Local and regional contributions to springtime ozone in the Osaka metropolitan area, estimated from aircraft observations

Vertical profiles of springtime ozone (O3) over the Osaka metropolitan area in Japan were investigated by means of aircraft-based observations in March 2001 (Period I) and March 2003 (Period II). The observations in Period I were carried out during an episode of high O3 concentration under stagnant meteorological conditions, whereas in Period II, pollution levels were low and the weather was cloudy with strong winds. The elevated O3 concentration in the surface mixing layer in Period I was a result of local photochemical production as well as trans-boundary air pollution transported from East Asia. In contrast, the O3 concentration in Period II was very low, corresponding to the O3 level of ‘clean continental’ air masses from the northern Asian continent. On the basis of the vertical O3 profiles observed in Period I, we estimated the contributions of local photochemical production and trans-boundary air pollution to the O3 within the mixing layer, local photochemical production contributed −3% to 33% and trans-boundary air pollution contributed 8%–27%. We found that the trans-boundary air pollution made a notable contribution to the springtime O3 in the Osaka metropolitan area, its contribution being comparable to that of local photochemical production.