Point Monitoring of Ambient Concentrations of Atmospheric Gases Using Tunable Lasers

A laser absorption spectrometer (LAS) is described which employs a wavelength tunable Pb 1-xSnxSe laser diode in conjunction with a multi-pass White cell. Absorption coefficients as low as 3 x 10-3 m-1 can be measured as a result of using a laser wavelength modulation technique to suppress noise in the detection system. This constitutes an improvement of several orders of magnitude in sensitivity compared with earlier reported applications of the direct absorption technique employing tunable diode lasers. The performance of the LAS is illustrated by describing the measurement of three gases commonly found in the atmosphere: SO2 and N2 0 with sensitivities of 1 ppb, and NH3 with a sensitivity of better than 0.1 ppb. The linearity of re-sponse and calibration of the instrument are also discussed in some detail.