The Mercury Sodium Atmosphere Spectral Imager (MSASI) on the BepiColombo/Mercury Magnetospheric Orbiter (MMO) is a high-dispersion Fabry-Perot imager. MSASI will address a wealth of fundamental scientific questions pertaining to the Mercury's exosphere. Together, our measurement on the overall scale will provide ample new information on regolith-exospheremagnetosphere coupling as well as new understanding of the dynamics governing the 'surfacebounded exosphere'. It arises quite clearly from continuous ground-based observations that the regolith of Mercury releases a fraction of its content to Mercury's exosphere. Some processes are identified up to now as leading to this ejection. These processes are associated with different energies of ejection, behavior in different regions of Mercury's surface and eject different types of population from the surface. The responsible processes are (1) Chemical sputtering, (2) Thermal desorption, (3) Photon-stimulated desorption, (4) Ion sputtering, and (5) Micro-meteoroid impact/vaporization. Each candidate seems to be fairly operative, but any cannot completely explain phenomena observed from the Earth. Also, the fate of ejecta from the regolith is still unknown. Some are expected to return to the lithosphere, the other are lost to interplanetary space. Circulation of lithospheric sodium atoms via exosphere-magnetosphere might bring a significant change in the composition of surface layer on Mercury. The MSASI measurements clearly and definitely can identify the release mechanism, how exospheric sodium is born from the regolith, and bring comprehensive picture of global circulation of regolith materials. Also, BepiColombo is the first and unique opportunity to study the formation, circulation, maintenance of this surface-bounded exosphere, which is a different type of terrestrial atmosphere. Below we describe in somewhat more detail the primary scientific objectives of MSASI.