Modeling coagulation of externally mixed particles: Sectional approach for both size and chemical composition

Three-dimensional chemical transport models of air quality often assume particles to be internally mixed, that is, particles of a given size are associated to a unique chemical composition. However, measurements show that this assumption is not valid, especially close to sources. We present here the dynamic equation for Brownian coagulation of externally mixed particles (i.e., particles of a given size may have different chemical compositions). Both size distribution and mass fraction of particle chemical components are discretized. Either one or several families of particle chemical components may be discretized to reduce computational cost. The model is validated by comparison to results obtained for internally mixed particles and the effect of mixing is investigated for aerosol mixtures of two, three and four components. Mixing increases with the number of components, because the probability for a particle to coagulate with a particle of different composition increases.