Abstract Quantifying the climate effects of black carbon (BC) requires resolving how atmospheric aging modifies the mixing state and light absorption of BC‐containing particles. Here, we optimize a tandem DMA‐SP2 system to obtain size‐resolved coating amount (MR) and morphology of BC‐containing particles. As the BC core diameter increased from 100 to 300 nm, MR decreased from 6.0 to 3.0, whereas the non‐spherical fraction increased from 23% to 59%. In addition, size‐dependent morphological evolution pathways were identified for BC‐containing particles during coating growth: non‐BC materials mainly drive size growth for particles with smaller BC cores but predominantly structural compaction for particles with larger cores. Considering both core size and air‐mass aging, a differentiated morphological transition MR threshold (ranging from ∼8 to ∼1) is identified. Optical simulations constrained by these thresholds show that the core‐shell Mie model overestimates BC absorption enhancement during the initial aging stage, particularly for particles with larger BC cores.