Temporal variations in black carbon in the Yangtze River Delta of China: Long-term observations and trends under different pollution processes

Abstract

As an important absorptive component of aerosols in the atmosphere, black carbon (BC) plays a key role in climatic processes. Continuous BC measurements in the atmosphere of Nanjing, China, were performed from 2015 to 2022. By combining data from PM_2.5, PM₁₀, SO₂, CO, NO₂, O₃ and meteorological variables, the temporal variations in BC during different pollution events were determined in this study. The annual mean BC concentration increased from 2015 to 2018 (0.80 ± 0.05 μg/(m³ a)) and decreased from 2019 to 2022 (0.30 ± 0.001 μg/(m³ a)). The daily variations in BC concentrations during different processes (haze, dust, ozone, fireworks and fog) in Nanjing showed different characteristics. The BC concentration increased the most during haze events (50 %–100 %), while it changed little during fog events, so BC can be used to distinguish between these two processes. During O₃ pollution events, the BC concentration increased by 6 %–44 %. During dust pollution events, BC concentration decreased owing to the high wind speed. Besides, BC wasn't a major pollutant originating from Nanjing fireworks. During haze and O₃ pollution events, BC concentrations tended to increase (0.45 μg/m³–1.51 μg/m³) as the pollution levels increase. It's worth noting that BC concentrations were higher in autumn (4.85 μg/m³) than other seasons during moderate haze events because of enhanced long-range BC transport. Furthermore, during O₃ pollution events, high BC concentrations exacerbated BC-boundary layer (BL) interactions and sustained high O₃ concentrations. Bimodal distribution of haze (O₃) pollution at all levels, but the nocturnal BC concentration peak weakened during heavy haze events.