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

摘要

Oxidation flow reactors that use low-pressure mercury lamps to producehydroxyl (OH) radicals are an emerging technique for studying the oxidativeaging of organic aerosols. Here, ozone (O) is photolyzed at 254 nmto produce O(D) radicals, which react with water vapor to produceOH. However, the need to use parts-per-million levels of O hindersthe ability of oxidation flow reactors to simulate NO-dependentsecondary organic aerosol (SOA) formation pathways. Simple addition of nitricoxide (NO) results in fast conversion of NO(NO + NO) to nitric acid (HNO), making it impossible tosustain NO at levels that are sufficient to compete withhydroperoxy (HO) radicals as a sink for organic peroxy (RO)radicals. We developed a new method that is well suited to thecharacterization of NO-dependent SOA formation pathways inoxidation flow reactors. NO and NO are produced via the reactionO(D) + NO  →  2NO, followed by the reactionNO + O  →  NO + O. Laboratorymeasurements coupled with photochemical model simulations suggest thatO(D) + NO reactions can be used to systematically varythe relative branching ratio of RO + NO reactions relative toRO + HO and/or RO + RO reactionsover a range of conditions relevant to atmospheric SOA formation. Wedemonstrate proof of concept using high-resolution time-of-flight chemicalionization mass spectrometer (HR-ToF-CIMS) measurements with nitrate(NO) reagent ion to detect gas-phase oxidation products ofisoprene and -pinene previously observed inNO-influenced environments and in laboratory chamberexperiments.