Chemical ionization mass spectrometer technique for the measurement of HNO3 in air traffic corridors in the upper troposphere during the SONEX campaign

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 10(6) molecules cm(-3), under i deal conditions. During the SONEX mission the precision of the experiment w as considerably lower due to inlet fluctuations. Ten-second integration per iods were used to obtain a precision of typically 10 parts per trillion by volume. A description is given of the instrument and the technique, includi ng 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.