Thank you Jeremy for the response.  It makes sense now.  I tested it on a unit cube, with quiescent water and a specified rate of release of particles from one of the sides of the cube.  I did a transient simulation for 10 seconds.  After the simulation, I calculated the volume integral of Scalar-0 inside the cube and it is matching with the rate of release times the simulation time and the area.   

The quantity that I am looking for is the number of particles that cross a given surface, say an interior boundary or a physical boundary.  I couldn't find an option to do this.  I tried surface integral of Scalar-0, but it gives a large number because Scalar-0 is the number of particle per unit volume.  How do I calculate the actual number of particle crossing a surface?

Thank you,