Improving Urban Air Quality in China: Beijing Case Study

TECHNICAL PAPER

ISSN 1047-3289 J. Air & Waste Manage. Assoc. 55:1298 –1305 Copyright 2005 Air & Waste Management Association

Improving Urban Air Quality in China: Beijing Case Study Jiming Hao and Litao Wang Department of Environmental Science and Engineering, Tsinghua University, Beijing, China

ABSTRACT China is undergoing rapid urbanization because of unprecedented economic growth. As a result, many cities suffer from air pollution. Two-thirds of China’s cities have not attained the ambient air quality standards applicable to urban residential areas (Grade II). Particulate matter (PM), rather than sulfur dioxide (SO2), is the major pollutant reflecting the shift from coal burning to mixed source pollution. In 2002, 63.2 and 22.4% of the monitored cities have PM and SO2 concentrations exceeding the Grade II standard, respectively. Nitrogen oxides (NOx) concentration kept a relatively stable level near the Grade II standard in the last decade and had an increasing potential in recent years because of the rapid motorization. In general, the air pollutants emission did not increase as quickly as the economic growth and energy consumption, and air quality in Chinese cities has improved to some extent. Beijing, a typical representative of rapidly developing cities, is an example to illustrate the possible options for urban air pollution control. Beijing’s case provides hope that the challenges associated with improving air quality can be met during a period of explosive development and motorization. INTRODUCTION A dramatic increase in the rate of urbanization is taking place throughout the world. In 1975, only a little more than one-third of the world population lived in cities, but by 2025 this fraction is estimated to be increased to twothirds. The population in cities will double between 1990

IMPLICATIONS In the last 10 years, air quality problems in Chinese cities have been transformed from those that associated with traditional coal burning to mixed source pollution because of the continuous, rapid increase of energy consumption and vehicle population. Although urban air quality has shown recent improvement, integrated air pollution control is an important issue that city governments must consider and pursue over a long period of time. The mitigation of air pollution in Beijing, a typical representative of large Chinese cities, demonstrates that air quality can be enhanced coincident with high-speed urban development.

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and 2025, and 90% of that increase will be in developing countries. Every 1% increment of city population in developed countries equates to about a 3.5% increment in developing countries.1 China reflects this rapid urbanization. Figure 12 depicts the rate of increase of total population and urbanization in China from 1978 to 2003. In the 11-yr period of 1990 –2001, the urban population increased 10%, from 26 to 36% of the population. In developed countries, when the urbanization level reaches 30%, the rate of city growth will markedly increase. Urbanization in China accompanies industrialization and modernization and improves societal development with benefits to the population. But at the same time, it puts substantial pressure on public facilities and natural resources. Cities consume natural resources and produce a large quantity of wastes to be digested within and outside the cities that results in large-scale environmental problems. Along with the unprecedented high-speed economic growth, many cities in China suffer from various environmental problems, such as air-quality degradation, water pollution and water shortage, excess solid waste, resource depletion, and so on. Urban air pollution is one of the major environmental issues. This article gives a brief introduction to the characteristics of urban air pollution problems in China and presents the challenges and the possible solutions by using Beijing as a case example. Air Pollution in Chinese Cities Current Status. In general, urban air pollution in Chinese cities is serious. According to the Report on the State of Environment in China,3 in 2002, of the 343 monitored cities, only about one third met the Grade II National Ambient Air Quality Standard (Table 1) for general residential areas, and 107 cities (31.2%) were heavily polluted exceeding the Grade III standard (Figure 2). Nearly threefourths of China’s urban population was living under harmful air quality conditions (Figure 3). Air pollution is one of the important factors that influence people’s health and welfare in the cities. Figure 4 shows the air pollution status of Chinese cities of different sizes. Air pollution in mega- and large cities is more severe than in medium and small cities, Volume 55 September 2005

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Figure 3. Percent of urban population subject to different air quality in China in 2002.

Figure 1. Population and urbanization rate increase in China.

especially in large cities with the population between one and two million. These cities are provincial capitals or more economically developed cities and often have a large energy, chemical, or iron and steel industry. They mostly suffer from coal-combustion pollution and have less experience, technical support, and economic power to prevent air pollution compared with the megacities, Table 1. Concentration limits for some pollutants in national ambient air quality standard.

