Contribution of Photochemical Ozone Formation in Cities to Total Ozone Levels by Comparing Background Ozone Levels in Mountainous Areas.

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We measured O3 levels from May to September 1997 at 3 stations set at different altitudes on Mt Happo, which is located in a remote area of the Japan Alps. At night, O3 concentrations were correlated positively with altitude, but during the day, O3 levels at the lower station increased as the mixing layer developed, thus reducing the vertical differences in O3 concentration. Since it was rare for O3 concentrations at the lower station to exceed those at the upper station, we concluded that photochemical O3 formation on the mountainside was negligible, and that the data represented background ozone levels. We then calculated the contribution of photochemical O3 formation in city areas to total O3 (Ox) levels by subtracting the background O3 concentrations obtained on the mountainside from the Ox levels measured in the cities. These calculations were performed for periods when Ox concentrations exceeded the environmental standard (more than 60 ppb). In the cities of Nagano, Omachi, and Matsumoto, which are located around Mt Happo, photochemical O3 contributed a minimum of 18%, 19%, and 20%, respectively, to Ox levels in May. These percentages increased during summer, and reached a maximum of 40%, 37%, and 41%, respectively, in September. In contrast, the Ox dose (ppb h) showed a spring maximum and a summer minimum .
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