Adapting transport of diluted species interface using mathematical PDE

Hello!

I'm trying to adapt the transport of diluted species interface using custom PDE:s in order to open up for further modification later on. I have been trying to copy the functionality of the interface using coefficient form PDE or the specialized classical Convection-Diffusion equation. The purpose is to model the concentration of a dilute species in a laminar flow of a carrier gas. No electric field.

Using the PDE interfaces to implement Fick’s second law proved no problem, but the diluted species transport interface also uses Fick's first law, it seems. I wasn't able to implement it with any of the PDE interfaces (I don't know how to use weak form equations). The interface gives almost no indication to which part it plays a role. Why isn’t it a part of the classical Convection-Diffusion equation if it is needed? It doesn’t seem to work without it. Is it only important for boundary conditions?

Another thing I haven’t been able to implement is the outflow boundary condition of the diluted species transport. The diluted species should disappear with the carrier fluid. If I try to implement it with the flux/source boundary condition in the Convection-Diffusion interface with the flux set to zero, it is identical to the outflow condition, apart from a minus sign. But it turns out this is the same as the zero flux condition. The question is, how do I implement an outflow boundary condition in the Convection-Diffusion equation?

Thanks!