Air Pollution as a Climate Forcing: A Workshop
Day 1 Presentations
Recent Changes of Ozone in the Far East Asia and Europe
Hajime Akimoto, Pakpong Pochanart, and Manish Naja
Frontier Research System for Global Change
Regional-scale ozone change in one continent is controlled by both the meteorology/photochemistry within the continent and intercontinental long-range transport on the hemispheric scale. For example, ozone in Far East Asia is determined by regional scale photochemical buildup superimposed on the "northern-hemispheric background" that is controlled by the precursor emissions in Europe, North America and Asia itself. Therefore, in order to elucidate the characteristics of ozone change in the Far East on the basis of observational data, it is necessary to separate the within-continental component from the inter-continental one by developing a suitable technique for analysis. Here we present the methodology and results of analyzing observed ozone in the Far East and Europe. It is found that the regional background ozone level in remote areas in the Far East is nearly 10 ppbv higher than in Europe, which is reflected in the regionally polluted ozone levels also. Thus, it is suggested that the Far East is already suffering from the highest near-surface ozone level on a regional scale in the Northern Hemisphere. When the ozone impact index of AOT40 for vegetation used in Europe is applied, a critical level for forest trees is exceeded by nearly a factor of two in remote areas in the main island of Japan.
Methodology of analyzing ozone change
It has been well established that sector classification of isentropic backward trajectory can successfully characterize the observed ozone levels in the background and regionally polluted air mass in Far East Asia (Pochanart et al., 1999). In Europe, however, it has been found that sector classification doesn't work except for Atlantic coastal stations such as reported at Mace Head (Derwent et al., 1998). Instead, residence time classification of backward trajectory can successfully characterize the observed ozone levels at a continental European station such as Arosa (Pochanart et al., 2001). There, ozone levels in the background air mass can be obtained by extrapolating to zero residence time. On the other hand, the ozone levels in the regionally polluted air mass can be defined by averaging data of aged air mass with residence time of 4-6 days.
Higher regional ozone levels in East Asia than in Europe. Figures 1 and 2 shows the comparison of seasonal change of surface ozone in the background and regionally polluted air masses in East Asia and Western Europe. As shown in Fig. 1, a spring maximum and a summer minimum of ozone are clearly seen in the background air mass in both continents. Absolute values at two sites in each of the regions, Mondy (East Siberia) and Happo (central Japan) in East Asia, and Arosa and Mace Head in Europe agree excellently, which confirms the validity of the methodology. Comparing the different continents, the absolute ozone level in East Asia is up to ca. 10 ppbv higher than in Europe. This would be due to the different nature of the background air mass in both continents; Atlantic oceanic air in Europe and Eurasian continental air in East Asia. The higher ozone level in East Asia would be attributed partly to the ozone buildup within Europe whose outflow affects East Asia by inter-continental long-range transport, and partly to additional ozone formation in central Eurasia. As shown in Fig. 2, the difference in ozone level in the background air mass is somehow reflected in the mixing ratio of the regionally polluted air mass in the two continents. Thus, while seasonal variations at Arosa and Happo commonly show a spring-summer maximum due to regional photochemical buildup in the photoactive season, the absolute concentration is still higher in East Asia. The difference in ozone mixing ratios between the regionally polluted and background air mass at each site, which elucidates photochemical ozone buildup within each continent, is 20±5 ppbv being comparable between the Far East and Europe.
Although comparison to the North American continent has not been made, the reported ozone background in United States is 20-45 ppbv, which is comparable to or lower than in Europe. Therefore, it may be concluded that the ozone concentration in East Asia is already the largest from a global perspective. When ozone impact index of AOT40 for vegetation, as used in Europe, is applied, it is revealed that the critical level for forest trees is exceeded by nearly a factor of two in remote areas in the main island of Japan. When an urban plume overlaps with regional ozone pollution, it can easily exceed the critical level by a factor of three.
Ozone Trend in Europe and Far East Asia. Ozone trends in Europe and the Far East have been analyzed by using ozone-sounding records at Hohenpeissenberg and Payern in Europe, and Okinawa and Kagoshima in Japan by applying the trajectory classification technique described above. The leveling off of the ozone amount in Europe after 1985 and the still increasing trend in the Far East have been revealed. Detailed results were presented.
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