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 NO2 on 11 flights and HNO3 on five flights. Detection limits were determined to be 25 parts per trillion by volume for NO2 and 75 pptv for HNO3 with response times of 3 min. This was sufficient to permit measurements of NO2 in the free troposphere and HNO3 in the continental boundary layer a significant fraction of the time. ¿American Geophysical Union 1990