Pollutants SO2

TSP PM10 NO2

CO O3

Averaging Time

Grade I (mg/m3)

Grade II (mg/m3)

Grade III (mg/m3)

Annual Daily Hourly Annual Daily Annual Daily Annual Daily Hourly Daily Hourly Hourly

0.02 0.05 0.15 0.08 0.12 0.04 0.05 0.04 0.08 0.12 4 10 0.16

0.06 0.15 0.50 0.20 0.30 0.10 0.15 0.08 0.12 0.24 4 10 0.20

0.10 0.25 0.70 0.30 0.50 0.15 0.25 0.08 0.12 0.24 6 20 0.20

Notes: The Grade I standard is for natural reserves, national parks, and other protected areas. The Grade II standard is for urban residential, commercetraffic-resident mixed, common industrial, and rural areas. The Grade III standard is for special industrial areas.

Figure 2. Grading of urban air quality in China in 2002. Volume 55 September 2005

which are the most economically developed in China and have had a history of serious air pollution. Air pollution in the megacities began to transform from coal-combustion products to the mixed-source type in recent years. The medium and small cities still suffer from coal-combustion pollution, and, in general, more population equates to worse air quality. Although 30% of the area in China is included in acid rain-controlled zones, and 90% of the cities in these areas recorded acid rains in 2002,3 the major pollutant in Chinese cities has switched from SO2, a major constituent of acid rain, to particulate matter (PM).4 In 2002, 63.2% of the monitored cities had PM concentrations exceeding the national Grade II ambient standard. PM pollution in northern cities is more severe than in southern cities, especially in the cities in the north-central, northwest, northeast, central plains, and the eastern part of Sichuan Province and Chongqing City. This is partly because northern cities have to burn much more coal in the cold season for heating, and more fugitive dust exists, leading to PM pollution because of the dryer climate and lower vegetation coverage. Of all cities, 22.4% had sulfur dioxide (SO2) concentrations above the Grade II standard, mainly in Shanxi, Hebei, Guizhou, Sichuan, Gansu Provinces, and Chongqing City.3 The annual average of Air Pollution Index (API),5 a pollutant-integrated indicator of air quality, is another indicator that air pollution in northern cities was more severe than in southern cities. There is a high-level API

Figure 4. Air pollution in cities of different size in China in 2002. Journal of the Air & Waste Management Association 1299

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Figure 5. Air quality comparison of some world cities in 2000.

area in northern China from Urumqi, Xining city in the west, to Shenyang city in the east, including Xining, Lanzhou, Xi’an, Taiyuan, Shijiazhuang, Beijing, and Tianjin. The lowest API appeared in southern coastal cities.6 Figure 5 compares the annual average concentrations of total suspended particulates (TSPs), SO2 and nitrogen dioxide (NO2), of some cities in the world in 2000.7 PM pollution in Chinese cities was severe compared with cities in either developed countries or developing countries. The concentration of SO2 and NO2 were also high in China, indicating the appearance of a mixed pollution pattern in large cities caused by coal combustion and vehicle emissions. Chinese cities need to substantially improve their urban air quality to compare favorably with cities in developed countries. Trends in Recent Years. In the past 2 decades, China experienced rapid economic development, and, since 1996, the annual gross domestic product has grown between 7 and 10% (Figure 68). This development was accompanied by the rapid growth of energy consumption and vehicle population (Figure 79). The total energy consumption in China increased ⬃50% from 987 million tons of coal equivalent in 1990 to 1,482 million tons of coal equivalent in 2003, and the motor vehicle population growth rate increased by ⬃10% annually in this period. This rapid development placed a large burden on the urban atmosphere. Fortunately, apparent progress was achieved in air pollution prevention and control in this period. The air pollutants did not increase as quickly as the energy consumption and vehicle population, as shown in Figure 8.10 The national SO2 emissions increased ⬃30% from 1991 to 2003, and fly ash was even reduced by 20%. 1300 Journal of the Air & Waste Management Association

