Sources and trends of Black Carbon Aerosol in a Megacity of Nanjing, East 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 the sources, particularly considering primary emissions of BC, which is crucial for developing effective mitigation strategies. Here, based on three-year observations (2019–2021), random forest (RF) algorithms were employed to reconstruct BC concentrations in Nanjing from 2014 to 2021. Source apportionment was conducted on the reconstructed data to investigate long-term trends of BC and its sources. The results showed that the three-year average BC concentration was 2.5±1.6 μg m^-3, peaking in winter, with approximately 80 % attributed to liquid fuel combustion. Notably, the reconstructed time series revealed a significant decrease (p < 0.05) in BC levels over the eight-year period, primarily due to reduced emissions from liquid fuels. The comparison between two control polices periods (P1:2014–2017 and P2:2018–2021) indicate that BC concentrations decline more steeply during S2 since significant (p < 0.05) reduction in biomass burning. The seasonal analysis showed significant reductions (p < 0.05) in BC, BC_liquid (black carbon from liquid fuel combustion) and BC_solid (black carbon from solid fuel combustion) during winter, with BC_liquid accounting for 77 % of the reduction. Overall, emission reduction was the dominant factor in reducing BC levels, contributing between 62 % and 86 %, as revealed by Kolmogorov-Zurbenko (KZ) filter. However, during P2, meteorological conditions played a more significant role, especially in reducing BC and BC_liquid, with an increase in their impact on BC_solid compared to P1. Our results demonstrated that target control measures for liquid fuel combustion are necessary, as liquid fuel combustion is a major driver of decreasing BC, especially in summer, while the influence of meteorological factors on BC variations cannot be overlooked.