Methane (CH4) is a greenhouse gas and CH4 concentration of the air has been increasing every year at a rate of about 1%. Plant ecosystems act as both a source and a sink of atmospheric methane, but there have been some difficulties in determining the actual CH4 flux over each type of vegetation because a proper gas analyzer had never been available. We developed a movable infrared methane (NDIR-CH4) analyzer for field measurements. The analyzer can be applied to field measurements, and has a 10ppm realizable limit and good reproducibility by using both a cross-modulation type of NDIR analyzing system and a pre-processing unit with a selective caalyzer.
The analyzer was applied to measurement of the CH4 concentration and its flux over grasslands. The CH4 gradient was measured by alternately sampling air from two different heights. The CH4 concentration gradient over the dry grassland was positive meaning that is a CH4 sink, and gradient was negative over the pasture-land at night time where cattle excrement had been spread. Continuous measurement showed that CH4 concentrations changed remarkably over the pasture-land when the CH4 enriched parcel advected from source points such as excrement near cattle sheds.
CH4 flux was evaluated micro-meteorologically using CO2 flux data obtained by the eddy correlation technique. There was an efflux (source) of 0.1-0.3mg CH4/m2h over the pastureland during night time, while it was 0.1-0.3mgCH4/m2h of downward flux (sink) in the morning. These levels of sink were higher than previous results which were obtained by the chamber method. The performance of the present NDIR-CH4 analyzer was confirmed by field tests over grasslands. Thus, the analyzer is useful for studying the characteristics of CH4 in ecosystems.