A chemical ionization mass spectrometer (CIMS) was used for rapid detection of HNO3 in air traffic corridors, primarily over the North Atlantic region, during the NASA Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) campaign in the fall of 1997. The sensitivity of the CIMS instrument approaches 1 ion count per second for each 106 molecules cm-3, under ideal conditions. During the SONEX mission the precision of the experiment was considerably lower due to inlet fluctuations. Ten-second integration periods were used to obtain a precision of typically 10 parts per trillion by volume. A description is given of the instrument and the technique, including inflight calibration using a permeation tube. Comparisons are made with NOy data and with the University of New Hampshire HNO3 data obtained with a mist chamber method. ¿ 2000 American Geophysical Union