An Alternative Calibration Method for Measuring N₂O₅ with an Iodide-Chemical Ionization Mass Spectrometer and...

In this work, we developed an alternative calibration method for measuring N₂O₅ with an iodide adduct mass spectrometer (I-CIMS). In this calibration method, N₂O₅ is heated and then quantified based on the decrease in the amount of NO due to its reaction with the pyrolysis product (NO₃). This alternative calibration method was compared with the commonly used method utilizing NO analyzers equipped with a photolytic converter, which gauge NO₂ reduction as a result of its reaction with O₃ to quantify N₂O₅. It is notable that the two methodologies demonstrate favorable consistency in terms of calibrating N₂O₅, with a variance of less than 10 %. The alternative calibration method is a more reliable way to quantify N₂O₅ with CIMS, considering the instability of the NO₂ conversion efficiency of photolytic converters in NO analyzers and the loss of N₂O₅ in the sampling line. The effects of O₃ and relative humidity (RH) on the sensitivity toward N₂O₅ were further examined. There was minimal perturbation of N₂O₅ quantification upon exposure to O₃ even at high concentrations. The N₂O₅ sensitivity exhibited a nonlinear dependence on RH as it initially rose and then fell. Besides I(N₂O₅)^-, the collisional interaction between I(H₂O)^- and N₂O₅ also forms I(HNO₃)^-, which may interfere with the accurate quantification of HNO₃. As a consequence of the pronounced dependence on humidity, it is advisable to implement humidity correction procedures when conducting measurements of N₂O₅.