An intercomparison of two tunable diode laser spectrometers used for eddy correlation measurements of methane flux

Tunable Diode laser spectroscopy has, in the last few years, become an important technique for the measurement of atmospheric trace gases and pollutants. This technique is useful not only for concentration monitoring of a particular species, but can be used to measure the flux of trace gas species released or absorbed by an area. At the present time, mainly two types of instruments are being manufactured and used. In the summer of 1993, the Center for Laser Analytical Studies of Trace Gas Dynamics (CLAS) at the University of Nebraska-Lincoln made an intercomparison of these two types of tunable diode laser spectrometers (TDLS) as used to measure the flux and concentration of naturally produced methane. Fluxes were measured using the eddy correlation (or eddy covariance) method. This method requires both a high sensitivity to the species being measured, and a fast instrument response (on the order of 100 msec) and so was deemed a stringent test of instrument performance. The study was conducted at a prairie wetland site near Valentine, Nebraska (43 °N, 100.5 °W, 788 m elev.) in late July and early August, 1993. The two instruments compared were built by Unisearch Associates Inc. of Concord, Ontario, and by Campbell Scientific Inc. of Logan, Utah and were representative of the two main types in general use. The study showed that both instruments exhibited similar detectabilities, and had almost identical noise characteristics when measuring both fluxes and concentrations. The study also brought out the need for careful calibration procedures and precise thermal management of the instruments to achieve optimal performance.