The air quality in Chinese cities was improved to some extent. Figures 9–11 show the annual average concentrations of SO2, TSP, and nitrogen oxides (NOx) in Chinese cities from 1990 to 2002.11–14 In general, air pollution in northern cities was more severe than in southern cities. The average SO2 concentration was reduced by 44.3% from 93 ␮g/m3 in 1990 to 52 ␮g/m3 in 2002, and, after 1998, it reached the National Grade II ambient standard. This improvement was mainly because of the industrial SO2 emission control and clean energy actions taken in most Chinese cities. TSP pollution also showed an apparent mitigation with the average concentration decreasing from 387 ␮g/m3 in 1990 to 290 ␮g/m3 in 2002. This represents a TSP reduction of 33.8% in southern cities, which met the national standard in 1998, and 27% in northern cities. Despite the progress in the control of TSP pollution, the traditional control equipment developed in this period were not highly effective in removing fine PM, the control of which needs to be one of the key tasks in urban air quality improvement in the future. NOx pollution was not as serious as SO2 and TSP in Chinese cities; however, its concentration was not

Figure 6. Gross domestic product growth in China from 1990 to 2003.

Figure 7. Energy consumption and vehicle population growth in China. Volume 55 September 2005

Hao and Wang with elsewhere in China. NOx concentration in megacities grew from 0.058 mg/m3 to 0.075 mg/m3 from 1990 to 1998.11 One of the major reasons was the explosive growth of the vehicle population. Because NOx has multiple impacts on the atmosphere, such as acid deposition, photochemical pollution, and secondary particles, its pollution control will be an important problem in mega- and large cities. Figure 12 shows the record of urban air quality in China from 1998 to 2003.10 An improved tendency can be seen, because the percentage of cities reaching the Grade II standard increased from 27.6% in 1998 to 41.6% in 2003, although, in general, air pollution status is still serious in Figure 8. SO2 and fly ash emission in China. Note: The SO2 and fly ash Chinese cities. Regarding the API, northern cities emissions of 1991–1994 didn’t include the emissions from Township and Village generally showed improvement (decreasing API), Industrial Enterprises (TVIE). whereas southern cities had an API increase from 2001 to 2003, although the absolute level of the API in reduced in these years. Since 1996, NOx concentration in northern cities was higher than in southern urban censouthern cities slightly increased because of the higher ters.6 speed of industrialization and motorization compared

Figure 9. Annual average SO2 concentrations in Chinese cities in 1990 –2002.

Figure 10. Annual average TSP concentrations in Chinese cities in 1990 –2002. Volume 55 September 2005

Options: The Case of Beijing Status and Challenges. Beijing has undergone a rapid economic growth and motorization in the last several decades. As shown in Figure 13,15,16 the rising gross domestic product kept the vehicle growth rate above 10% for most years since 1978, and the average rate of increase of the motor vehicle population was ⬃15% since 1997. In August 2003, the motor vehicle population in Beijing exceeded 2 million, and it is estimated that this number will rise to 3.5 million in 2008 when the 24th Olympic Games will be held (Figure 13). This rapid development and motorization put a heavy pressure on the urban atmosphere of Beijing. In the1990s, it was listed among the world’s top 10 most polluted cities and suffers from the mixed-source air pollution caused by coal combustion, vehicle exhaust, fugitive dust, and other sources. Beijing’s case reflects the problems that other rapidly developing cities in China must face. Air pollution in Beijing was very severe in the 1990s. In 1998, Beijing began to publish weekly air quality reports according to which air quality for ⬃85% of the days exceeded the Grade II standard. The urban average concentrations of TSP, SO2, NOx, and carbon monoxide (CO) in the heating season were 431, 252, and 201 ␮g/m3, and 4.4 mg/m3, respectively, all of which exceeded the Grade II standard. NOx and CO pollution in traffic areas were severe (Table 2). Ozone (O3) concentration in 101 days or 504 hr in 1998 was beyond the standard, and the highest hourly concentration was 384 ␮g/m3, 1.4 times the standard.17 As the political, economic, cultural, and educational center of China, Beijing cannot avoid rapid geographical Journal of the Air & Waste Management Association 1301

