Highly sensitive and mobile TDL-PA spectroscopic measuring device for methane

A mobile, reliable and highly sensitive measuring system for on-line outdoor measurements of methane is described using tunable diode laser resonant photoacoustic (TDL-PA) spectroscopy. The light source is a distributed feedback laser diode for selective detection of methane in ambient conditions via the $2\nu_3$-band transition at 1650.957 nm. A strong influence of humidity on the photoacoustic signal was observed due to a kinetic cooling effect caused by second order collisions of methane in the presence of oxygen. The measuring device has been used successfully in various field measurements. Very good correlations have been found between TDL-PA and commercial none dispersive infrared spectroscopic as well as gas chromatographic measurements. We realized an excellent linearity (R$^2 $= 0.999988) over three orders of magnitude for the concentration measurements with a detection limit (triple standard deviation) of 115 ppbv methane