Seasonal cycles of ozone and oxidized nitrogen species in northeast Asia 2. A model analysis of the roles of chemistry and transport

The dominant factors controlling the seasonal variations of ozone (O₃) and three major oxidized nitrogen species, peroxyacetyl nitrate (PAN), nitrogen oxides (NO_x), and nitric acid (HNO₃), in northeast Asia are investigated by using a three-dimensional global chemical transport model to analyze surface observations made at Rishiri Island, a remote island in northern Japan. The model was evaluated by comparing with observed seasonal variations, and with the relationships between O₃, CO, and PAN. We show that the model reproduces the chemical environment at Rishiri Island reasonably well, and that the seasonal cycles of O₃, CO, NO_y species, and VOCs are well predicted. The impact of local emissions on some of these constituents is significant, but is not the dominant factor affecting the seasonal cycles. The seasonal roles of chemistry and transport in controlling O₃ and PAN are revealed by examining production/destruction and import/ export/deposition fluxes in the boundary layer over the Rishiri region. For O₃, transport plays a key role throughout the year, and the regional photochemical contribution is at most 10% in summer. For PAN, in contrast, transport dominates in winter, while in-situ chemistry contributes as much as 75% in summer. It is suggested that the relative contribution of transport and in-situ chemistry is significantly different for O₃ and PAN, but that the wintertime dominance of transport due to the long chemical lifetimes of these species is sufficient to drive the seasonal cycles of springtime maximum and summertime minimum characteristic of remote sites.