Numerical study of the amplification effects of cold-front passage on air pollution over the North China Plain

Cold-front systems provide scavenging mechanisms for air pollution in the North China Plain (NCP), but the transport of pollutants with cold fronts aggravates air quality downstream. The impact of cold fronts on PM_2.5 concentrations over the NCP during 8-14 December 2019 was studied using the WRF-Chem model. Results indicate that cold fronts directly influence PM_2.5 concentration through regional transport of pollutants and adjustment of meteorological systems, and they indirectly affect air quality by influencing aerosol-radiation interaction. Pollutants affecting downstream areas may be transported to altitudes of ~3 km along the frontal surface, with near-surface PM_2.5 concentrations increasing temporarily at up to 15 μg·m^-3·h^-1 behind the surface frontal line owing to the inversion layer triggered by the oblique frontal surface. The transport process plays an essential role in affecting air pollution levels, more than vertical mixing and chemical reaction processes. Changes in the meteorological system (eastward shift of the high-pressure center) occurring with the passage of cold fronts facilitate the accumulation and transport of pollutants in the NCP, reducing air quality in the western and northern NCP. Cold fronts may also indirectly exacerbate near-surface pollutant diffusion conditions by affecting solar radiation incidence, with a reduction of the 2-m temperature by as much as 1 °C, increasing near-surface (<1 and 0.5 km agl on the pre- and post-frontal sides, respectively) PM_2.5 concentrations by up to 40 μg·m^-3, while reducing upper-layer concentrations by up to 30 μg·m^-3. This study emphasizes the amplification effect of cold fronts on air pollution, with inter-regional cooperation being essential in improving air quality in the NCP region.