Source apportionments of black carbon induced by local and regional transport in the atmospheric 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 emphases on model simulations, the characteristics and regional sources of black carbon (BC) aerosol profiles have been relatively understudied. 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 subregions over the Yangtze River Delta (YRD)) during a November 2017 pollution event in the YRD, China. In this event, the YRD mainly experienced 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 that region itself (70.6 %-98.2 %), with little intra- and inter-regional transport. During the uniform low-pressure period, highly variable contributions to the surface BC from intra-regional transport within the YRD (0.05 %-65.9 %) and inter-regional transport outside the YRD (mostly from Anhui (AH) to the west 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 atmospheric boundary layer (ABL). The inter-transport contributions increased extensively above the ABL. As a westerly trough crossing, a long-range inter-regional transport from South China (SCHN, 3.3 %) and the North China Plain (NCP, 2.7 %) was simulated above the ABL. We found that when the surface experienced similar stable weather conditions, the weather conditions in the upper air may have been quite different, resulting in significant differences in the regional transport of BC in the upper ABL. This study provides a reference for improving air quality from the local scale to the regional scale.