Recent trends in precursor concentrations and oxidant distributions in the Tokyo and Osaka areas

Photochemical air pollution is still a serious environmental issue in Tokyo and Osaka areas. To clarify the past trend and present status of photochemical air pollution, the annual trends of daily maximum oxidant concentration covering Kanto (Tokyo and surrounding prefectures) and Kansai (Osaka and surrounding prefectures) areas were analyzed. Trends in the photochemical air pollution and NOx and VOC precursors showed identical characteristics both in Tokyo and Osaka areas. Trends in volatile organic hydrocarbons and NOx were analyzed; NOx is incresing and NMHC/NOx ratio is decreasing in the long term over both Kanto and Kansai areas, indicating an increase in ozone forming potential and a decrease in photochemical reactivity, which may be responsible for the air pollution patterns observed in the Tokyo and Osaka areas. The location of the daily maximum oxidant concentration has moved further from the emission areas.