甲苯和间二甲苯在近实际大气条件下的光氧化机制研究

Secondary organic aerosol (SOA) was formed from photooxidation of atmospheric volatile organic compounds. Research on SOA in the atmosphere has played an important role in the field of atmospheric chemistry because of its impacts on the atmospheric visibility, climate changes and health effects. The chemistry and partitioning processes involved in SOA formation are complex and dependent on a variety of atmospherically conditions such as the species and concetrations of particle seed and VOC etc.. Toluene and m-xylene are reported to be among the most abundant compounds in the urban volatile hydrocarbon mixture and have high hydroxyl radical (OH) rate coefficients; and a lot of mechanism study on their photooxidation mechanism were conducted in recent years due to the development of detecting instruments. However, the current OH initialed toluene and m-xylene oxidation mechanism were usually conducted under high levels of VOCs (typically 10 times more than the real air pollution levels) and low concentrations (or without) of particle seed, it's difficult to know whether these conclusions could be used to model the SOA formation under real pollution conditions. Here in this study，the OH-initiated photooxidation of toluene and m-xylene that near the same level of atmospheric were conducted in the 56m3 outdoor smog chamber. Experiments were performed under NOx and high concentrations of seed aerosols (~100μg m-3) that also approaching the real air pollution conditions of Beijing city. By using the various detecting instruments, the kinetic of toluene and m-xylene, the size distribution and the possible components of the formed SOA were investigated.In addition, the photooxidation mechanism of toluene and m-xylene were discussed by combining the MCM3.1.

挥发性有机物（VOCs）在大气中光氧化产生的二次有机气溶胶（SOA）对区域乃至全球大气环境具有显著影响。然而，由于受研究手段及检测技术限制，已有的研究大多采用较高初始VOCs浓度及单一体系进行模拟研究，其结果已被证明很难在实际大气中得以应用。中国环境科学研究院率先在国内建成的大型室外光化学模拟舱配备了多种高灵敏度现代仪器设备，可满足近实际大气环境的模拟及检测。为此，本项目拟将以城市大气中污染浓度较高且反应活性较强的甲苯和二甲苯为研究对象，利用该模拟舱,在接近北京实际污染状况下（甲苯和二甲苯为ppbv量级，(NH4)2SO4种子气溶胶浓度约100 ug m-3）进行模拟研究，采用O3和HONO的光解产生OH自由基，引发甲苯和二甲苯氧化，根据甲苯和二甲苯的消耗量以及SOA生成量，获得甲苯和二甲苯在近实际大气条件下的SOA产率，并结合化学模型MCM3.1，认识其在复合反应体系中的光化学反应机制

采用大型室外烟雾箱研究典型苯系物的光化学反应是目前国际和国内的研究热点和难点，一方面由于检测手段的限制而导致的苯系物的光化学反应产物还没有完全认识，另一方面是因为采用较高初始VOCs浓度及单一体系的已有研究结果已被证明很难在实际大气中得以应用。因此本项目首先调研了典型城区大气环境中VOCs的浓度变化特征和不同影响因素（雾霾和阅兵期间大气污染控制对策）对VOCs及其臭氧和SOA生成的影响；然后选择了接近实际大气污染程度的VOCs和NOx浓度，进行了甲苯和1,3,5-三甲苯的大型室外光化学烟雾箱模拟，研究了接近实际大气浓度的反应物和生成的O3和SOA随时间的变化规律，获得SOA的粒径分布、浓度变化特征及产率，并尝试采用HPLC-MS/MS的方法对膜采样获得的SOA组分进行了离线定量检测；最后运行化学模型MCM3.1，为探讨在接近真实大气浓度条件下的甲苯与OH自由基的光化学反应机制奠定了理论基础。本项目的主要突破为：第一，通过分析外场观测数据，发现苯系物的浓度冬季最高（13.1 μg m-3），夏季最低（8.6 μg m-3），冬季和夏季各BTEX化合物雾霾天的浓度是非雾霾天的1.9-5.7倍和1.0-3.0倍。乙苯/间对二甲苯的值均是雾霾天大于非雾霾天，表明了雾霾天较高的化学反应活性。第二，烟雾箱表征实验结果表明烟雾箱性能良好，用该大型室外烟雾箱获得的实验结果是可信的。烟雾箱的混合时间小于180s，反应器本底NMHCs和NOx的浓度均小于1ppb，夏季光辐射最强的时候最大臭氧浓度小于30ppb，O3壁损失速率常数为(3.19±0.39)×10-6s-1，NO2壁损失速率常数为(7.0±0.39)×10-6s-1。第三，固定甲苯的初始浓度，调节NOx的浓度，使两者的比值分别为0.280，1.110和2.850，结果表明，当两者比值为1.10时，生成O3浓度最大(271ppb),NO过量条件下基本没有SOA形成，VOCs过量时SOA的生成率最高（2.31%）。第四，在均三甲苯与NOx浓度大于1的条件下，O3的形成属于NOx敏感，降低VOCs的浓度，反而会使O3的浓度升高，六次实验SOA的产率小于1.1%。相同VOCs/NOx比值条件下，均三甲苯生成的O3浓度大于甲苯。