Ground-based measurements of gas-phase nitric acid (HNO3) and particulate nitrate (NO3-) were performed in Tokyo during 2003--2004. These measurements provide a comprehensive data set for investigating the diurnal and seasonal variations of gas-phase HNO3 and particulate NO3- and the thermodynamic equilibrium of these compounds. HNO3 and NO3- have distinct diurnal and seasonal variations, especially in summer. This study shows that the thermodynamic equilibrium of HNO3 and NO3- and the production rate of total nitrate (TNO3 = HNO3 + NO3-) are the major controlling factors affecting these seasonal and diurnal variations. A thermodynamic equilibrium model (ISORROPIA) is newly coupled with a one-dimensional (1-D) model to take into account the effect of vertical mixing during daytime on the partitioning of HNO3 and NO3- by constraining the TNO3 concentrations to the observations. The 1-D model reproduces the NO3-/TNO3 ratios observed during daytime, whereas the equilibrium model significantly underestimates these ratios. The agreement between the observed and calculated NO3-/TNO3 ratios is improved over the observed temperature range (1¿--34¿C) and relative humidity (18--95%) by the 1-D model. These results suggest the importance of vertical mixing in determining HNO3-NO3- partitioning in the boundary layer. It is also found that the mass accommodation coefficient for HNO3 needs to be approximately 0.1 to explain the observed HNO3-NO3- partitioning at the surface.