Calibration Method for N2O5, ClNO2, and HONO with Iodide Chemical Ionization Mass Spectrometry (I-CIMS)

Abstract Nighttime reactive nitrogen reservoir species such as dinitrogen pentoxide (N₂O₅), nitryl chloride (ClNO₂), and nitrous acid (HONO) play critical roles in nocturnal nitrate formation yet remain understudied due to the absence of certified standard gases. Accurate calibration of iodide chemical ionization mass spectrometry (I-CIMS) for these species is therefore essential to improve quantitative understanding of nighttime chemistry. This study establishes a reproducible in situ calibration framework for N₂O₅, ClNO₂, and HONO under controlled laboratory conditions. N₂O₅ was generated through the gas-phase reaction of O₃ with NO₂ and quantified from the stoichiometric decrease in NO₂, yielding a slope of 0.5 (R 2  = 0.99) and confirming that the NO₃ self-reaction was negligible under excess-NO₂ conditions. ClNO₂ was subsequently generated by introducing the produced N₂O₅ into a NaCl slurry, resulting in a strong linear relationship (R 2  > 0.99) between the I-CIMS signal and calculated ClNO₂ concentration. HONO was generated via the reaction of HCl gas with NaNO₂-coated annular denuder, which provided uniform coating and stable production. This unified calibration approach provides a practical and reproducible basis for accurate quantification of key nighttime reactive nitrogen reservoirs and enhances understanding of nighttime nitrate formation processes. Graphical Abstract