Characteristics of aerosol within the nocturnal residual layer and its effects on surface PM2.5 over China

Aerosol loading within the surface layer is an important aspect in studying air quality. However, a comprehensive understanding of the characteristics and effects of aerosol in the residual layer (RL) over China is yet to be achieved. In this study, the characteristics of aerosol in the RL and its effects on the surface PM2.5 over China are investigated using ten-year Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data from January 2008 to December 2017. Our results show that the high aerosol optical depth in the RL (RAOD) is generally found in the north-central and southwestern parts, whereas low values are observed in the northeastern and northwestern areas. The RAOD accounts for >50% of the total columnar aerosols in most regions, and the main components of aerosols in RL are polluted continental aerosol and polluted dust. A decreasing trend of RAOD was observed in the Yellow River Delta (YRD), North China Plain (NCP), Central China (CC) and Sichuan Basin (SCB), which are related to the decreasing emission of aerosols. Moreover, the effects of meteorological parameters on the RAOD were investigated. The relative humidity and the latent heat flux has a positive correlation with the RAOD in most areas, while the wind speed and the sensible heat flux has a negative correlation with the RAOD. The relationship between the RAOD and the following daytime surface particulate matters (PM2.5) were assessed. The correlation coefficients between RAOD and following daytime surface PM2.5 in YRD, NCP, CC and SCB were 0.29, 0.34, 0.4 and 0.39, respectively. The results proved that a high RAOD can promote the concentration of surface PM2.5. These findings are significant to the improvement of our understanding of the effects of aerosols in the RL on air quality.