I want to use the DPM to track the concentration vs. time for different particle sizes. I have some cells marked out as volume monitors. I want to track the concentration at these points as a function of time. Any ideas?

Have you tried to solve it as an unsteady case? In the 'solver' panel choose the 'unsteady' simulation. Fix an appropriate time (sec) and than choose 1 for time step(s). Then use a big number for the 'itereation at each step time'. Big enough to converge. After you reached the convergence, you can monitor the chosen cells. If you are ready just click on the 'iteration' button (without initialize). It will calculate the next time step.