Annual and seasonal variations in oxidant concentration in Matsue, Japan

Annual and seasonal trends in surface oxidant concentrations were investigated between January 1989 and December 1997 at Matsue in the Japan Sea coastal margin where the westerly winds predominate. The mean value of oxidant concentration was 39.1 ppb during this period. The monthly oxidant concentrations in spring were at their annual maximum values, while those recorded in summer and autumn showed their lowest levels for the year as a whole. These seasonal variations were observed throughout the period. Oxidant concentrations above 80 ppb were observed between February and October, especially in April and May. Trajectory analysis indicated that when photochemical oxidant levels in excess of 80 ppb were observed in Matsue, the region was affected by a greater number of air masses flowing in from Korea and China than from other directions. It can therefore be concluded that the elevated oxidant concentration in Matsue is due to an increase in the amount of NOx emissions in East Asia, especially in Korea and China to the west of Matsue. The yearly mean oxidant concentration showed a steady annual increase at a rate of 0.89±0.21 ppb (or 2.0%: R=0.87) a year on average during this period. The rate of annual increase in oxidant concentration for the westerly wind (with Korea and China to the windward) was greater in comparison to SE-SW winds (with Japan to the windward). The deposition flux of nitric ion also showed an annual increase at a rate of 2.39±0.37 meq m−2 (or 5.9%: R=0.92) a year on average from 1989 to 1997. The NOx emissions have been rapidly increasing in the windward region.