The PAN-CIGAR chemical ionization mass spectrometer which measures up to 7 PAN species simultaneously and semi-continuously with a time resolution of ~2 seconds. The method is based on the detection of the acylperoxy radicals formed from thermal decomposition of the PAN species at the inlet by reacting them with iodide ions, which are formed by passing methyl iodide diluted in nitrogen through an α–particle source. The reaction of the peroxy acyl radicals with I- forms IO and the acyl ion, which is detected using a quadrupole mass spectrometer (Extrel) at a mass to charge ratio of 59 in the case of PAN. The method is very specific for PAN type compounds and the limit of detection is ~1 pptv/s or better for most PAN species. The instrument employs a realtime continuous calibration using isotopically labeled PAN produced in-situ by a photolytic calibration source.