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The advantages of tunable diode laser absorption spectrometry (TDLAS) for measuring trace atmospheric gases are universality, positive identification, good sensitivity, and rapid response time. A TDLAS system is described which is capable of making measurements on board an aircraft. The instrument was employed during the Chemical Instrumentation Test and Evaluation (CITE) 2 program to measure NO 2 , on 11 flights and HNO 3 , on five flights. Detection limits were determined to be 25 parts per trillion by volume pptv for NO 2 and 75 pptv for HNO 3 with response times of 3 min. This was sufficient to permit measurements of NO 2 in the free troposphere and HNO 3 , in the continental boundary layer a significant fraction of the time.
Schiff et al. (Wed,) studied this question.