Contrasting pathways of spring black carbon pollution in Northeastern Sichuan basin: Biomass burning transport versus local fossil fuel accumulation

Liangping District, located in the northeastern part of the Sichuan Basin (SCB), is a representative site for studying black carbon (BC) aerosol pollution within the basin. Field measurements of BC mass concentration were conducted continuously from March to May 2021 using an Aethalometer (AE33). The mean BC mass concentration was determined to be 2.4 μg m −3 . The aerosol optical properties were derived through application of the Aethalometer model, yielding an average absorption coefficient of 36.7 ± 20.1 Mm −1 at 520 nm with a corresponding absorption Ångström exponent (AAE) of 1.40 ± 0.12. During the observation period, two six-day pollution events (designated as Event Mar and Event May) were recorded, with PM 2.5 concentrations exceeding the first-class ambient air quality standards threshold of 35 μg m −3 . The maximum AAE was 1.70 during Event Mar compared to 1.43 during Event May. The elevated AAE value in Event Mar suggested increased biomass burning constituted a significant pollution source, whereas the lower AAE during Event May reflected a mixed source dominated by fossil fuel combustion. Non-parametric wind regression (NWR) analysis revealed that high BC concentrations were spatially concentrated in the southwestern region during Event Mar, whereas during Event May, elevated BC levels originated from multiple directions. Concentration-weighted trajectory (CWT) analysis indicated that during Event Mar, long-range transport played a more dominant role, whereas local emissions were the primary pollution source during Event May. The study of the two pollution events identified the primary sources of BC, providing a method for analyzing the characteristics of aerosol pollution in SCB and offering a basis for developing pollution control measures. • Non-BC components accounted for 31.7 % of the total absorption coefficient at 370 nm. • The pollution event that occurred in March was driven by long-range transported biomass burning with a peak AAE of 1.70. • The pollution event that occurred in May was dominated by local fossil fuel emissions with a stable AAE of 1.40. • BC emissions from fossil fuel combustion are almost twice as large as those from biomass combustion.