Impact of the Return Flow on Heavy Pollution in Winter over the Beijing–Tianjin–Hebei Region

In North China, the return flow (RF) refers to the airflow at the rear of an inshore high pressure, bringing southerly wind to the Beijing–Tianjin–Hebei (BTH) region when the high pressure pushes deeper from coast into the mainland. Many studies have pointed out the significant contribution of southerly anomalies to the transport and accumulation of pollutants in the BTH region. However, the relationship between RF and heavy pollution episodes (HPEs) in the BTH region requires more in-depth analysis, and this study will focus on this topic. By objectively identifying RFs and HPEs based on the ERA5 reanalysis data and observed hourly PM_2.5 concentration data during 9 winters of 2012–2020, it is found that almost two-thirds of the HPEs in the BTH region coincide with the occurrence of RFs. The northward transport of warm and humid air is stronger in the HPEs under RF conditions, whereas the sinking motion and the decrease in boundary layer height dominate the HPEs without any RF. We then classify the RFs into north and south types by a demarcation line of 32°N. Both types of RFs are featured with a zonal circulation pattern, stable atmosphere, and southerly airflow favoring the development of HPEs, but the south type RFs bring warmer and wetter air masses to the BTH region, forming a more stable and thicker inversion layer and causing more severe HPEs. With occurrences of the RF, low-level winds are observed to accelerate, and the ageostrophic wind components contribute mainly to this acceleration. During the presence of RFs, the kinetic energy generation at the high level decreases, and the weakened downward transport results in weak low-level northerly winds, weak turbulence, and a shallow boundary layer, thus hindering the diffusion of atmospheric pollutants in the BTH region.