Critical role of single-particle partial coating fractions in reproducing black carbon absorption and refining climate simulations

Black carbon (BC) significantly influences global warming, but uncertainties in its absorption enhancement ( E abs ) cause large discrepancies in radiative forcing estimates. This study aims to explain the low absorption enhancement observations in the field that current models tend to overestimate by identifying incomplete particle coating, quantified by coating fraction F , as a key regulator. Observations show environmental constraints maintain F ≪ 1, reducing lensing efficiency. A new unified mixing framework incorporates a size-resolved parameterization linking F and coating-to-core mass ratio R coating , bridging model-observation gaps. Integrating empirical F distributions into Mie-based models retains complex simulation accuracy while reducing computational cost drastically, enabling scalable climate model implementation. Validation with single-particle soot photometer data confirms reduced absorption overestimation across diverse regimes. Including F distributions lowers global BC direct radiative forcing by ~40% (from +0.53 ± 0.13 to +0.32 ± 0.08 W m −2 ) relative to uniform mixing models. This establishes particle-scale coating coverage as a critical missing variable in BC forcing assessment, providing an observationally anchored pathway to reduce climate projection uncertainties.