MEASUREMENTS OF NITRIC-OXIDE, NITROGEN-DIOXIDE AND NITRIC ACID IN AMBIENT AND CAPTIVE AIR BY SECOND HARMONIC DETECTION WITH TUNABLE DIODE LASERS

A Tropospheric Air Monitoring System (TAMS) was designed and constructed to measure trace gases in ambient air. The second harmonic component of the modulated light from a tunable diode laser due to an absorption line was measured to determine the mixing ratio of a trace gas in the sample. The technique of second harmonic detection increased the signal-to-noise ratio so that a species with mixing ratios in the sub-ppbv range could be detected.;The TAMS was intercalibrated with a chemiluminescence instrument of the University of Michigan. The system was transported to Los Angeles in fall of 1981 to join a captive air measurement programme. Both the captive and ambient air in Los Angeles was monitored for a one month period. The agreement with the University of Michigan was excellent for NO and satisfactory for NO(,2) and HNO(,3). The results of the intercalibrations and measurements are described.;Second harmonic signals of an absorption line was calculated at different pressures (line shapes) and different modulation amplitudes to understand the optimum conditions for trace gas measurements. Also examined were the properties of the second harmonic signals due to etalon fringes, wavelength dependence of the laser power, tails of other absorption lines and interferences by these factors. The reduction of random noise by second harmonic detection was also investigated.