Abstract:Due to the unique geographical location of the Beijing-Tianjin-Hebei Region, topography and mesoscale weather play a crucial role in the formation of regional air pollution in this region. A typical air pollution process was observed in the region from 29 to 30 December 2023. To explore the causes and deep formation mechanism for this pollution process, the influences of mesoscale weather and boundary layer structure on the intensification of heavy pollution under the warm front and special terrain background are studied on the basis of the numerical simulation of RMAPS-CHEM system, the PM2.5 concentration observation and the meteorological data. It is found that under the influence of North China topographic trough, the southwest wind not only transported the pollutants, but also brought the warm and humid air mass from south to north, resulting in the formation of an obvious low-level warm front on the southeast side of the Beijing-Tianjin-Hebei Plain. Synergistically, impacted by warm front and topographic thermal characteristics, the polluted cold air mass was driven by warm front in the southeast side and restricted by terrain in the northwest side, forming accumulation called cold pool near the surface of the plain area close to the mountains. The strong stable atmospheric condition of the temperature inversion in the cold pool, calm wind and extremely low boundary layer height led to the pollutants to be trapped in a limited vertical space. At the same time, the weak wind caused the stagnation of air mass and inhibited the horizontal transmission and diffusion of pollutants. These adverse factors ultimately resulted in the continuous accumulation of pollutant concentrations.