Concurrent Formation of Low-Maturity EC and BrC in Biomass and Coal Burning: O-PAH as a Precursor

Black carbon (BC) significantly influences climate change through light absorption. Traditional emission inventories equate BC with elemental carbon (EC) and overlook the variability in its properties across sources, leading to uncertainties in climate predictions. This study shows that EC from solid fuel combustion contains substantial low-maturity EC (char), whose emissions increase alongside the light absorption of soluble organic carbon (OC) as the fuel aromaticity rises. Concurrently, the abundance of oxygenated polycyclic aromatic hydrocarbons (O-PAHs) in soluble OC also increases. This suggests that char and brown carbon (BrC) share similar formation pathways with O-PAHs as key precursors. Time-resolved analysis during combustion cycles revealed a significant positive correlation between O-PAHs, the light absorption of soluble OC, and char emissions, further supporting this shared pathway. The nonbonding orbitals in BrC and char facilitate <i>n</i> → π* transitions in the visible region, which are more wavelength-dependent than the π → π* transition in high-maturity EC (soot). This study highlights char as a light-absorbing intermediate, influencing light absorption of EC emitted from solid fuel combustion. These insights into the formation pathways and optical properties of carbonaceous aerosols enhance our understanding of their climate impacts and underscore the need to differentiate between char and soot in climate models to improve accuracy.