10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Organic Peroxy Chemistry in Oxidation Flow Reactors and Chambers and Their Atmospheric Relevance
ZHE PENG, Julia Lee-Taylor, John Orlando, Geoffrey Tyndall, Jose-Luis Jimenez, University of Colorado-Boulder
Abstract Number: 140 Working Group: Oxidation Flow Reactor: Development, Characterization, and Application to Aerosols
Abstract Oxidation flow reactors (OFR) are a promising alternative to environmental chambers for investigating atmospheric oxidation processes and secondary aerosol formation. Because of their portability, short residence times, and ability to reach high photochemical ages, they have been increasingly employed in both lab and field studies. However, questions have been raised about how representative the chemistry within OFRs is of that in the troposphere. We investigate the fates of organic peroxy radicals (RO2), which play a central role in atmospheric organic chemistry, in OFRs, environmental chambers, and under ambient conditions by chemical kinetic modeling. We simulate RO2 fate in OFRs over a wide range of conditions (both low-NO and high-NO) and compare the results with RO2 fates in key chamber and atmospheric conditions. For most types of RO2, their bimolecular fate is mainly RO2+HO2 and RO2+NO, similar to those in chambers and the atmospheric studies. For α,β-substituted primary (SPRO2) and acyl RO2 (aRO2), RO2+RO2 can be significant in RO2 fate. However, the relative importance of RO2+RO2 is sensitive to HOx recycling ratio of the reaction system. At our best estimate of HOx recycling ratio (~0.3), RO2+RO2 can be the most important bimolecular loss pathway of SPRO2 and aRO2, but still can generally cover the range of the relative importance of RO2+RO2 of SPRO2 and aRO2 in chambers and the atmosphere. This is can be explained by HO2 increased together with OH in OFRs. The OH-to-HO2 ratio in OFRs covers the ambient values (on the order of 0.01), but can be significantly higher (on the order of 0.1) at high water vapor concentrations, high UV settings, and low precursor concentrations. A low-NO OFR experiments under these conditions could lead to substantial relative contribution of recently proposed, but highly uncertain RO2+OH to RO2 fate, much higher than the highest ambient value (~10%). For aRO2, if they do not undergo very fast isomerization in OFRs, they would be substantially (even dominantly) consumed by NO2 as long as significant amounts of NO precursor(s) are injected into OFRs. RO2+NO2→RO2NO2 is not a sink of aRO2 in chambers and the atmosphere, but it is a sink in OFRs, where the thermal decomposition lifetimes of the product, peroxynitrates (hours), is longer than OFR residence times (minutes). Short residence times of OFRs also prevent RO2 isomerization in OFRs from proceeding to the same extent as in the atmosphere. Nevertheless, as long as the isomerization of an RO2 is important in the atmosphere, this isomerization is significant in OFRs, and far from negligible as some researchers have speculated. Overall, chambers generally can obtain atmospherically relevant RO2 fates; so can OFRs to a large extent. Most importantly, careful attention to OFR (and chamber) operating conditions needs to be paid to simulate specific RO2 fates, such as avoiding atmospherically irrelevant organic photolysis when increasing VOC concentrations to achieve higher RO2+RO2 and/or NO precursor concentrations to achieve higher RO2+NO.