Perhaps you have a look into DEFINE_DIFFUSIVITY Macro and check if it is helpful for your case.

of course,the Ds term in equation below is calculated in DEFINE_DIFFUSIVITY macro having unit to be m^2/s and included in species transport model under mass diffusivity and FLUENT will include it in species conservation equation by multiplying with density and gradient of mass fraction.nBut in second term highlighted I need to multiply the individual species density in the form of PMs/RT (instead of total density) and mass fraction of 's' species in itnI don't know how and where to include itn