We have developed a thermal dissociation–chemical ionization mass spectrometry (TD-CIMS) technique for fast measurements of a series of peroxyacyl nitrates and dinitrogen pentoxide. Thermally generated acylperoxy radicals react with I− to produce a carboxylate ion that is unique for each parent species. NO3 resulting from the decomposition of N2O5 reacts with I− to form NO3−. The measurement technique was verified for PAN and PPN during an informal comparison with a gas chromatograph (GC) equipped with an electron capture detector (ECD) in Boulder, Colorado, during October 2002. Good agreement was obtained between the two instruments with R2 = 0.91 for PAN (n = 657) and R2 = 0.89 for PPN (n = 655). Detection limits of 7 pptv and 4 pptv were determined for PAN and PPN, respectively, for a 1 s integration period and a signal-to-noise ratio of 3. The TD-CIMS simultaneously detected ambient PiBN and N2O5 + NO3 in addition to PAN and PPN during the intercomparison period. We estimate a detection limit of 3 pptv in 1 s for PiBN. PnBN would be detected at the same mass, so we cannot rule out a contribution from PnBN to our PiBN estimate. Sensitivity to MPAN was found to be lower than to PAN in laboratory experiments, but a detection limit of 10 pptv can still be achieved by integrating for 15 s. Assuming the same sensitivity as for PAN, the detection limit for the sum of N2O5 and NO3 was estimated to be 12 pptv in 1 s. The fast time response of the TD-CIMS combined with a sensitivity and limit of detection comparable to the GC/ECD make this a promising technique for PAN flux measurements by eddy covariance. PAN uptake by different types of vegetation could be important input for global and regional ozone models, and PAN deposition to snow is of interest in polar regions.