A novel photochemical conversion technique for reliable calibration of peroxyacetyl nitrate (PAN) analyzers

Photochemical synthesis of peroxyacetyl nitrate (PAN) through irradiating air mixtures of NO and acetone is prevailingly adopted for calibrating PAN analyzers, but few users of PAN analyzers provide evidence to certify the calibration reliability. Here we report a nonnegligible variation (up to ~50 %) of PAN synthesized in the calibration unit of a commercial PAN analyzer, whereas PAN synthesized in the two custom-made reactors could achieve stable values with variations of <2.5 %. Compared with a straight quartz tube flow reactor (SQTFR), PAN synthesized by a coiled quartz tube flow reactor (CQTFR) could achieve more stable (relative standard deviation: <0.66 % versus 2.49 %) and larger (PAN_CQTFR/PAN_SQTFR: 1.04-1.10) values. The residence time and reaction temperature of photochemical mixtures in CQTFR were found to be the key factors affecting PAN synthesis, with their optimal values of 30-60 s and 30-35 °C for achieving the highest PAN levels. The photochemical conversion efficiencies of NO to PAN in CQTFR under the optimal conditions were successfully measured to be 98.5 ± 0.5 % based on the alkaline-absorption method. Therefore, CQTFR is suggested to be adopted for calibrating PAN analyzers to reduce calibration uncertainties.