A new technique for measuring peroxyacetyl nitrate (PAN) at parts per trillion by volume (pptv) levels has been developed using gas chromatography with negative ion chemical ionization mass spectrometry (GC/NICI MS). Four fragment ions, CH3C(O)O- (mass-to-charge ratios (m/z)=59), NO3-(m/z=62), CH3C(O)OO-(m/z=75), and NO2-(m/z=46), were identified by NICI mass spectra. NO3-(m/z=62) was used in selected ion monitoring (SIM) mode among these fragment ions, since it gave the best signal-to-noise (S/N) ratio, and a potential interference from isopropyl nitrate (IPN) was excluded. Optimization of mass spectrometry and use of capillary gas chromatography enabled us to detect PAN at a retention time of 2.6¿0.1 min in ambient air mixing ratio range. The present detection limit is 15¿4 pptv (S/N=3), which is comparable to those of previously published GC/ECD methods, with good linearity at pptv levels. The instrumental accuracy and precision are estimated to be ¿20% and ¿15%, respectively. Other PAN analogs (i.e., peroxypropionyl nitrate and peroxymethacryloyl nitrate) were also measured within 10 min. Exploratory ambient air measurements were made at our laboratory in a suburban area of Tokyo, Japan. These results demonstrated the performance of GC/NICI MS for ambient air measurements. ¿ 1999 American Geophysical Union