The molar concentration of surface
species 
in a particle population can be calculated by
 | (19–111) |
where 
is the surface molar concentration of 
in [mole/cm2 ] and the summation corresponds to
the total particle surface area. Conservation of surface species 
leads to the transport equation for 
as
 | (19–112) |
Note that because surface species only exist on the particle surface, transport of surface species is solely due to the relocation of particles. The production rate term on the right-hand side consists of contributions due to surface chemistry that does not involve particle area change and the processes that modify particle surface area— nucleation, gas-particle interaction (growth/etching), and particle coagulation.
 | (19–113) |
The surface chemistry contribution due to reactions that do not involve particle area change (that is, no growth or etching) is simply net production rate due surface reactions and is computed as the product of surface molar production rate and the existing particle surface area.
 | (19–114) |
Nucleation adds new surface area and correspond surface species are specified by the nucleation reaction. The rate of surface species production can be simply computed using the reaction rate of progress and stoichiometry.
 | (19–115) |
Notice that this will be exactly the same if computed from the net area production rate due to nucleation. Thus
 | (19–116) |
In the above equation, Y S and σs are the site fraction and occupancy of the surface species, respectively, and γ is the surface site density. For example, for the nucleation reaction shown in Figure 19.6: Formation of particles from gas species C16H10 , Y H(s) = 20/48.72. Note that this number will be more exact than this due to the native site stoichiometry computation internally done by Ansys Chemkin and as explained in Native Surface Sites , The area production rate due to nucleation can be computed as
 | (19–117) |
In the above equation 
is the rate of progress of the nucleation reaction and 
is the size of the nucleated particle.
The growth/etching reactions and particle coagulation modify the total particle surface area. As shown in Equation 19–118 the corresponding contribution to surface species production is grouped together as indicated by the curly brackets. When the net area generation due to these processes is positive, Ansys Chemkin considers that this area is occupied by the native species (see Native Surface Sites ). (Note that for correct elemental balance, the native sites should be "empty" sites.) On the other hand, when the net area generation due to growth/etching reactions and coagulation is negative, Chemkin assumes that all surface species are "destroyed" in an amount proportional to their surface coverage. The destroyed species are considered to be "trapped" in the bulk particle and are not released in the gas phase.
Using the expressions for particle diffusivity and thermophoretic velocity described in Transport Equations for Size Moments , Equation 19–111 is written as
 | (19–118) |