Abstract
The mixing state of black carbon (BC) aerosols, namely, the degree to which BC particles are coated with other aerosol components, has been recognized as important for evaluating aerosol radiative forcing. In order to resolve the BC mixing state explicitly in model simulations, a two‐dimensional aerosol representation, in which aerosols are given for individual particle diameters and BC mass fractions, is introduced. This representation was incorporated into an aerosol module, the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID), and a new box model, MADRID‐BC, was developed. MADRID‐BC can accurately simulate changes in the entire BC mixing state resulting from condensation/evaporation processes. Aircraft observations conducted in March 2004 show that the mass fraction of thickly coated BC particles increased in air horizontally transported out from an urban area in Japan over the ocean. MADRID‐BC generally reproduces this feature well when observed bulk aerosol concentrations are used as constraints. The model simulations in this particular case show that for particles with BC core diameters of 100–200 nm, the particle diameters, including both core and coating materials, had already increased by a factor of 1.6 on average when they left the source region and by as large as a factor of 1.9 of the BC core diameters after their transport over the ocean for a half day. The model simulations also show that 58% of the total condensed mass was partitioned onto BC‐free particles during transport, indicating their importance for the BC mixing state. Although the model simulations are applied to a limited number of the observations in this study, they clearly show the time evolution of the coating thicknesses of BC‐containing particles, which is necessary for calculating aerosol optical properties and cloud condensation nuclei activities.
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