Black carbon (BC) is a light-absorbing particle that contributes to atmospheric warming, but its radiative impact remains highly uncertain. We conducted field measurements in Hangzhou, China, to examine how the mass ratio (M_R, coating-to-BC) affects light absorption enhancement (E_abs). Our results show that widely used optical models overestimate E_abs, especially under clean conditions. A new MR-dependent parameterization improves model accuracy and reduces this uncertainty.