成都市及周边地区严重臭氧污染过程成因分析

结合天气形势，地面观测资料和WRF-CMAQ模式，分析了2017年7月8~15日成都市一次罕见持续O₃污染过程的特征及成因，量化了各个物理化学过程对此次污染过程的相对贡献，并通过敏感性实验分析了四川盆地内O₃及其前体物的区域传输和本地光化学反应对此次污染过程的影响.结果表明，此次O₃持续污染过程主要是因为四川盆地内盛行偏东风，导致盆地东部城市群的O₃及其前体物经区域输送到成都及周边地区，加之成都市出现小风、气温升高等气象条件进而形成，属于典型的传输性爆发污染.持续污染形成的主要物理化学机制体现为日间气相化学过程贡献为稳定的正值，加之输送过程贡献出现爆发式升高，进而导致近地面O₃小时净增量迅速上升且高达50μg/（m³·h），随之O₃浓度迅速响应，产生爆发式增长.此外，敏感性实验结果显示此次成都市O₃持续污染的形成受区域输送影响较受本地光化学反应影响更为明显.O₃污染爆发前上游地区高浓度O₃及其前体物沿流场输送并在成都及周边地区不断积累，导致日间O₃浓度不断升高.

Characteristics and formation mechanism of a severe O3 episode in Chengdu and surrounding areas

Based on the synoptic pattern, ground observation data and WRF-CMAQ model, this study analyzed the characteristics of a sustained O₃ episode in Chengdu and surrounding areas from July 8-15, 2017. And the physical and chemical processes of this episode was quantified. Sensitivity experiments were used to calculate the relative contribution of regional transport and local photochemical reactions on this episode. The results showed that the sustained O₃ episode was mainly caused by the prevailing easterly wind in the Sichuan Basin, which leaded to the O₃ and its precursors in the eastern urban agglomerations being transported to Chengdu and surrounding areas. Unfavorable meteorological conditions, such as high temperature and low wind speed, were favorable for the formation of this O₃ episode. The process analysis indicated that the contribution of the gas phase chemical reaction was a positive source of O₃ in the daytime, and the contribution of the transport process was explosively increased during severe O₃ episode, which leaded to a rapid increase (up to 50μg/(m³·h)) in the net O₃near the ground. In addition, the sensitivity experiments showed that this episode was dominated by regional transport rather than the local photochemical reaction. Before the outbreak of O₃ concentration, high concentrations of O₃ and its precursors were transported along the wind field and accumulated in Chengdu and surrounding areas, resulting in enhanced O₃ formation during daytime.