Controlled nitric oxide production via O(¹D)  + N₂O reactions for use in oxidation flow reactor studies

Oxidation flow reactors that use low-pressure mercury lamps to produce hydroxyl (OH) radicals are an emerging technique for studying the oxidative aging of organic aerosols. Here, ozone (O₃) is photolyzed at 254 nm to produce O(¹D) radicals, which react with water vapor to produce OH. However, the need to use parts-per-million levels of O₃ hinders the ability of oxidation flow reactors to simulate NO_<i>x</i>-dependent secondary organic aerosol (SOA) formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NO_<i>x</i> (NO + NO₂) to nitric acid (HNO₃), making it impossible to sustain NO_<i>x</i> at levels that are sufficient to compete with hydroperoxy (HO₂) radicals as a sink for organic peroxy (RO₂) radicals. We developed a new method that is well suited to the characterization of NO_<i>x</i>-dependent SOA formation pathways in oxidation flow reactors. NO and NO₂ are produced via the reaction O(¹D) + N₂O  →  2NO, followed by the reaction NO + O₃  →  NO₂ + O₂. Laboratory measurements coupled with photochemical model simulations suggest that O(¹D) + N₂O reactions can be used to systematically vary the relative branching ratio of RO₂ + NO reactions relative to RO₂ + HO₂ and/or RO₂ + RO₂ reactions over a range of conditions relevant to atmospheric SOA formation. We demonstrate proof of concept using high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) measurements with nitrate (NO₃⁻) reagent ion to detect gas-phase oxidation products of isoprene and <i>α</i>-pinene previously observed in NO_<i>x</i>-influenced environments and in laboratory chamber experiments.