Abstract

A 2f tunable diode laser absorption spectrometer has been used to measure the concentrations of NO₂ and HNO₃ in the atmosphere with detection limits of 25 and 75 pptv, respectively. The laser, which is held in a liquid nitrogen Dewar, is modulated and ramped across an absorption feature. Measurements are made on ambient air contained in a Horn and Pimentel type White cell. Radiation is detected by a cooled HgCdTe detector, and the signal is recovered by a lock-in amplifier. The system can be placed entirely under computer control. There is a master computer and a dedicated microprocessor in the laser control module. The dedicated micro scans the laser, averages the output from the lock-in amplifiers, downloads the spectra into the master computer, and actively stabilizes the laser frequency. The master computer initiates measurement cycles and analyzes the data by fitting the measured spectrum minus the background to the calibration spectrum. The instrument was used extensively on the NASA GTE CITEII mission where detection limits of 25 pptv for NO₂ (absorptions of 5 × 10^-6 over 141 m) were obtained.

© 1987 Optical Society of America