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in the titration effect of NO due to its decreasing emissions, as suggested by Chou et al. (2006). We note that the increasing trends at three background stations are nearly equal to one another (at 14% per decade for O3) for three different locations (Wanli is along the northern coast, Lanyu is on an island off the southeastern coast, and Yangming is a mountain site about 1km above sea level). Moreover, the trends exist in all four seasons (not shown). The consistent trends for O3 and O3 + NO2 at the three background stations suggest that the trends are large-scale and most likely the result of long range transport of ozone from the Asian continent. This is supported by similar trends found in Hong Kong (Fig. 11) and Okinawa (Lee et al., 1998). The increases in Hong Kong (^40% per decade) and Okinawa (^25% per decade) are significantly greater than the 14% per decade increase observed in Taiwan. The greater increase in Hong Kong may be the result of the rapid development of the Pearl River Delta region. However, local effect should not be a factor in the ozone trend in Okinawa. Its increase must be because of the influence of long range transport from continental Asia. The same argument can be applied to the background stations in Taiwan. However, at this moment, we do not have a good explanation for the difference between Okinawa and the background stations in Taiwan.

Central/Western (CW) TsuenWan (TW) Tai Po (TP)

Central/Western (CW) TsuenWan (TW) Tai Po (TP)

Figure 11. Trends of ozone concentration observed at three stations in Hong Kong. The Tai Po station is a rural station, and the other two are urban stations.

3.2. Ozone trends in East Asia and Mauna Loa, Hawaii

The above discussion shows that the ozone trend at a background station is a valuable measure of the regional budget of ozone. Because of the large change in the photochemical lifetime of ozone from a few days in the boundary layer to a few months in the upper troposphere, the representativeness of a station depends critically on the location and altitude of the station. The ground station in Okinawa and the background stations in Taiwan should reflect mainly the ozone budget of East Asia. However, the influence of Southeast Asia, South Asia and further upwind regions could be significant too. In this regard, we notice that the increase of the ozone mixing ratio in the boundary layer measured by ozonesondes over Japan from the 1970s to the 1980s was about 30%, consistent with similar measurements in Hohenpeissenberg and Payern in Europe (Akimoto, 2003). The consistency suggests that the increase in ozone is extremely large-scale, probably over the entire midlatitudes in the Northern Hemisphere. The cause of the increase is most likely the increasing emissions of ozone precursors from industrial developments in Europe, the US and Asia. This increase must have had a significant impact (on the order of a few ppbv) on the concentration of ozone over Taiwan. Moreover, the impact should still exist today because the emissions of ozone precursors have been increasing since the 1980s (Street et al., 2001). Unfortunately, the impact on the concentration of ozone over Taiwan could not be verified because there was no reliable measurement of ozone in Taiwan before 1993, when the EPA established the current air quality monitoring stations.

The large scale impact of Northern Hemispheric industrial developments on the ozone concentration is probably best-recorded at the Mauna Loa, Hawaii observatory, which is located 3.4km above the sea level and 20°N. The observatory observes free tropospheric air during the night when down-slope flow prevails. It usually is downwind of Asia, especially in winter and spring. Nevertheless, given the long lifetime of ozone in the free troposphere

Mauna Loa

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