Intensified haze formation and meteorological feedback by complex terrain in the North China Plain region

The North China Plain has been suffering from severe haze pollution in the past few decades. In addition to rapid urbanization and intensive anthropogenic emissions, the complex terrain in this region greatly influences the atmospheric circulation, thereby weakening the ventilation of air pollutants. Nevertheless, the vertical responses of surface-emitted pollutants to the gradient of the terrain and its impact on secondary aerosol formation, as well as its interaction with boundary layer meteorology, have not yet been fully understood. Here, in-situ observations and satellite retrievals together with meteorology–chemistry coupled modeling are integrated to shed light on the terrain effects on atmospheric chemistry and its interaction with physical processes. It is found that the blocking effect of the terrain can result in haze accumulation over the plains and updrafts of pollution along the mountains. Long-term averaged PM_2.5 concentrations show that nearly 70% of plain stations exceeded 75 µg m^?3, compared with only 12% of high-altitude stations. In the highly polluted month of January 2018, the pollution layer was simulated to be elevated to an altitude of over 2 km. A higher oxidizing capacity in the upper boundary layer tends to accelerate secondary aerosol formation. Furthermore, the elevated pollution layer and the intense secondary formation due to the terrain effects jointly enhanced the aerosol–boundary layer interaction, weakening the vertical dispersion and further deteriorating the air pollution. This study highlights that there are intensified interactions between atmospheric chemistry and physics near complex terrain, which may substantially contribute to haze pollution in China.摘要华北平原地区冬季雾霾污染频发, 本研究结合地面观测, 卫星反演和大气动力-化学耦合模拟发现, 该地区复杂地形加剧了细颗粒物污染及其与大气边界层之间的相互作用. 一方面, 复杂地形导致污染在山麓平原积聚, 加强迎风坡上升气流. 在污染事件中, 污染层易被抬升至1–2公里高度, 高空较强的氧化能力利于二次气溶胶生成; 另一方面, 地形导致的污染层抬升和二次生成进一步加强气溶胶-边界层相互作用, 削弱垂直扩散并加剧近地面大气污染.