Since the Particle Tracking feature employs the method of moments to solve the size distribution, it is important to understand the concept of particle class and how this class concept is connected to surface reactions that describe particle nucleation and mass growth. We define the class of a particle by the number of bulk species in its core. The "soot" particle declared in Figure 6.4: Dispersed Silicon Dioxide Material Declaration With Chemical Composition provides an example. A "soot" particle of class 100 has a core consisting of 100 C(B) species regardless of whether or not other species and atoms might exist on the surface of the particle.

When a particle nucleus is created from gas-phase precursors, its class, that is, the inception class, is determined by the stoichiometric coefficient of the bulk species in the nucleation reaction. For example, the nucleation reaction in Figure 6.5: Nucleation Reaction Creating Particle Nuclei of Class 32 creates particle nuclei of class 32:

2A4 => 32C(B) + 20 H(se) + 28.72 open(se)        1.0E10   0.5     0.0
                NUCL

The syntax of the nucleation reaction will be discussed in a later section. A surface reaction resulting in a net gain in the number of bulk species increases the size of the particle and vice-versa.