Source-Dependent Aromatic Structure Helps Explain Variability in the Light Absorption of Freshly Emitted Black Carbon

Black carbon (BC) is a refractory, light-absorbing carbonaceous aerosol, yet how structural heterogeneity among combustion sources drives variability in light absorption remains poorly constrained. Here, we investigate the relationship between aromatic structure and light absorption of freshly emitted BC from major combustion sources by combining benzene polycarboxylic acid analysis with filter-based optical attenuation measurements. We observe substantial source-dependent differences in aromatic condensation, with estimated aromatic cluster sizes increasing from residential coal combustion to biomass burning, on-road diesel vehicles, and non-road engines. These differences are associated with combustion-related conditions, including modified combustion efficiency and engine operating state. Aromatic condensation was positively associated with (Formula presented.), and the association persisted after adjustment for source category, suggesting that differences in aromatic structure may partially explain source-dependent variability in freshly emitted BC light absorption.These results support a chemical continuum description of BC and highlight the need to account for source-related structural heterogeneity when interpreting BC optical properties and radiative effects.