Hao and Wang The first 6 stages of controls were mainly aimed at SO2, NOx, and PM pollution control from coal burning, industry, vehicle exhaust, and fugitive dust. The impact of these measures resulted in lowering the emissions of SO2, NOx, and PM10 (PM with an aerodynamic diameter of ⱕ 10 ␮m) by 22.15, 21.42, and 6.27%, respectively, and a reduction in the atmospheric concentrations by 36.3, 18.7, and 5.3%, respectively, by the end of the 4th stage in October 2000.18 The seventh to tenth stages of emergency control measures from November 2000 to the present mainly focus on PM pollution, total emission control, and ecological protection and development. In 2003, Euro-II emission standards were implemented for new vehicles in Beijing, and Figure 11. Annual average NOx concentrations in Chinese cities in 1990 –2002. Euro-III is scheduled to be implemented in 2005. Notes: NOx and NO2 concentrations in 2000 are not exact because of the lack of The changes of annual average concentrathe data of some cities. The concentration limit for NOx was abolished and tions of SO2, NOx/NO2, and TSP/PM10 in Beijing replaced by that of NO2 in 2000. are shown in Figures 15–17. A rapid decline of 44% is evident for SO2 concentration from 120 expansion and economic growth in the near future. At 3 ␮g/m in 1998 to 67 ␮g/m3 in 2002. NOx concentration the same time, as the most important center in China for decreased from 1998 to 2000 but then slightly increased international trade and communications, air quality in along with the vehicle population. The concentration of Beijing is very important for the benefit of its inhabitants PM remained high without noticeable change in Beijing. for the image of the country. Air pollution has been a The trends for O3 air quality standard attainment days high-profile issue to the local and national government and O3 nonattainment days are shown in Figure 18, and to the people living in the city especially, because it was selected to host the 2008 Olympics. Beijing will face a considerable challenge to improve its air quality while its economic development and vehicle population rapidly increase. Measures and Effects To achieve this goal, the Beijing municipal government has implemented 10 stages of comprehensive emergency control measures since December 1998 (Figure 14). In addition, a series of new local emission standards were promulgated and implemented in this period (Table 3).

Figure 13. Growth of gross domestic product and motor vehicle population in Beijing. Note: The motor vehicle population after 2003 was projected.

Table 2. Annual average concentrations of NOx and CO in traffic environment in beijing.17

NOx CO

Inside Second Ring (mg/m3)

Second to Third Ring (mg/m3)

Third to Fourth Ring (mg/m3)

Outside Fourth Ring (mg/m3)

Grade II Standard (mg/m3)

0.220 8.4

0.219 7.3

0.197 3.6

0.124 —

0.100 4.00

Figure 12. Grading of urban air quality in China in 1998 –2002. 1302 Journal of the Air & Waste Management Association

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Hao and Wang compares the energy consumption in six Chinese cities. It shows that Beijing still has an extensive economic growth pattern, and energy consumption in the industrial sector is very high. There is still a long way to go for Beijing to optimize its economic and energy structure. Another key issue is transportation. Table 6 compares road density in some world cities indicating the insufficient traffic infrastructure in China urban areas. Traffic congestion has been a severe problem and one of the key restrictions of economic development in many large cities, such as Beijing, Tianjin, Shanghai, and Xi’an.19 The serious traffic congestion and an increased vehicle population caused the deterioration of urban air quality. Approximately 74% of CO and 67% of NOx in the atmosphere in Beijing came from vehicle exhaust.20 Vehicle emissions not only increased the urban pollutant concentrations but also induced extremely high pollutant concentrations in traffic environments, which is harmful to public health. A distinction between ChiFigure 14. Ten stages of comprehensive emergency control measures in Beijing. nese cities and those in develwhich indicates that the O3 air pollution in Beijing was oped countries is that densely populated urban areas were mitigated since 1998. The comprehensive control meaalready formed before the rapid motorization, especially sures implemented by Beijing municipal government in Beijing, which has a long history and very congested were notably effective. old-city area. Chinese cities have much more difficulty in getting time and space to adapt the traffic infrastructure to the travel demand. Public transportation needs continFuture Considerations ued emphasis. It is estimated that in the mid 1990s in the Although inspiring progress was achieved to improve air urban Beijing area, ⬃56% of the road space is occupied by quality, important problems remain. Annual SO2 concenprivate vehicles, company cars, and taxis that carry ⬍10% tration in Beijing approached the Grade II standard, but of the passengers. The remaining 90% of travelers are its levels in the cold season were severe because of coal moved by buses, which occupy only 25% of the road combustion for heating (Figure 19). The Beijing governspace.21 Public transportation has higher transport effiment has put much effort into improving the energy structure, but coal consumption was not obviously reciency and lower environmental impact. Many Chinese duced from 1999 to 2003 because of the rapid growth of cities focus on the construction of a public transportation energy use (Table 4). Cleaner fuels, such as natural gas, system as a primary strategy to solve future transportation were mainly used to meet new energy demands. Table 5 problems. Volume 55 September 2005

