Abstract

Methane plays an important role in the chemistry and radiative transport of the atmosphere. Its sources in the biosphere are widely distributed, making it difficult to estimate the net annual flux of methane into the atmosphere or the reasons behind its increasing concentration. We describe eddy correlation methane flux instrumentation that uses a 1650 nm diode laser operating at 22°C to measure absorption in an open atmospheric path. The measurement region is formed by two mirrors, separated by 0.5 m, which are used for multipassing the laser beam (optical path length 53 m). A room temperature photodiode detects the transmitted laser power. Wavelength modulation techniques are used to achieve sensitivity to the 0.2% ambient methane absorbance. Results are presented from a prototype sensor that was deployed 3 m above a capped landfill in Oak Ridge, Tennessee, in November 1991. The measurements were conducted in collaboration with scientists at the National Oceanic and Atmospheric Administration Atmospheric Turbulence and Diffusion Division. We observed high fluxes of methane, 6.4 g m⁻² d⁻¹. More recent results also will be discussed.

© 1992 Optical Society of America