The role of radical chemistry in the product formation from nitrate radical initiated gas-phase oxidation of isoprene
Authors: Carlsson Philip T. M. , Vereecken Luc , Novelli Anna , Bernard François , Bohn Birger , Brown Steven S. , Cho Changmin , Crowley John , Hofzumahaus Andreas , Mellouki Abdelwahid , Reimer David , Rohrer Franz , Shenolikar Justin , Tillmann Ralf , Zhou Li , Kiendler-Scharr Astrid , Wahner Andreas , Fuchs Hendrik 
Published: March 2021

&lt;p&gt;Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO&lt;sub&gt;3&lt;/sub&gt; and its subsequent oxidation. Due to the production of NO&lt;sub&gt;3&lt;/sub&gt; from the reaction of NO&lt;sub&gt;2&lt;/sub&gt; with O&lt;sub&gt;3&lt;/sub&gt; as well as the formation of OH in subsequent reactions, the reactions of isoprene with O&lt;sub&gt;3&lt;/sub&gt; and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO&lt;sub&gt;2&lt;/sub&gt; to HO&lt;sub&gt;2&lt;/sub&gt; concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO&lt;sub&gt;2&lt;/sub&gt;. The reaction with HO&lt;sub&gt;2&lt;/sub&gt; or NO&lt;sub&gt;3&lt;/sub&gt; was found to be the main bimolecular loss process for the RO&lt;sub&gt;2&lt;/sub&gt; radicals under all conditions examined.&lt;/p&gt;&lt;p&gt;Yields of the first-generation isoprene oxygenated nitrates as well as the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) were determined by high resolution proton mass spectrometry using the Vocus PTR-TOF. The experimental time series of these products are compared to model calculations based on the MCM v3.3.1,&lt;sup&gt;1&lt;/sup&gt; the isoprene mechanism as published by Wennberg &lt;em&gt;et al.&lt;/em&gt;&lt;sup&gt;2&lt;/sup&gt; and the newly developed FZJ-NO&lt;sub&gt;3&lt;/sub&gt;-isoprene mechanism,&lt;sup&gt;3&lt;/sup&gt; which incorporates theory-based rate coefficients for a wide range of reactions.&lt;/p&gt;&lt;p&gt;Among other changes, the FZJ-NO&lt;sub&gt;3&lt;/sub&gt;-isoprene mechanism contains a novel fast oxidation route through the epoxidation of alkoxy radicals, originating from the formation of nitrated peroxy radicals. This inhibits the formation of MVK and MACR from the NO&lt;sub&gt;3&lt;/sub&gt;-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO&lt;sub&gt;3&lt;/sub&gt;-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, &lt;em&gt;Atmospheric Chem. Phys.&lt;/em&gt;, 2015, &lt;strong&gt;15&lt;/strong&gt;, 11433&amp;#8211;11459.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; P. O. Wennberg &lt;em&gt;at al.&lt;/em&gt;, Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, &lt;em&gt;Chem. Rev.&lt;/em&gt;, 2018, &lt;strong&gt;118&lt;/strong&gt;, 3337&amp;#8211;3390.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;3&lt;/sup&gt; L. Vereecken &lt;em&gt;et al.&lt;/em&gt;, Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, &lt;em&gt;Phys. Chem. Chem. Phys.&lt;/em&gt;, submitted.&lt;/p&gt;