Thanks again for your reply Rob,

I am not quite sure what you mean by "You're also calculating particles on some nodes". I chose surface injection and chose my inlet surface for that. I have about 2 Million cells in my entire domain and the inlet facets are about 700. 

Yes, reducing the number of time steps by reducing the stop time is a good idea. The main purpose is studying the efficiency of a filter, so I just need a few parcels in each injection group to judge the number of trapped particles at the filter. 

Correct me if I'm wrong: Based on what I understood from what you said, the "DPM time step" defines how often a parcel is injected from the designated injection location. In my case, it is creating a parcel every 1e-7 s. On the other side, the required time step for discretizing the equation of motion of particles (du_p / dt = Sigma F) is still calculated based on the "step length factor" or "length scale". right? If this is true, yes I will have an accumulation of multiple particles in my cells because the injection rate is higher than the speed of particles moving forward in comparison to my grid resolution. 

If that is the case, I do not need to set my "DPM time step" to such a small number not so ever! My particle relaxation time is about 6e-7 so I chose the DPM time step as 1e-7 while I should have calculated the "step length factor" or "length scale" in a way to satisfy my relaxation time. right?