A new inlet and instrument have been developed for the rapid measurement of gas phase nitric acid (HNO3) from an airborne platform. The inlet was kept near ambient temperatures with a very short sampling time (100 ms) to minimize desorption of particle nitrates. In addition, inlet surface adsorption problems were minimized by the use of extruded perfluoroalkoxy as a sampling material. Nitric acid was detected by selected ion chemical ionization mass spectrometry using deprontonated methanesulfonic acid as a reagent ion. Laboratory tests showed no interferences from NO, NO2, NO3, and N2O5 under wet (relative humidity (RH) = 100%) or dry (RH = 0%) conditions at levels exceeding those found in the troposphere. The inlet and instrument were flown on the NASA P-3B aircraft as part of the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign off the coast of Asia during February--April 2001. Nitric acid was measured every 5 s for a 3 s integration period with a limit of detection of ~10 parts per trillion by volume (pptv). The instrument was calibrated by the addition of isotopically labeled H15NO3 near the front of the ion source on a continual basis. Absolute uncertainties including systematic errors are the limit of detection (10 pptv) plus ¿20% for HNO3 > 200 pptv, ¿25% for HNO3 100--200 pptv, and ¿30% for HNO3 < 100 pptv (¿2 σ). Rapid changes in ambient HNO3 were resolved, suggesting minimal influences from instrument surfaces. Finally, the measurements compared favorably with the University of New Hampshire's mist chamber/ion chromatography instrument flown on board the NASA DC-8 aircraft during two intercomparison flights. The in-flight performance of the instrument is demonstrated under the wide range of conditions observed in TRACE-P.