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 PM2.5 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.