The role of radical chemistry in the product formation from nitrate radical initiated gas-phase oxidation of isoprene

<p>Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO₃ and its subsequent oxidation. Due to the production of NO₃ from the reaction of NO₂ with O₃ as well as the formation of OH in subsequent reactions, the reactions of isoprene with O₃ and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO₂ to HO₂ concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO₂. The reaction with HO₂ or NO₃ was found to be the main bimolecular loss process for the RO₂ radicals under all conditions examined.</p><p>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 v3.3.1,¹ the isoprene mechanism as published by Wennberg <em>et al.</em>² and the newly developed FZJ-NO₃-isoprene mechanism,³ which incorporates theory-based rate coefficients for a wide range of reactions.</p><p>Among other changes, the FZJ-NO₃-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 NO₃-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO₃-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates.</p><p>&#160;</p><p>¹ M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, <em>Atmospheric Chem. Phys.</em>, 2015, <strong>15</strong>, 11433&#8211;11459.</p><p>² P. O. Wennberg <em>at al.</em>, Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, <em>Chem. Rev.</em>, 2018, <strong>118</strong>, 3337&#8211;3390.<span>&#160;</span></p><p>³ L. Vereecken <em>et al.</em>, Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, <em>Phys. Chem. Chem. Phys.</em>, submitted.</p>