Steady-State Mixing State of Black Carbon Aerosols from a Particle-Resolved Model

Abstract. Black carbon (BC) exerts a notable warming effect due to its strong light absorption, largely influenced by its "mixing state". However, due to computational constraints, mixing state is challenging to accurately represent in large-scale models. In this study, we employ a particle-resolved model to simulate the evolution of BC mixing state based on field observation in Nanjing. Our result shows that aerosol compositions, coating thickness distribution, and optical properties of BC aerosols all exhibit a tendency toward steady-state with a characteristic time of less than one day. Using the steady-state simplifying assumption, BC absorption enhancement closely matches the result obtained through the particle-resolved method. Additionally, we discuss how to reconcile our finding of a universal distribution with the diversity in the distribution of BC coating thickness that has been documented in previous studies. This study simplifies the BC mixing state description and yields a precise evaluation of the BC optical properties, which facilitates the refinement of the assessment of BC's radiative effects in global and chemical transport models.