If Specify release times is selected from the Particle release specification list in the physics interface Particle Release and Propagation section, the Mass Deposition subnode is available from the context menu (right-click the Wall parent node) or from the Physics toolbar, Attributes menu. The Mass Deposition subnode uses an accumulated variable on each boundary element in the selection of the parent node to calculate the mass of particles deposited on the surface, expressed in mass per unit area.