Quantifying urban black carbon absorption enhancement under influence of open crop straw burning in Eastern China

Black carbon (BC) particles significantly impact the radiative balance of the atmosphere, introducing considerable uncertainties. This study addresses the limited quantitative observations of the atmospheric BC absorption enhancement effect, particularly influenced by diverse BC sources in urban environments. Online measurements of atmospheric BC absorption coefficient, mixing state, and aerosol chemical composition were conducted in a representative city within the Yangtze River Delta region, focusing on the repercussions of open crop straw burning. The results revealed a clear positive relationship between the coated BC size to BC core diameter (D_p/D_c) ratio and the mass concentration of secondary inorganics and more oxidized organics in aerosols from open straw burning. This emphasizes that secondary oxidation processes in biomass burning plumes could stimulate the abundant formation of internally mixed urban BC particles. Observations in open straw burning plumes exhibited an average BC absorption enhancement factor (E_{abs-870 nm}) at 870 nm of 1.52 ± 0.19. Discrepancies between observed and simulated E_abs values, based on the core-shell Mie theory model, diminished with an increase in the f₄₄ to f₆₀ ratio – the signal fractions of aerosol mass spectrometer m/z (m/z 44 and m/z 60) in total OA. This suggests that the morphology of BC in aged biomass burning plumes aligns more closely with the core-shell model. Furthermore, we provide a comprehensive summary of E_abs value levels from measurements across various regions and sources on a global scale.