Surface reaction source definition.

windhair

There is a porous wall in my simulation of surface reaction, with such reaction A + B = C + D, A, B and C are always in the flow domain, but D will penetrate through reaction wall to the outside, this cause the lose of total mass in the flow domain. Under such situation, how to define the boundary source, shall I define only the mass source for each components or the source for the continuity equation is also necessary?