Vereecken L, Carlsson P T M, Novelli A, Bernard F, Brown S S, Cho C, Crowley J N, Fuchs H, Mellouki W, Reimer D, Shenolikar J, Tillmann R, Zhou L, Kiendler-Scharr A, Wahner A
Institute for Energy and Climate Research, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany.
Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE), CNRS (UPR 3021)/OSUC, 1C Avenue de la Recherche Scientifique, 45071 Orléans CEDEX 2, France.
Phys Chem Chem Phys. 2021 Mar 11;23(9):5496-5515. doi: 10.1039/d0cp06267g.
The initial stages of the nitrate radical (NO3) initiated oxidation of isoprene, in particular the fate of the peroxy (RO2) and alkoxy (RO) radicals, are examined by an extensive set of quantum chemical and theoretical kinetic calculations. It is shown that the oxidation mechanism is highly complex, and bears similarities to its OH-initiated oxidation mechanism as studied intensively over the last decade. The nascent nitrated RO2 radicals can interconvert by successive O2 addition/elimination reactions, and potentially have access to a wide range of unimolecular reactions with rate coefficients as high as 35 s-1; the contribution of this chemistry could not be ascertained experimentally. The chemistry of the alkoxy radicals derived from these peroxy radicals is affected by the nitrate moiety, and can lead to the formation of nitrated epoxy peroxy radicals in competition with isomerisation and decomposition channels that terminate the organic radical chain by NO2 elimination. The theoretical predictions are implemented in the FZJ-NO3-isoprene mechanism for NO3-initiated atmospheric oxidation of isoprene. The model predictions are compared against peroxy radical (RO2) and methyl vinyl ketone (MVK) measurements in a set of experiments on the isoprene + NO3 reaction system performed in the SAPHIR environmental chamber (IsopNO3 campaign). It is shown that the formation of NO2 from the peroxy radicals can prevent a large fraction of the peroxy radicals from being measured by the laser-induced fluorescence (ROxLIF) technique that relies on a quantitative conversion of peroxy radicals to hydroxyl radicals. Accounting for the relative conversion efficiency of RO2 species in the experiments, the agreement between observations and the theory-based FZJ-NO3-isoprene model predictions improves significantly. In addition, MVK formation in the NO3-initiated oxidation was found to be suppressed by the epoxidation of the unsaturated RO radical intermediates, allowing the model-predicted MVK concentrations to be in good agreement with the measurements. The FZJ-NO3-isoprene mechanism is compared against the MCM v3.3.1 and Wennberg et al. (2018) mechanisms.
通过一系列广泛的量子化学和理论动力学计算，研究了硝酸根自由基（NO₃）引发的异戊二烯氧化的初始阶段，特别是过氧自由基（RO₂）和烷氧自由基（RO）的归宿。结果表明，氧化机制高度复杂，与过去十年深入研究的OH引发的氧化机制有相似之处。新生的硝化RO₂自由基可以通过连续的O₂加成/消除反应相互转化，并可能参与一系列单分子反应，其速率系数高达35 s⁻¹；这种化学反应的贡献无法通过实验确定。这些过氧自由基衍生的烷氧自由基的化学性质受硝酸根部分的影响，并且在与通过NO₂消除终止有机自由基链的异构化和分解通道竞争时，会导致硝化环氧过氧自由基的形成。理论预测被应用于FZJ-NO₃-异戊二烯机制中，用于NO₃引发的大气中异戊二烯的氧化。在蓝宝石环境舱中进行的一组异戊二烯 + NO₃反应系统实验中，将模型预测结果与过氧自由基（RO₂）和甲基乙烯基酮（MVK）的测量结果进行了比较（异戊二烯硝酸盐实验）。结果表明，过氧自由基形成NO₂会阻止很大一部分过氧自由基被激光诱导荧光（ROxLIF）技术测量，该技术依赖于过氧自由基向羟基自由基的定量转化。考虑到实验中RO₂物种的相对转化效率，观测结果与基于理论的FZJ-NO₃-异戊二烯模型预测之间的一致性显著提高。此外，发现不饱和RO自由基中间体的环氧化抑制了NO₃引发氧化过程中MVK的形成，使得模型预测的MVK浓度与测量结果吻合良好。将FZJ-NO₃-异戊二烯机制与MCM v3.3.1和温伯格等人（2018年）的机制进行了比较。
