Effectiveness of emission control in sensitive emission regions associated with local atmospheric circulation in O-3 pollution reduction: A case study in the Beijing-Tianjin-Hebei region

Currently, mitigation of persistent severe O3 pollution is the key for further improving the air quality in the Beijing-Tianjin-Hebei (BTH) region, China. The source contributions and transboundary transport of O3 during pollution episodes are closely related to weather conditions. The effective reduction in emissions to control O3 pollution under typical local atmospheric circulation conditions highly correlated with continuous pollution processes in the North China Plain (NCP) is rarely reported. In this study, a sensitive area in the BTH region was identified based on the analysis of the diurnal evolution features of local atmospheric circulation and the distribution characteristics of anthropogenic emissions in the NCP region. Multiple emission reduction scenarios were designed to explore the impacts of hotspot anthropogenic emissions in this sensitive area on O3 pollution formation during a regional prolonged O3 pollution episode from 30 August to 6 September 2019. Compared to large-scale anthropogenic emission control across the NCP region, a smaller emission reduction and much larger O3DM (the daytime mean ozone concentration, from 10 a.m. to 5pm) reduction were obtained through emission control in the sensitive area. The results also indicated that anthropogenic emission hotspots in this sensitive area play a decisive role in O3 photochemical formation chemistry across the whole BTH region with high emissions. The actual impact of hotspot anthropogenic emissions originating in the BTH region was not limited to its locality but occurred within a certain wide range, with a relatively significant contribution to O3DM ranging from approximately 20-80 mu g/m3 in most of the plain area, accounting for approximately 15%~40% of O3DM. Among all anthropogenic sources, the industry is the most important contributor to O3 formation, and traffic is the second most important contributor. In terms of regional emission control, it is suggested that under extremely unfavorable weather conditions, the BTH government should focus on local emission reduction, particularly in the industrial source category, and it should simultaneously consider the synergistic impact between the industry and traffic to maximize the effectiveness of control policies to achieve long-term ozone reduction.