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Hao and Wang Table 3. Beijing local emission standards. Implementing Time 10/01/2003 10/01/2003 10/01/2003 04/01/2003 04/01/2003 03/01/2003 03/01/2003 03/01/2003 03/01/2003 03/01/2002 07/15/2001 01/01/2001 01/01/2000 04/01/1999 04/01/1999 01/01/1999

Standards Emission controls and limits for oil-gas from gas station Emission controls and measurement standard for oil-gas from fuel depot Emission controls and measurement standard for oil-gas from tank truck Limits and measurement methods for exhaust pollutants from nonroad diesel engines Limits and measurement methods for exhaust visible pollutants from nonroad diesel engines Limits and measurement methods for exhaust smoke under lugdown test from agricultural vehicles Limits and measurement methods for exhaust pollutants from motorcycles and mopeds under steady-state loaded mode Emission standard for exhaust pollutants from gasoline vehicles under steady-state loaded mode Exhaust smoke standard for diesel vehicle under lug-down test Integrated emission standard of boilers pollutants Emission standard for smoke at free acceleration from vehicles with diesel engines Emission standard for exhaust emissions from motorcycles and mopeds Emission standard for exhaust pollutants from gasoline engines of vehicles Emission standard for pollutants at double idle speed from vehicle with petrol engine Emission standard for smoke at free acceleration from farm vehicles and tracker with diesel engines Emission standard for exhaust pollutants from light-duty vehicles

Figure 16. Annual average NOx/NO2 concentrations in Beijing. Note: The concentration limit for NOx was abolished and replaced by that of NO2 in 2000.

reach the national ambient air quality standards. PM was the major pollutant affecting the urban air quality, particularly in northern cities. Nevertheless, the rapid growth of the economy and energy consumption in Chinese cities did not bring the same speed increase of air pollutants emission and air quality degradation. As a whole, air quality in Chinese cities was improved in recent years, especially urban SO2 and TSP concentrations. As the capital city of China and the city responsible for the 2008 Olympic games, Beijing has put a lot of effort in air

CONCLUSIONS Air pollution problems induced by high-speed urbanization, rapid economic growth, and explosive motorization in Chinese cities pose a direct threat to long-term economic sustainability and social benefit. Air pollution problems in Chinese cities are serious, especially in large cities. About two-thirds of the monitored cities did not

Figure 17. Annual average TSP/PM10 concentrations in Beijing.

Figure 15. Annual average SO2 concentrations in Beijing.

Figure 18. Air quality attainment days and O3 nonattainment days in Beijing.

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Hao and Wang development in other Chinese cities. Additional optimization of the economic and energy structure and improvement of the transportation system needed. The control and mitigation of urban air pollution is a laborious task that China will have to deal with for a long time. ACKNOWLEDGMENTS This project research is sponsored by National Basic Research Priorities Programme (Project No. G1999045700). REFERENCES Figure 19. Temporal variation of SO2 concentration in Beijing.

pollution control from coal burning, industry, vehicle exhaust, and fugitive dust, and the effort has had a positive impact. It provides a hope for rapid economic growth without environmental degradation and also a model for urban Table 4. Coal Consumption in Beijing. Year

1999

2000

2001

2002

2003

Coal consumption (Mt)

26.5

27.1

26.8

25.3

26.8

Table 5. Comparison of energy consumption in six cities in China.

Cities Beijing Tianjin Shenyang Xi’an Jinan Taiyuan

GDP Energy Energy Industrial per Consumption Intensity Energy Capita per by GDP Consumption GDP (108RMB) (RMB) Capita (tce) (tce/104RMB) Fraction (%) 2478.80 1639.36 1116.10 688.01 952.18 346.53

22,460 17,900 16,291 10,000 16,999 11,386

3.35 2.88 1.35 1.05 1.43 4.18

1.50 1.60 0.83 1.05 0.84 3.72

71.35 61.51 49.10 52.13 — 80.80

Notes: Reprinted with permission from Wang, X.H., et al. (2004). Copyright 2004 Energy of China. RMB ⫽ Renminbi; GDP ⫽ gross domestic product. Table 6. Comparison of urban road density

City Beijing Shanghai Guangzhou Dalian Tokyo Osaka London New York

Road Density (km/km2)

Percentage of Road Area

Road Area per Capita (m2)

