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

Vertical profiles of springtime ozone (O-3) over the Osaka metropolitan area in Japan were investigated by means of aircraft-based observations in March 2001 (Period 1) and March 2003 (Period 11). The observations in Period I were carried out during an episode of high O-3 concentration under stagnant meteorological conditions, whereas in Period 11, pollution levels were low and the weather was cloudy with strong winds. The elevated O-3 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 O-3 concentration in Period 11 was very low, corresponding to the O-3 level of 'clean continental' air masses from the northern Asian continent. On the basis of the vertical O-3 profiles observed in Period 1, we estimated the contributions of local photochemical production and trans-boundary air pollution to the O-3 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 O-3 in the Osaka metropolitan area, its contribution being comparable to that of local photochemical production. (c) 2005 Elsevier Ltd. All rights reserved.