Microphysical complexity of black carbon particles restricts their warming potential

Black carbon (BC) strongly absorbs solar radiation, but its warming effect on climate is poorly quantified. A key challenge is to accurately assess BC light absorption after BC is mixed with non-BC components. However, there has consistently been a large observation-modeling gap in BC light absorption estimation, reflecting the insufficient understanding of realistic BC complexity. Here, we conduct comprehensive in situ measurements of BC single-particle microphysics, e.g., size, coating amounts, density, and shape, along with optical closure calculation. Specifically, the observed particle-to-particle heterogeneities in size and coating and the non-spherical BC shape only explain the lower observed BC absorption by ∼20% and ∼30%, respectively. A remaining gap for fully aged spherical BC-containing particles is related to the off-center BC-core position. The global climate model assessment shows that fully accounting for the observed BC complexity in the aerosol microphysical representation reduces the global BC direct radiative forcing by up to 23%.