Partitioning of HNO₃ and particulate nitrate over Tokyo: Effect of vertical mixing

Ground‐based measurements of gas‐phase nitric acid (HNO 3 ) and particulate nitrate (NO 3 − ) were performed in Tokyo during 2003–2004. These measurements provide a comprehensive data set for investigating the diurnal and seasonal variations of gas‐phase HNO 3 and particulate NO 3 − and the thermodynamic equilibrium of these compounds. HNO 3 and NO 3 − have distinct diurnal and seasonal variations, especially in summer. This study shows that the thermodynamic equilibrium of HNO 3 and NO 3 − and the production rate of total nitrate (TNO 3 = HNO 3 + NO 3 − ) 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 HNO 3 and NO 3 − by constraining the TNO 3 concentrations to the observations. The 1‐D model reproduces the NO 3 − /TNO 3 ratios observed during daytime, whereas the equilibrium model significantly underestimates these ratios. The agreement between the observed and calculated NO 3 − /TNO 3 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 HNO 3 ‐NO 3 − partitioning in the boundary layer. It is also found that the mass accommodation coefficient for HNO 3 needs to be approximately 0.1 to explain the observed HNO 3 ‐NO 3 − partitioning at the surface.