The radiative forcing of PM_2.5 heavy pollution, its influencing factors and importance to precipitation during 2014–2023 in the Bohai Rim, China

Abstract. There were PM_2.5 heavy pollution events in the Bohai Rim regions in China over the past decade, which can significantly affect radiative forcing (RF). However, the characteristics and influencing factors of RF on heavy pollution days, and its relative importance to precipitation remain unclear. This work combined ground-based and satellite observations and reanalysis data to investigate the RF characteristics of regional PM_2.5 heavy pollution in the Bohai Rim regions during the fall and winter of 2014–2023. Additionally, the impact of meteorological vertical profiles on surface PM_2.5 and pollution RF, and the importance of various factors to pollution RF and precipitation, were explored based on machine learning algorithms. The results showed that the RF on PM_2.5 regional heavy pollution days can be up to approximately -70 Wm^-2 at the surface, ±8 Wm^-2 at top of atmosphere (TOA), and +80 Wm^-2 in the atmosphere in clear-sky, with lower absolute values in all-sky. Low- to medium-altitude inversions of temperature (T) profiles in the boundary layer favored higher surface PM_2.5 concentration, whereas isothermal stratification and medium- to high-altitude inversions corresponded to higher surface RF. Lower horizontal speeds and upward motion at low levels can induce higher surface PM_2.5 and surface RF. Surface PM_2.5 was the most important factor to surface and atmosphere RF in clear-sky, but V wind in high level (500 hPa) in all-sky. Moreover, pollution RFs in all-sky were as important as vertical winds to the total precipitation. Notably, there was considerable regional heterogeneity in the important factors affecting the RF and precipitation in the Bohai Rim regions.