Articles | Volume 10, issue 6
Research article
22 Jun 2017
Research article |  | 22 Jun 2017

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

Andrew Lambe, Paola Massoli, Xuan Zhang, Manjula Canagaratna, John Nowak, Conner Daube, Chao Yan, Wei Nie, Timothy Onasch, John Jayne, Charles Kolb, Paul Davidovits, Douglas Worsnop, and William Brune

Abstract. 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 NOx-dependent secondary organic aerosol (SOA) formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NOx (NO + NO2) to nitric acid (HNO3), making it impossible to sustain NOx 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 NOx-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 α-pinene previously observed in NOx-influenced environments and in laboratory chamber experiments.

Short summary
This work enables the study of NOx-influenced secondary organic aerosol formation chemistry in oxidation flow reactors to an extent that was not previously possible. The method uses reactions of exited oxygen O(1D) radicals (formed from ozone photolysis at 254 nm or nitrous oxide photolysis at 185 nm) with nitrous oxide (N2O) to produce NO. We demonstrate proof of concept using chemical ionization mass spectrometer measurements to detect gas-phase oxidation products of isoprene and α-pinene.