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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium The Characteristics of Urban Air Pollution in China XIAOYAN TANG Center of Environmental Sciences Peking University China has experienced rapid economic growth (7 to 8 percent of gross domestic product [GDP] per year) since the mid-1980s. This rapid growth in such a short period of time has not only led to a remarkable increase in material wealth and a higher standard of living, but has also caused severe environmental pollution, particularly atmospheric pollution (He et al., 2002). The problem was first observed in the 1970s with industrial emissions of sulfur dioxide (SO2) and total suspended particulates (TSP). In the 1980s, acid rain was detected in major cities in the southern part of the country (Feng et al., 2002; Hao et al., 2001; Lei et al., 1997; Li and Gao, 2002; Qin and Huang, 2001; Tanner et al., 1997; Wang and Wang, 1995, 1996). This was caused mainly by SO2 from coal combustion, which accounts for more than 70 percent of fuel consumption in China. In the 1990s, the number of vehicles on roads increased very rapidly, especially in medium-sized and large cities. In Beijing, the number of vehicles increased by a factor of 4, from 0.5 million in 1990 to 2 million in 2002. In addition, the emission factor (the amount of pollution emitted by one car) in China is much higher than in developed countries, because China has much lower emissions standards for automobiles. Thus, the drastic rise in the number of vehicles and rapid development of industries in cities has led to worsening air quality (Xie et al., 2000, 2003; Zhang et al., 1999), particularly higher concentrations of nitrogen oxides (NOx) (Wang et al., 2001) and particulates (Hu et al., 2002a,b; Kan and Chen, 2004; Song et al., 2002). High levels of ozone concentration were frequently observed in the summer and fall in several big cities (Ma, 2000; Tang et al., 1989, 1995; Wang et al., 2003; Zhang et al., 1998), and visibility in urban
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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium FIGURE 1 The chemical behavior of pollutants in the air. areas continues to deteriorate (Song et al., 2003a). For the past three years, PM10 has been the predominant pollutant in most Chinese cities. Economic growth and rapid urbanization in China have caused a tremendous increase in the consumption of energy and emissions of SO2, NOx (Hao et al., 2002), volatile organic compounds (VOCs) (Shao et al., 2000), TSP, and other pollutants. These primary pollutants not only disperse in the air and move to surrounding areas, but they also react photochemically to generate secondary pollutants. Photochemical smog (ground-level ozone) is produced from the reaction of NOx and VOCs with ultraviolet radiation. Gas-phase SO2, NOx and VOCs can be transformed into fine particles (PM2.5), which have large surface areas and can catalyze further reactions on the particle surface. Thus, reactions between pollutants are cyclical, leading to buildups of secondary pollutants in the air and causing severe air quality problems (Figure 1). This phenomenon has been observed not only in several megacities, such as Beijing, Guangzhou, and Shanghai, but also in many medium-sized and large cities. This kind of air pollution has three distinguishing characteristics: (1) a high concentration of fine particles that adversely affect visibility; (2) a high capacity for atmospheric oxidation; and (3) regional environmental effects. HIGH CONCENTRATIONS OF FINE PARTICLES AND ADVERSE EFFECTS ON VISIBILITY The visibility problem has affected Beijing for several years (Song et al., 2003a). Citizens often complain that they rarely see blue skies or white clouds in
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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium FIGURE 2 The diurnal profile of PM2.5 mass concentration in winter (above) and summer (below) in 1999 in Beijing. the Beijing area. A study of the relationship between visibility and concentrations of PM2.5 in 1999–2000 showed a direct correlation in every season (Song et al., 2003b). Results also showed that the concentration of PM2.5 in the summer and winter (a daily average of 60 to 80 µg/m3) was higher than the national air quality standard in the United States (65 µg/m3). In winter, the PM2.5 concentration was highest at midnight (Figure 2), probably because heavy vehicles were permitted to pass through the metropolitan area only after 8:00 p.m. HIGH CAPACITY FOR ATMOSPHERIC OXIDATION Since the 1990s, high concentrations of surface ozone have been observed in Beijing from May to October. The diurnal profiles of ambient ozone concentration observed at the Peking University monitoring site for several years are shown in Figure 3 (Ma and Zhang, 2000; Zhang et al., 1998). The peak ozone concentration has increased every year, no doubt as a result of economic and population growth. In other megacities, such as Guangzhou and Shanghai, high ozone concentrations have also been observed in residential areas. In June and July 2000, intensive measurements were taken of fine particles
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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium FIGURE 3 The diurnal profile of ozone concentration measured in different years at the Peking University site. Source: Zhang, 1998. and ozone levels (and precursor levels) at six sites in the Beijing area. A high ozone episode occurred between June 21 to July 3, and the daily ozone peak (more than 200 ppb) occurred at about noon on most days. Ozone as a secondary product is produced simultaneously with a series of oxidants, such as atmospheric free radicals (e.g., OH, HO2, RO, RO2, etc.). High concentrations of ozone and free radicals in the atmosphere lead to a high potential for the oxidation of primary pollutants (SO2 and NOx) to secondary pollutants (sulfate [SO4–2] and nitrate [NO3–]). The increased concentration of secondary pollutants is probably the major reason for the increasingly serious visibility problem. Short-term and long-term exposure to ozone-rich air can cause eye irritation, plant damage, respiratory problems, and the deterioration of rubber and paint. The World Health Organization has suggested that one hour of exposure should not exceed 75 to 100 parts per billion (ppb). Most cities in China cannot meet that standard. Therefore, reducing the number and severity of ozone episodes and lowering the levels of atmospheric ozone have become major health concerns of city governments.
