Source apportionment and driving mechanisms of black carbon in a mountainous megacity: insights from urban–suburban observations in Chongqing, China

Abstract

Black carbon, a strongly light-absorbing aerosol from incomplete combustion, has raised global concern due to its substantial impacts on climate forcing, air quality degradation, and public health risks. This study investigates the distribution and sources of equivalent BC (eBC) across urban–suburban sites in Chongqing, a mountainous megacity in southwestern China, based on year-long multi-wavelength Aethalometer (AE33) observations. Four sites were deliberately selected to represent distinct source regimes—port, airport, rail freight, and dense urban traffic—in a spatial source separation design. Annual mean eBC concentrations ranged from 2.4 ± 1.3 µg m⁻³ at the urban site to 2.8 ± 1.9 µg m⁻³ in suburban areas. Using absorption Ångström exponent diagnostics, eBC was apportioned into traffic-related (eBC_liquid) and biomass/coal-burning (eBC_solid) components. Results showed distinct spatiotemporal patterns between the two components: eBC_liquid dominated urban area (up to 85%) and was significantly elevated during weekdays (p < 0.0001), while eBC_solid exhibited a pronounced wintertime enhancement in suburban areas, accounting for nearly 40% of total eBC, indicating the influence of seasonal solid fuel combustion. Seasonally resolved correlation analysis further revealed that source–tracer relationships, such as the eBC_liquid–AAE correlation at the port site, shifted markedly with season, capturing the transition from shipping-dominated emissions in summer to mixed solid fuel influences in winter. Notably, riverine freight activity during the flood season contributed to episodic increases in eBC_liquid. Local meteorological factors, especially wind and temperature, substantially modulated eBC accumulation and dispersion. Strong correlations with NO_x, SO₂, and NMHCs further improved the source attribution. These findings underscore the heterogeneous behaviors of eBC components under complex terrain–meteorology interactions and provide region-specific evidence for emission control. The results suggest that strengthening traffic management in urban cores and promoting clean energy substitution in suburban areas are essential to reduce BC pollution and improve air quality in mountainous megacities.