(Kg/s) * (1/T) *(1/density of air) * (1/volume of domain) *10^6 =PPM
(Kg/s) = co source term given with this units in the passive scalar box
T = Passive scalar quantity (1/s)
Density of air = kg/m^3
Volume of domain = m^3
please conform if the above approach is correct. @jousefm