Vertical Distributions Of Black Carbon Aerosol Over Yangtze River Delta In Winter And Autumn

A systematic study of BC vertical profiles and PBL structures over a rural area and a suburb of the Yangtze River Delta was conducted.Relevant campaigns lasted from December 14,2016to January 4,2017 and from October 19,2017 to November 15,2017.58 and 81vertical profiles were measured respectively,including PM_2.5,m_BC and a full set of meteorological parameters.Four profile types were categorized in Shouxian:I:uniform vertical distributions（38%）,II:higher values at lower altitudes（29%）,III:bimodal distributions with high values near the ground and at higher altitudes（17%）,and IV:unimodal distributions with high values at higher altitudes（11%）.A further analysis confirmed that all types were mainly influenced by the PBL diurnal evolution and local emissions,while types III and IV were strongly associated with the temperature inversions at low altitudes.The diurnal variations of the BC vertical profiles mainly followed the evolution of the PBL.In the early morning,the average m_BC within the PBL(M_BL,BC)increased significantly,reaching the highest level in the diurnal cycles.The pollutants were confined to a thin layer about 0.2 km above the ground,which contributed to the smoke produced by local residential biomass burning.Around noon,the accumulated BC in the layer was diluted as a result of the development of the PBL.The height of the PBL(H_PBL)reached its maximum in the afternoon,with an average of 0.65 km,while M_BL,BC dropped to its minimum.As evening approached,the BC produced by local residential biomass burning gradually accumulated near the ground and linearly declined along the standardized height（H_S）within the nocturnal boundary layer（NBL）.There were large differences in the BC concentration within and above the PBL both in the daytime and at night.Three profile types were categorized in northern Suburb of Nanjing:I:higher values at higher altitudes（46%）,II:bimodal distributions with high values near the ground and at higher altitudes（31%）,III:unimodal distributions with high values at higher altitudes（14%）.Pollutants emitted from the surrounding elevated sources were transmitted to the observation station under the influence of easterly winds,resulting in the occurrence of high-concentration stratification.Similar characteristics of diurnal variations of the BC vertical profiles were observed in both stations.The differences between the two lies in,the emission from the observation station in the northern Suburb of Nanjing has little contribution to the pollutants in PBL,and the daily variation range of the near-ground BC concentration was small;under the influence of the surrounding elevated sources,the frequency of high concentration of stratification was higher in Nanjing station.