Mercury is valuable to us because we can see the interaction between the planet and its space environment. This research aims to clarify how Mercury’s neutral Na exosphere was produced. Data from MESSENGER/MASCS and model calculations that examine possible generation, transportation and dissipation processes will be compared. First, seasonal variability of the amount of Na exosphere is analyzed for each local time (LT) using MASCS data. Previous research has shown that the amount of Na above LT12 reaches its maximum at aphelion, and it is found that this maximum is recorded only at LT12. Following this result, we construct 3-D Na exosphere model to understand the key seasonal variability processes occurring around LT12. The numerical calculation produced results that are consistent with the MASCS observations regarding the vertical profile and the seasonal variability at LT06 and LT18. However, the peak that occurs around aphelion at LT12 could not be reproduced. Yet the model produced results suggesting that less than 10 kg particles of comet stream dust per Mercury year could be the local and short-term source of Na.