Abstract
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.
Highlights
We developed a new method that is well suited to the characterization of NOx-dependent secondary organic aerosol (SOA) formation pathways in oxidation flow reactors
We demonstrate proof of concept using high-resolution timeof-flight chemical ionization mass spectrometer (HR-ToFCIMS) measurements with nitrate (NO−3 ) reagent ion to detect gas-phase oxidation products of isoprene and α-pinene previously observed in NOx-influenced environments and in laboratory chamber experiments
Recent atmospheric observations supported by experimental and theoretical studies show that highly oxidized molecules (HOMs), together with sulfuric acid, are involved in the initial nucleation steps leading to new particle formation (NPF) (Donahue et al, 2013; Riccobono et al, 2014; Kurten et al, 2016)
Summary
Recent atmospheric observations supported by experimental and theoretical studies show that highly oxidized molecules (HOMs), together with sulfuric acid, are involved in the initial nucleation steps leading to new particle formation (NPF) (Donahue et al, 2013; Riccobono et al, 2014; Kurten et al, 2016). Chambers have relatively low throughput and are limited to residence times of several hours due to chamber deflation and/or loss of particles and oxidized vapors to the chamber walls (Zhang et al, 2014) This restricts environmental chambers to simulating atmospheric aerosol particle lifetimes and SOA yields only up to 1 or 2 days, limiting the study of the formation of highly oxygenated SOA characteristic of aged atmospheric organic aerosol particulate matter (PM) (Ng et al, 2010) unless very low VOC precursor concentrations are used (Shilling et al, 2009; Pfaffenberger et al, 2013). Oxidation flow reactors have recently been developed to study SOA formation and evolution over timescales ranging from hours to multiple days of equivalent atmospheric OH exposure. We validate the concept using high-resolution timeof-flight chemical ionization mass spectrometer measurements (HR-ToF-CIMS) to detect gas-phase oxidation products of isoprene and α-pinene that have been observed in NOx-influenced environments and laboratory chamber experiments
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