Seasonal Variations and Potential Sources of Black Carbon at a Mountain Site in the Central Yunnan Plateau, China

To investigate the factors influencing air quality over the Central Yunnan Plateau (CYP), China, black carbon (BC) concentrations were continuously monitored from February 2023 to January 2024 at the Yunya Atmospheric Environment Research Observatory (YAERO), a regional background site located on the slopes of the Woyun Mountains. BC levels in pre-monsoon (2.35 ± 1.77 µg/m³) were 3–4 times higher than those observed in monsoon (0.54 ± 0.28 µg/m³), post-monsoon (0.65 ± 0.36 µg/m³), and winter (0.79 ± 0.44 µg/m³) seasons. Significant differences were also observed in ΔBC/ΔCO ratios, biomass burning contributions, and BC/PM_2.5 ratios between pre-monsoon and the other three seasons. Concentration-weighted trajectory analysis showed that the highest BC emissions occurred in pre-monsoon, with potential source regions located in Southeast Asia. In contrast, emissions in the other three seasons were much lower, with source regions located within China. The seasonal average biomass burning contributions exceeded 50% in all four seasons, highlighting biomass burning as the dominant BC source over the CYP. Air mass back-trajectory analysis and fire spot imagery revealed that high biomass burning contributions during the pre-monsoon night-time were mainly attributed to cross-boundary transport of crop residue burning from Southeast Asia, with peak contributions occurring between 04:00 and 06:00 LST. These cross-boundary biomass burning inputs explained the higher BC concentrations in pre-monsoon compared to the other seasons. Their contributions to BC concentrations were quantified and discussed in this study. These findings emphasise significant impact of biomass burning in Southeast Asia on air quality over the CYP in pre-monsoon.