Help needed in determining multilayer resuspension rate

Hi,

         I am trying to determine the resuspension rate, Λi (t) , in the i th layer for a multi layer resuspension model. One needs to determine the number of exposed particles , ni (ra',t), in the ith layer to determine the resuspension rate. I am kind of stuck here. I would greatly appreciate the help. I am attaching the file (Check the last page). If you feel any parameter is missing, fill in an arbitrary value. 

Regards