Accessing the previous cell
 

I am simulating flow through porous media. Mass source term that I am planning to use is m=hA(delta c)

where m is the mass source
h is the mass transfer coefficient
A is the area
delta c is the concentration gradient = c(mixture in porous media) - c(inlet air)

Now delta c varies with each cell in x direction as inlet air absorbs moisture. I can use the c(out) of previous cell as the c(inlet) for the next cell. However, how should I specify the previous cell for complex geometries.