Sources and trends of black carbon aerosol in the megacity of Nanjing, eastern China, after the China Clean Action Plan and Three-Year Action Plan

Abstract. Black carbon (BC) is an essential component of particulate matter (PM), with a significant impact on climate change. Few studies have investigated the long-term changes in BC and its sources, particularly considering primary emissions of BC, which is crucial for developing effective mitigation strategies. Here, 3-year BC observations (2019–2021) are reported in Nanjing, a polluted city in the Yangtze River Delta (YRD) region, eastern China. The results revealed that the average BC concentration was 2.5 ± 1.6 µg m−3, peaking in winter, with approximately 80 % attributed to liquid fuel combustion. Based on 3-year monitoring data, the random forest (RF) algorithm was employed to reconstruct BC concentrations in Nanjing from 2014 to 2021. Source apportionment was conducted on the reconstructed time series, which revealed a significant decrease (p < 0.05) in BC levels over the 8-year period, primarily due to reduced emissions from liquid fuels. Compared to the earlier control policy period (P1: 2013–2017), BC concentrations declined more steeply after 2018 (P2) due to reduced solid fuel burning. The seasonal analysis indicated significant reductions (p < 0.05) in BC, BCliquid (black carbon from liquid fuel combustion) and BCsolid (black carbon from solid fuel combustion) during winter, with BCliquid accounting for 77 % of the reduction. Overall, emission reduction was the dominant factor in lowering BC levels, contributing between 62 % and 86 %, though meteorological conditions played an increasingly important role in P2, particularly for BC and BCliquid. Our results demonstrate that targeted control measures for liquid fuel combustion are necessary, as it is a major driver of BC reduction, and highlight the non-negligible influence of meteorological factors on long-term BC variations.