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Figure 12. Trends of ozone concentration observed at Mauna Loa, Hawaii (3.4 km above the sea level and 20° N) between 1973 and 2004.

(more than a month), ozone trends observed at the observatory should reflect the integrated impact of Northern Hemispheric industrial developments. Figure 12 shows the ozone trends in different seasons and the annual mean from 1973 to 2004. The increasing trends are obvious except in the spring. The lack of increase in the spring is probably due to the relatively strong influence of stratospheric intrusion, which brings down stratospheric ozone to the troposphere. The persistent increases in ozone at Mauna Loa are consistent with the upward emission trends of NOx in the US, Europe and East Asia (Street et al., 2003). The fact that NOx emissions in East Asia have increased the most may also contribute to the persistent ozone trends because East Asia is directly upwind of Mauna Loa. In addition there are substantial increases in NOx emissions in South Asia and Southeast Asia, which may also have a significant impact on the ozone trends at Mauna Loa.

What does the persistent increase in ozone at Mauna Loa imply for Taiwan? Taiwan is slightly north of Mauna Loa but much closer to the Asian continent. So we expect a substantially greater influence of the emissions from East Asia for Taiwan, i.e. a substantially greater increase in ozone. This is borne out by the fact that the annual average trend for background stations in Taiwan (^14% increase per decade between 1993 and 2004) is about a-factor-of-3.5 greater than the trend observed at Mauna Loa (^4.1% increase per decade between 1973 and 2004). If this ratio of 3.5 could be extended back to 1973 for Taiwan, it would imply a 43% increase for the background concentrations of ozone over Taiwan from 1973 to 2004.

In summary, the increasing trends of ozone concentration at background stations in Taiwan, Hong Kong and Japan from the 1980s to 2005 strongly suggest that photochemical production of ozone in Asia has been increasing due to increasing anthropogenic emissions of ozone precursors. In addition, we believe that the increasing trend of ozone concentration (4.1% increase per decade between 1973 and 2004) at Mauna Loa (3.4 km) is strong evidence supporting the theory that the background ozone level over Asia and most of the Northern Hemisphere has been rising in the same period. An observation-constrained three-dimensional photochemical transport model should be able to use the measurements at Mauna Loa to inverse-model the trends of background ozone concentration in Asia. More importantly, one can even inverse-model the global trend of ozone concentration between 1973 and 2004.

4. Concluding Remarks

In this work we have reviewed the formation of high levels of ozone in large metropolitan areas or megacities of East Asia and explored

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