METHANE EMISSIONS FROM A LANDFILL MEASURED BY EDDY-CORRELATION USING A FAST-RESPONSE DIODE-LASER SENSOR

We describe a fast response methane sensor based on the absorption of radiation generated with a near-infrared InGaAsP diode laser. The sens or uses an open path absorption region 0.5 m long; multiple pass optic s provide an optical path of 50 m. High frequency wavelength modulatio n methods give stable signals with detection sensitivity (S/N = 1, 1 H z bandwidth) for methane of 65 ppb at atmospheric pressure and room te mperature. Improvements in the optical stability are expected to lower the current detection limit. We used the new sensor to measure, by ed dy correlation, the CH4 flux from a clay-capped sanitary landfill. Sim ultaneously we measured the Aux of CO2 and H2O. From seven half-hourly periods of data collected after a rainstorm on November 23, 1991, the average flux of CH4 was 17 mmol m(-2) hr(-1) (6400 mg CH4 m(-2) d(-1) ) with a coefficient of variation of 25%. This measurement may underre present the flux by 15% due to roll-off of the sensor response at high frequency. The landfill was also a source of CO2 with an average flux of 8.1 mmol m(-2) hr(-1) (8550 mg CO2 m(-2) d(-1)) and a coefficient of variation of 26%. A spectral analysis of the data collected from th e CH4, CO2, and H2O sensors showed a strong similarity in the turbulen t transfer mechanisms.