2022 Volume 57 Issue 5 Pages 109-118
To study recent changes in the photochemical oxidant (Ox) concentration, analyses were conducted using indices focusing on the daily maximum Ox concentration and diurnal Ox change during spring and summer over the Kanto Plain. Among the meteorological factors, the daily maximum temperature was strongly correlated with Ox in both seasons and a higher temperature indicated the possibility of a higher Ox. Under constant meteorological conditions in terms of sunshine duration and daily maximum temperature, these Ox indices were compared between two periods—2011–2014 and 2016–2019—and it was shown that the Ox decreased in summer with an especially significant decrease in the inland areas. The decrease in Ox during summer was presumably due to the decrease in the photochemical production caused by the decrease in the precursors (NOx and NMHC). In spring, even though the precursors decreased as in summer, these Ox indices did not decrease. It was considered that there are other factors besides local photochemical production in spring, and the effect of the decrease in the precursors was less apparent as the decrease in Ox. Furthermore, the Ox concentration reached the Ox alert level on days with meteorological conditions having a sunshine duration ≥6 h and a daily maximum temperature ≥27°C in spring and ≥35°C in summer.
