Source Apportionments of Black Carbon Induced by Local and Regional Transport in the Boundary Layer of the Yangtze River Delta Under Stable Weather Conditions

Long-range transport (LRT) and local accumulation (LA) are key atmospheric physical processes affecting air pollution formation, and their impacts on surface air pollution have been extensively researched.Due to the lack of vertical observations and emphaseson model simulations, the characteristics and regional sources of black carbon (BC) aerosol profiles have been relatively under studied. In this study, the chemistry-coupled Weather Research and Forecasting model (WRF-Chem) with a BC source-tagging method was used to quantify BC source contributions (considering 18 geographical regions over east Asia, including 3 sub-regions over the Yangtze River Delta (YRD)) during a November 2017 pollution event in the YRD, China. In this event, the YRD was mainly covered by a uniform pressure field, stable weather and weak wind fields. During the uniform high-pressure period, the dominant contribution to surface BC in each sub-region was from itself respectively (70.6%-98.2%), with little intra- and inter-regional transport. During the uniform low-pressure period, highly varying contributions to surface BC from intra-regional transport within the YRD (0.05%-65.9%) and inter-regional transport outside the YRD (mostly from Anhui (AH), the western neighbour of the YRD, 0.37%-23.9%) were simulated. In the vertical direction, the dominant contributors were local YRD sources (73.8%-94.2%) below the boundary layer (BL). The inter-transport contributions increased extensively above the BL. As a westerly trough crossing, long-range inter-regional transport from South China (SCHN, 3.3%) and the North China Plain (NCP, 2.7%) were simulated above the BL. In addition, we determined that the BC peaks observed in the central BL over Nanjing (NJ) resulted from the transport of BC from northern Zhejiang (ZJN) and AH via dynamic lifting in the upper BL. This study visually quantified BC and its regional sources in 3 dimensions and provides a reference for improving air quality from the local scale to the regional scale.