels of methane with RMS noise below 6 ppb at 10 Hz sampling. Field maintenance is minimized by a self-cleaning mechanism to keep the lower mirror free of contamination.
Results/Conclusions
Eddy Covariance measurements of methane flux using this prototype open-path methane analyzer are presented for the period between 2006 and 2008 in three ecosystems with contrasting weather and moisture conditions:
(1) Fluxes over a short-hydroperiod sawgrass wetland in the Florida Everglades were measured in a warm and humid environment with temperatures often exceeding 25oC, variable winds, and frequent heavy dew at night;
(2) Fluxes over coastal wetlands in an Arctic tundra were measured in an environment with frequent sub-zero temperatures, moderate winds, and ocean mist;
(3) Fluxes over pacific mangroves in Mexico were measured in an environment with moderate air temperatures high winds, and sea spray.
Presented eddy covariance flux data were collected from a co-located prototype open-path methane analyzer, LI-7500, and sonic anemometer at a 10 Hz rate. Data were processed using EdiRe software following standard FluxNet methodology, including stationarity tests, frequency response, and Webb-Pearman-Leuning density terms.
