Spectroscopic Issues Related to Trace Gas Measurements in the Stratosphere Using the FM-Modulated Tunable Diode Laser Absorption Spectrometer

The fm-modulated tunable diode laser absorption spectrometer has been shown to be capable of detecting fractional absorptions of 1 part in 105 for NO under stratospheric conditions.1 This capability implies that trace species concentrations as low as 108 molecules/ cm3 can be detected at 32 Km in conjunction with absorption path lengths of 100m. The filtering properties of the fm/2nd harmonic detection tech nique which make these measurements possible2 also distort the IR signature being detected and make calibration of the measurement necessary. Calibration of fractional absorption is most generally accomplished through the use of calibration cells containing a known amount of gas under known, constant conditions. The trace gases being studied in the stratosphere are subject to a wide range of pressure and temperature conditions. The interplay of distortion, variation of ambient conditions, and calibration technique make interpretation of detected signatures complex. This interpretation can be made more certain by selecting appropriate lines when possible, by recording the ambient conditions of each measurement, and by drawing upon a high resolution spectroscopic data base for each species being studied.