Since particles are in a condensed phase, interactions between particles and a surrounding gas mixture must take place on the particle surface. These particle-gas interactions include adsorption of gas species onto the particle surface, desorption of surface species into the gas, growth of the particle by deposition of bulk species, consumption of the particle by gasification of bulk species, and chemical reactions between surface species. Particle Tracking treats all chemical processes involving particles as surface reactions. This surface chemistry concept reflects the fact that net rates of these particle-gas interactions are proportional to the total active particle surface area. Thus all chemical processes on particle surfaces can be limited by the availability of particle surface area. For example, if there is no particle present in the system, the net rate of particle mass growth by surface reactions is zero. Moreover, Ansys Chemkin Surface Kinetics has many special formulations, such as sticking coefficients and coverage-dependent activation energy that can provide flexibility in defining the particle-gas processes. Furthermore, since they are incorporated into the surface mechanism, contributions of individual surface processes to particle properties can be determined using standard rate-of-production and sensitivity analyses.

Because there are typically many particles in the system, it is impossible to track the surface condition of individual particles. Particle Tracking instead employs a statistical approach to describe the surface condition of a particle population. The particles are assumed to have the same surface coverage and surface temperature if they are exposed to the same gas mixture locally. When particles from other locations are added, the local surface coverage and surface temperature will change to reflect the new "average" local gas values.

In this section, the connection between particle-gas reactions and particle size distribution (or size moments) is described in detail. In addition, a special surface reaction keyword is introduced. This new keyword allows the size, that is, diameter, of a gas-phase species to be taken into account when the reaction rate between the gas species and particles is calculated.