Unraveling the Impact of Heterogeneity and Morphology on Light Absorption Enhancement of Black Carbon-Containing Particles

Abstract. 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 (E_abs). In this study, we employed a Centrifugal Particle Mass Analyzer and Single Particle Soot Photometer tandem system to characterize mass ratio (M_R, coating-to-BC) and morphology of BC-containing particles in Hangzhou, China. Fortunately, low, medium, and high E_abs values were observed during a single field campaign. Results show that the uniform core-shell Mie model overestimated E_abs especially in clean conditions (low E_abs). A morphology-dependent correction scheme was developed to improve optical property estimates of BC in the “transition state.” This improved model better reproduces measured E_abs in different pollution conditions and reveals that the concentrations of particle chemical composition affect the M_R threshold defining this state. Our findings highlight the need to account for real-world particle complexity in climate-relevant BC modeling.