低NOx浓度条件下挥发性有机物的大气氧化机理研究

The oxidation of volatile organic compounds (VOCs) is one of the main reasons for the regional high ozone and PM2.5 concentrations in the atmosphere. The atmospheric oxidation mechanism of VOCs is therefore essential in developing reasonable strategy for air quality control. Although there have been plenty of studies on the atmospheric oxidation mechanism of VOCs and a few models on the oxidation mechanism of VOCs have also been developed, deficiency persists in this model, i.e., the big gap between model predictions and field measurements on the ambient hydroxyl radical concentration under low-NOx conditions. We believe, one of the important reasons is that the isomerization reactions of peroxyl raidcals (RO2) were missing in these models. In the existing models, RO2 would react only with NOx and HO2/R'O2. However, our preliminary studies show that the isomerizations through H-shift are fast enough to compete with the bimolecular reactions with NOx and HO2/R'O2 for some RO2 radicals, especially when NOx concentrations are low. The H-shift will be more important with the increased temperature; but the widely used VOCs oxidation models such as SAPRC and MCM completely ignored the isomerization reactions of RO2 radicals and the ensuing reactions, and the literature report on the isomerization of RO2 under the atmospheric conditions was rather limited as well. We propose to study the atmospheric oxidation mechanism of some oxygenated VOCs and some conjugated dialkenes. We plan to examine the isomerization reactions of RO2 radicals formed during the atmospheric oxidation of these VOCs, computing their reaction rates and the temperature effect, and to improve their atmospheric oxidation mechanism. The mechanism for each VOC species will be evaluated and incorporated into the existing models, and a new mechanism model for the atmospheric oxidation of VOCs will be developed. The new model will be suitable for predicting the effect of VOCs on the air quality under different NOx concentrations and temperature conditions.

挥发性有机物(VOCs)的大气氧化是引起区域性大气臭氧超标和PM重度污染的关键因素之一。对VOCs的大气氧化有很多研究，也建立了诸多机理模型，但仍存在很多缺陷，导致如低NOx条件下模型预测和大气实测之间的巨大差异。我们认为机理模型中有机过氧自由基(RO2)异构化反应的缺失是可能的重要原因之一。现有的机理模型中RO2仅与NOx和HO2/R'O2反应；但我们的初步研究表明，一些RO2的H-转移异构化反应可与这些反应相竞争，低NOx时更是如此，且随温度升高变得更重要。但广泛使用的VOCs大气氧化机理模型如SAPRC和MCM完全忽略了RO2的异构化反应，文献中也极少有报导。本项目拟研究几类含氧VOCs和含共轭烯烃的VOCs大气氧化中RO2的异构化及温度的影响，完善其大气氧化机理。逐步修改现有的机理模型，发展新的VOCs大气氧化机理模型，以预测不同NOx浓度和温度条件下VOCs对空气质量的影响。

挥发性有机物(VOCs)的大气氧化是引起区域性大气臭氧超标和PM重度污染的关键因素之一。对VOCs的大气氧化有很多研究，也建立了诸多机理模型，但仍存在很多缺陷，导致如低NOx条件下模型预测和大气实测之间的巨大差异。.受项目资助，我们研究了几类VOCs和含共轭烯烃的VOCs大气氧化中RO2的异构化及温度的影响，完善其大气氧化机理。我们研究的VOCs别包括：芳香化合物、异戊二烯、萜烯、呋喃类化合物、醚与二甲硫醚、和醛酮中的RCO自由基等，其中芳香化合物和异戊二烯机理进行了实验验证。研究发现，在取代苯、异戊二烯、萜烯、醚和二甲硫醚的大气氧化过程中，中间体RO2自由基均有较为快速的单分子氢转移过程。这些化合物中，芳香苯化合物是典型的人为排放VOC污染物，异戊二烯和萜烯是典型的生物源排放VOCs，呋喃类化合物是可再生燃料和化工原料的典型代表，而二甲硫醚则是海洋排放的最大量含硫有机物。.在取代苯、异戊二烯、和萜烯中，快速的RO2单分子氢转移过程生成高度氧化的多官能团产物(Highly Oxidized Molecules,HOMs)。这些化合物具有极低的挥发性，是对大气二次气溶胶具有重要的贡献。萜烯氧化中HOMs产物在NO浓度较低的低污染地区可能对大气新颗粒生成也有较大的影响。在取代苯氧化中，具有快速分子内氢转移的过氧自由基为双环过氧自由基，在异戊二烯中是4-ZH自由基，在萜烯中则为Criegee中间体分解得到的类乙烯基氧自由基与O2结合得到的过氧自由基。对这些反应体系，我们首先采用理论计算预测到这些过氧自由基的单分子反应过程；随后，采用在线化学电离质谱法测得了取代苯和异戊二烯氧化中的过氧自由基中间体和HOMs产物。理论和实验相结合，证明了过氧自由基单分子反应在这些VOCs大气氧化中的重要性。.在二甲硫醚的研究中，我们发现CH3SCH2OO自由基可能具有快速的单分子异构过程，得到CH2SCH2OOH自由基。后者与O2结合后得到OOCH2SCH2OOH，再次发生氢原子转移异构，得到HOOCH2SCHOOH，并快速分解为HC(O)SCH2OOH + OH 自由基。.本项目研究有力地证明了RO2单分子异构在VOCs大气氧化中的重要性，包括HOMs产物的生成、VOCs的自氧化、和OH自由基的再生等。这些研究对于理解和预测大气氧化性以及臭氧和颗粒相的生成有着重要的意义。