Effects of mass ratio heterogeneity and coating-related optical characteristics on the light absorption enhancement of black carbon-containing particles

Black carbon (BC) is a strong climate forcer, but considerable uncertainty remains in estimating its radiative impact, largely due to persistent gaps between observed and modeled light absorption enhancement (Eabs). In this study, we employed a Centrifugal Particle Mass Analyzer and Single Particle Soot Photometer tandem system to characterize mass ratio (MR, coating-to-BC) of BC-containing particles in Hangzhou, China. Observations across a field campaign revealed low, medium, and high Eabs values under varying atmospheric conditions. The uniform core-shell Mie model overestimated Eabs, particularly during clean periods (low Eabs). To address this, we developed an observationally constrained parameterization for transition-state particles based on MR-dependent optical transitions behaviors. This approach effectively reconciles modeled and measured Eabs across varying pollution conditions. It also emphasizes the importance of incorporating MR heterogeneity and transition-state optical behavior to improve BC light absorption estimates and reduce uncertainties in assessing radiative effects.