A new chemical ionization mass spectrometer (CIMS) technique has been developed for the measurement of HNO3. The technique has been field tested and informally intercompared with a standard filter pack method, and a new CIMS technique developed by the National Oceanic and Atmospheric Administration Aeronomy Laboratory. The technique has been shown under lab conditions to have sensitivities as high as 0.2 parts per trillion volume (pptv) HNO3 for a 1 s integration of each the reagent and product ions, with a typical clean air field detection limit of 1--3 pptv for the same integration times. The new technique also responds quickly to changes in HNO3 concentrations. The high sensitivity and fast response time make this technique well suited for airborne measurements. ¿ 1998 American Geophysical Union