Black carbon mixing-state heterogeneity influences absorption enhancement: Results from a field study across three seasons

Abstract. Black carbon (BC) is the major light-absorbing aerosol and an important contributor to radiative forcing. Atmospheric aging often coats BC with non-absorbing material, enhancing its mass absorption cross-section (MAC). However, this absorption enhancement depends not only on the bulk amount of coating material but also on how the coatings are distributed across individual BC particles. Here, we represent BC mixing-state heterogeneity using particle-resolved two-dimensional mixing-state distributions. This representation provides a more complete explanation of MAC variability than a single bulk mixing-state parameter. During summer, mixing-state heterogeneity changed little, and MAC remained nearly constant despite a substantial increase in the bulk coating-to-core mass ratio. By contrast, larger changes in mixing-state heterogeneity observed during autumn and winter coincided with MAC increases of up to 35 % and 30 %, respectively. MAC was most sensitive when initially homogeneous BC populations became more heterogeneous, whereas further changes within already heterogeneous populations produced comparatively small responses. A higher fraction of large-core BC particles (>300 nm), associated with biomass and coal combustion, further reduced MAC because of their lower absorbing efficiency. Mie calculations further showed that MAC differed by ~24 % on average under homogeneous and heterogeneous mixing-state assumptions. Our results highlight the importance of explicitly representing BC mixing-state heterogeneity for a better understanding in aerosol optical property variations and improving their simulation in models, thereby reducing uncertainties in estimates of BC radiative forcing.