Controlled nitric oxide production via O(<sup>1</sup>D)  + N<sub>2</sub>O reactions for use in oxidation flow reactor studies

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 (O 3 ) is photolyzed at 254 nm to produce O( 1 D) radicals, which react with water vapor to produce OH. However, the need to use parts-per-million levels of O 3 hinders the ability of oxidation flow reactors to simulate NO x -dependent secondary organic aerosol (SOA) formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NO x (NO + NO 2 ) to nitric acid (HNO 3 ), making it impossible to sustain NO x at levels that are sufficient to compete with hydroperoxy (HO 2 ) radicals as a sink for organic peroxy (RO 2 ) radicals. We developed a new method that is well suited to the characterization of NO x -dependent SOA formation pathways in oxidation flow reactors. NO and NO 2 are produced via the reaction O( 1 D) + N 2 O → 2NO, followed by the reaction NO + O 3 → NO 2 + O 2 . Laboratory measurements coupled with photochemical model simulations suggest that O( 1 D) + N 2 O reactions can be used to systematically vary the relative branching ratio of RO 2 + NO reactions relative to RO 2 + HO 2 and/or RO 2 + RO 2 reactions over a range of conditions relevant to atmospheric SOA formation. We demonstrate proof of concept using high-resolution timeof-flight chemical ionization mass spectrometer (HR-ToF-CIMS) measurements with nitrate (NO − 3 ) reagent ion to detect gas-phase oxidation products of isoprene and α-pinene previously observed in NO x -influenced environments and in laboratory chamber experiments.

Response to Reviewer 2

“…It is worthwhile noting that the chemistry modeled by Peng and Jimenez may find a workaround by utilizing N 2 O as NO precursor (Lambe et al, 2017) rather than NO itself, potentially minimizing RTD-related errors. Palm et al (2018) report data from OFR field deployment where the same comparison (ideal plug flow vs. the RTD experimentally obtained by Lambe et al, 2011a) suggests RTD-related errors overpredict (for CO) or underpredict (for toluene and monoterpenes) photochemical age (that is, the ratio of OH exposure to tropospheric average OH number concentrations) in the reactor, generally within a factor of 3 of model error. Considering this work employs the compartmental model RTD described by Lambe et al (2011a), which for reasons mentioned in the previous section may not be the true PAM RTD, and given that non-ideality in RTDs affects certain OFRs more than others, implementing the method presented here to obtain a more representative reactor RTD can either help constrain error uncertainty in the models, or possibly extend the OH exp range in which OFRs can be operated, a reportedly nontrivial task (Palm et al, 2018).…”

“…3f), as observed by Huang et al (2017). Lambe et al (2011a) modeled the Pennsylvania State University PAM (PSU-PAM) reactor using a compartmen- tal model consisting of two parallel tubular reactors that exhibit Taylor dispersion (Taylor, 1953), suggesting that their reactor (whose geometry is identical to that of the WU-PAM OFR) has two main volumes: an active reactor volume and another volume with entrainment. The model output matches their experimental data reasonably well, but they did not decouple the reactor's E curve from that of the setup, implying the match may include phenomena occurring in other pipes of the setup.…”

“…Finally, we compare results from tracer tests and CFD for the base case. We compare this approach to that of previous studies by Lambe et al (2011a), Huang et al (2017), andSimonen et al (2017), which to the best of our knowledge are the only other studies to date that report experimentally derived RTDs in OFRs. We do not provide predictive configurations for the PAM reactor because there are many avenues different groups can take depending on their focus, and this study is central to the current design.…”

“…For example, Ortega et al removed the inlet plate of their PAM reactor during a deployment in the Fire Lab at Missoula Experiment (FLAME-3) while keeping the inlet baffle to reduce particle loss, and in doing so observed a reduction in jetting of centerline flow (Ortega et al, 2013). In a different study, Lambe et al (2011a) ran experiments keeping the inlet plate on the PAM coupled with a sparger (a cap with large holes in the side in fixed onto the inlet, so that the flow does not jet into the chamber), because laboratory experiments required a closed system (Lambe et al, 2011a).…”

“…However, recent work incorporates the effect of non-ideal RTDs on model outputs (Palm et al, 2017, 2018, 2017. Peng et al (2015) show that for three OFR operational modes (that is, modes of different oxidant formation mechanisms denoted by OFR185, OFR254-70, and OFR254-7), a comparison between model output for ideal plug flow vs. non-ideal RTDs (using the RTD experimentally obtained by Lambe et al, 2011a) for OH exposure (OH exp ) generally agree within a factor of 2 for low OH exp ; the model disagreement is exacerbated at high OH exp beyond a factor of ∼ 4. then extend OFR operational modes to include N-containing chemistry (in modes referred to therein as OFR185-iNO, OFR185-7-iNO, and OFR185-70-iNO) where at moderate-to-high OH exp , the deviations are exacerbated significantly, although the authors argue those conditions represent unrealistic chemical pathways. It is worthwhile noting that the chemistry modeled by Peng and Jimenez may find a workaround by utilizing N 2 O as NO precursor (Lambe et al, 2017) rather than NO itself, potentially minimizing RTD-related errors.…”