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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium REGIONAL EFFECTS OF AIR POLLUTION Because of geographic conditions, rapid economic growth has taken place mainly in the eastern part of the country, particularly in coastal areas. The Yangtze River delta region, the Pearl River delta region, and the Beijing-Tianjin-Bohai Bay region are typical of economically developed zones in China. In just 20 years, urbanization has increased dramatically in those regions, and the distances between cities have steadily decreased. As the density of cities has increased, the regional impact of air pollution has become increasingly noticeable. In recent years, field measurements of air pollutants with meteorological observations and model simulations of air quality on a regional scale have been conducted in these three regions. The results showed pollutants, particularly secondary pollutants such as fine particles and ozone, were distributed regionally under certain meteorological conditions (Municipal Area Project Report, 2002; Pearl River Delta Region Project Report, 2002). Figure 4 shows the regional effects of ozone in the Pearl River delta region measured in an intensive study (PGSAQPRD, 2002). A very strong negative relationship was found between concentrations of ozone and NOx from upwind to downwind sites. At the site in the downtown area of Guangzhou, the ozone FIGURE 4 The upwind-to-downwind distribution of ozone and NOx showing daytime average concentrations in Guangzhou and contiguous areas. Source: PGSAQPRD, 2002.
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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium concentration was very low because of the titration of NOx with ozone. At the downwind remote site (50 km), the ozone concentration was higher because of pollution transport by wind. The same phenomenon was found in the summer in the same region. In Beijing, similar research has also shown regional effects (PGAPC, 2002). Figure 5 shows the regions included in the three-dimensional numerical simulation of air quality in Beijing and the surrounding area. The model shows that ozone episodes vary and the contributions to ozone formation come from different sources depending on meteorological conditions. The simulation results for June 27, 2000, are shown in Table 1. The daily maximum concentration of 200 ppb appeared at 1 to 2 p.m. in the Beijing downtown area; high ozone covered a large region, including part of Tianjin and surroundings. The model calculation indicates that the main contributor to the ozone episode at that time was precursors from Tianjin city and residential areas (Municipal Area Project Report, 2002). FIGURE 5 The simulation area, including Beijing municipality and surrounding areas, in the three-dimensional air quality model in three scales of nesting grids. Source: PGAPC, 2002.
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Urbanization, Energy, and Air Pollution in China: The Challenges Ahead - Proceedings of a Symposium TABLE 1 Contributions of Precursors (NOx and VOCs) from Various Locations in the Formation of Ozone in Downtown Beijing (1–2 p.m., June 27, 2000) Source Region NOx Contribution VOC Contribution Total Contribution Beijing downtown 9.9% 14.7% 24.6% Southeast part of Beijing 5.6% 3.5% 9.1% Tianjin and vicinity 9.3% 23.8% 33.1% Southern part of Hebei Province 3.3% 7.9% 11.2% Source: Municipal Area Project Report, 2002. CONCLUSION Rapid economic growth combined with urbanization has caused severe air pollution problems in China, especially in urban areas. These problems are proving very difficult to solve because of the complex nature of the pollution. Studies have been focused on identifying the formation mechanisms, sources, and impacts on health and the ecosystem. Much attention has also been focused on control measures and policy issues. China expects that the air pollution problem will be reduced in the next 10 to 15 years. REFERENCES Feng, Z.W., H. Miao, F.Z. Zhang, and Y.Z. Huang. 2002. Effects of acid deposition on terrestrial ecosystems and their rehabilitation strategies in China. Journal of Environmental Sciences-China 14(2): 227–233. Hao, J.M., H.Z. Tian, and Y.Q. Lu. 2002. Emission inventories of NOx from commercial energy consumption in China, 1995–1998. Environmental Science and Technology 36(4): 552–560. Hao, J.M., S.X. Wang, B.J. Liu, and K.B. He. 2001. Plotting of acid rain and sulfur dioxide pollution control zones and integrated control planning in China. Water Air and Soil Pollution 130(1-4): 259–264, Part 2. He, K.B., H. Huo, and Q. Zhang. 2002. Urban air pollution in China: current status, characteristics, and progress. Annual Review of Energy and the Environment 27: 397–431. Hu, M., L.Y. He, Y.H. Zhang, M. Wang, Y.P. Kim, and K.C. Moon. 2002a. Seasonal variation of ionic species in fine particles at Qingdao, China. Atmospheric Environment 36(38): 5853–5859. Hu, M., F.M. Zhou, K.S. Shao, Y.H. Zhang, X.Y. Tang, and J. Slanina. 2002b. Diurnal variations of aerosol chemical compositions and related gaseous pollutants in Beijing and Guangzhou. Journal of Environmental Science and Health, Part A-Toxic/Hazardous Substances and Environmental Engineering 37(4): 479–488. Kan, H.D., and B.H. Chen. 2004. Particulate air pollution in urban areas of Shanghai, China: health-based economic assessment. Science of the Total Environment 322: 71–79. Lei, H.C., P.A. Tanner, M.Y. Huang, Z.L. Shen, and Y.X. Wu. 1997. The acidification process under the cloud in southwest China: observation results and simulation. Atmospheric Environment 31(6): 851–861.
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