6.8 7.6 7 12.6 18.9 18 18.1 8

7.1 12.6 7.7 6.46 14.9 17.5 24.1 16.6

4.7 5.4 5.2 5.7 10.9 14.4 28 26.3

Note: Reprinted with permission from He, K.B.; Chang, C. Copyright Asia Development Bank. Volume 55 September 2005

1. World Resources 1996 –97; World Resources Institute, United Nations Environmental Programme, United Nations Development Programme, World Bank, China Environmental Science: Beijing, China, 1996. 2. China Population Statistics Yearbook 2004; National Bureau of Statistics of China: Beijing, China, 2004. 3. State Environmental Protection Agency. 2002 Report on the State of Environment in China; Environ. Protect. 2003, 7, 3-13. 4. Ren, Z.H.; Wan, B.T.; Su, F.Q.; Gao, Q.X.; Zhang, Z.G.; Hong, Z.X.; Hu, F.; Cheng, S.Y. Several Characteristics of Atmospheric Environmental Quality in China at Present; Res. Environ. Sci. 2004, 17, 1-6. 5. He, K.B.; Huo, H., Zhang, Q. Urban Air Pollution in China: Current Status, Characteristics, and Progress; Annual Review of Energy Environment 2002, 27, 397-431 6. Tian, L.; Liu, X.D.; Lu, R.Y.; Wang, W. Temporal and Spatial Characteristics of Urban Air Pollution in Mainland China in 2001–2003. Proceedings of the Ninth Mainland-Taiwan Environmental Protection Academic Conference 1, Xi’an, China, May 2004, pp 933-936. 7. Zhu, Z.X. International Statistics Yearbook; China Statistics: Beijing, China, 2002. 8. Editorial Committee of China Financial Yearbook. China Financial Yearbook 2003; China Finance: Beijing, China, 2003. 9. China Statistics Yearbook 2004; National Bureau of Statistics of China: Beijing, China, 2004. 10. Report on the State of Environment in China 1991–2003; State Environmental Protection Agency. http://www.zhb.gov.cn/ (accessed July 1, 2005). 11. Report on Air Pollution Prevention and Control in China; State Environmental Protection Agency, China Environmental Science: Beijing, China, 2000. 12. China Environmental Yearbook 2001; State Environmental Protection Agency, China Environmental Science: Beijing, China, 2001. 13. China Environmental Yearbook 2002; State Environmental Protection Agency, China Environmental Science: Beijing, China, 2002. 14. China Environmental Yearbook 2003; State Environmental Protection Agency, China Environmental Science: Beijing, China, 2003. 15. IMI Consumer Behaviors & Life Patterns Yearbook 02– 03, Beijing Chuangyan Research Institute, Beijing Broadcasting Institute: Beijing, China, 2003. 16. Beijing Statistical Yearbook 2003; Beijing Municipal Statistics Bureau, China Statistics: Beijing, China, 2003. 17. 1998 Report on the State of Environment in Beijing; Beijing Environmental Protection Bureau. http://www.bjepb.gov.cn/ (accessed July 10, 2005). 18. Yuan, J.; Fu, L.X.; Yu, X.C.; Hao, J.M.; He, K.B. Environmental Effect Analysis for Four Stage Measures of Short-Term Air Pollution Control in Beijing; China Environ. Sci. 2002, 23, 9-13. 19. National Plan for Controlling Motor Vehicle Pollution of China; GM-SEPA Collaboration Project, Tsinghua University: Beijing, China; 2001. 20. Hao, J.M.; He, D.Q.; Wu, Y.; Fu, L.X.; He, K.B. A Study of the Emission and Concentration Distribution of Vehicular Pollutants in the Urban Area of Beijing; Atmos. Environ. 2000, 34, 453-465. 21. Walter, H.; Michael, R. Motorization and Non-Motorized Transport in Asia: Transport System Evolution in China, Japan and Indonesia; Land Use Policy 1996, 13, 69-84.

About the Authors Jiming Hao is a professor and dean of the Institute of Environmental Science and Engineering at Tsinghua University. Litao Wang is a Ph.D. student in the Department of Environmental Science and Engineering. Address correspondence to: Litao Wang, Department of Environmental Science and Engineering, Tsinghua University, Beijing, 100084 China; phone: ⫹86-10-6279-4369; fax: ⫹86-10-6277-3650; e-mail: [email protected].

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