Yes, I did that. Please note that N_agg equation converges when its source term is active, and I have validated the results.I also dumped the source term for A_tot, and as you guessed it jump up and down throughout the domain. It is worth mentioning the physics of the problem. N_agg changes due to convection and diffusion, and A_tot is affected by diffusive flux of N_agg.
I tried to account for this effect two ways:
1) Calculating the derivatives using C_UDSI_G
2) Finding the diffusive flux using
None of these worked out for me!