This book presents the development and application of the new aerosol model (SCRAM), which is able to resolve aerosol mixing state by using a new size and composition resolved approach. This work is well-motivated by mounting evidence from field observations that ambient aerosol populations have complex mixing states. Such mixing states reflects the particles' history, and even freshly emitted particles can contain multiple chemical species depending on their source characteristics. Moreover, the initial particle composition is further modified in the atmosphere as a result of aging processes such as coagulation, condensation of secondary aerosol species, and heterogeneous reactions. This profoundly impacts the aerosol optical properties, their cloud condensation nuclei activity, and their lifetime in the atmosphere. This book tackles this problem and offers a solution.