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

Other literature type English OPEN
Lambe, Andrew ; Massoli, Paola ; Zhang, Xuan ; Canagaratna, Manjula ; Nowak, John ; Yan, Chao ; Nie, Wei ; Onasch, Timothy ; Jayne, John ; Kolb, Charles ; Davidovits, Paul ; Worsnop, Douglas ; Brune, William (2017)

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 (O<sub>3</sub>) is photolyzed at 254 nm to produce O(<sup>1</sup>D) radicals, which react with water vapor to produce OH. However, the need to use parts-per-million levels of O<sub>3</sub> hinders the ability of oxidation flow reactors to simulate NO<sub>x</sub>-dependent SOA formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NO<sub>x</sub> (NO + NO<sub>2</sub>) to nitric acid (HNO3), making it impossible to sustain NO at levels that are sufficient to compete with hydroperoxy (HO<sub>2</sub>) radicals as a sink for organic peroxy (RO<sub>2</sub>) radicals. We developed a new method that is well suited to the characterization of NO<sub>x</sub>-dependent SOA formation pathways in oxidation flow reactors. NO and NO<sub>2</sub> are produced via the reaction O(<sup>1</sup>D) + N<sub>2</sub>O→ 2NO, followed by the reaction NO + O<sub>3</sub> → NO<sub>2</sub>+ O<sub>2</sub>. Laboratory measurements coupled with photochemical model simulations suggest that O(<sup>1</sup>D) + N<sub>2</sub>O reactions can be used to systematically vary the relative branching ratio of RO<sub>2</sub> + NO reactions relative to RO<sub>2</sub> + HO<sub>2</sub> and/or RO<sub>2</sub> + RO<sub>2</sub> 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 (NO<sub>3</sub><sup>&minus;</sup>) reagent ion to detect gas-phase oxidation products of isoprene and α-pinene previously observed in NOx-influenced environments and in laboratory chamber experiments.
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