Modeling of the chemistry in oxidation flow reactors with high initial NO

Zhe Peng, Jose L. Jimenez
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Oxidation flow reactors (OFRs) are increasingly employed in atmospheric chemistry research because of their high efficiency of OH radical production from low-pressure Hg lamp emissions at both 185 and 254<span class="thinspace"></span>nm (OFR185) or 254<span class="thinspace"></span>nm only (OFR254). OFRs have been thought to be limited to studying low-NO chemistry (in which peroxy radicals (RO<sub>2</sub>) react preferentially with HO<sub>2</sub>) because NO is
more » ... y rapidly oxidized by the high concentrations of O<sub>3</sub>, HO<sub>2</sub>, and OH in OFRs. However, many groups are performing experiments by aging combustion exhaust with high NO levels or adding NO in the hopes of simulating high-NO chemistry (in which RO<sub>2</sub><span class="thinspace"></span>+<span class="thinspace"></span>NO dominates). This work systematically explores the chemistry in OFRs with high initial NO. Using box modeling, we investigate the interconversion of N-containing species and the uncertainties due to kinetic parameters. Simple initial injection of NO in OFR185 can result in more RO<sub>2</sub> reacted with NO than with HO<sub>2</sub> and minor non-tropospheric photolysis, but only under a very narrow set of conditions (high water mixing ratio, low UV intensity, low external OH reactivity (OHR<sub>ext</sub>), and initial NO concentration (NO<sup>in</sup>) of tens to hundreds of ppb) that account for a very small fraction of the input parameter space. These conditions are generally far away from experimental conditions of published OFR studies with high initial NO. In particular, studies of aerosol formation from vehicle emissions in OFRs often used OHR<sub>ext</sub> and NO<sup>in</sup> several orders of magnitude higher. Due to extremely high OHR<sub>ext</sub> and NO<sup>in</sup>, some studies may have resulted in substantial non-tropospheric photolysis, strong delay to RO<sub>2</sub> chemistry due to peroxynitrate formation, VOC reactions with NO<sub>3</sub> dominating over those with OH, and faster reactions of OH&amp;ndash;aromatic adducts with NO<sub>2</sub> than those with O<sub>2</sub>, all of which are irrelevant to ambient VOC photooxidation chemistry. Some of the negative effects are the worst for alkene and aromatic precursors. To avoid undesired chemistry, vehicle emissions generally need to be diluted by a factor of &amp;gt;<span class="thinspace"></span>100 before being injected into an OFR. However, sufficiently diluted vehicle emissions generally do not lead to high-NO chemistry in OFRs but are rather dominated by the low-NO RO<sub>2</sub><span class="thinspace"></span>+<span class="thinspace"></span>HO<sub>2</sub> pathway. To ensure high-NO conditions without substantial atmospherically irrelevant chemistry in a more controlled fashion, new techniques are needed.</p>
doi:10.5194/acp-17-11991-2017 fatcat:kxz5ijwxongdppbw2lew7ce66i