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Beijing, the capital of the People’s Republic of China, is located on the North China Plain at an elevation of 44 meters above mean sea level. Beijing is a typical Chinese megacity with a population of more than 11 million. The city, which has undergone rapid development since the 1980s, has high concentrations of particulate matter and poor visibility in spite of the adoption of numerous measures to control particle pollution. Measured annual mean mass concentrations of fine particles (PM10) exceed the Grade II (100 µg/m3) National Ambient Air Quality Standard (NAAQS) (Song et al., 2002a,b). Concentrations of PM2.5, particles with aerodynamic diameters of less than 2.5 µg/m3, are also much higher than the recommended standard for annual average ground-level PM2.5 in the United States (15 µg/m3). The ratio of PM2.5 concentration to PM10 concentration (PM2.5/PM10 ratio) ranges from 0.5 to 0.7, with an average of 0.6, which is about the same as the ratio observed in Europe and the United States. Measurements of PM2.5 performed along wind direction suggest that anthropogenic pollution in urban areas extends to a regional scale under some meteorological conditions and that the whole Beijing-Tianjin area is sometimes covered with a large polluted air mass. Studies in Beijing showed an inverse correlation between elevated concentrations of PM2.5 and visibility based on hourly measurements in June 1999 and January 2000 (Bergin et al., 2001).

High levels of particulate matter and adverse effects have inevitably increased concerns about how fine particles can be controlled. As a basis for developing effective control strategies for fine-particle pollution and improving air quality in Beijing, we must first determine the relative importance of the various sources that contribute to PM2.5. The goals of this study were: (1) to quantify the source contributions to PM2.5 in Beijing by source inventory and chemical mass balance (CMB) model; (2) to compare the results of these two methods; and (3) to investigate the spatial and seasonal variations of source contributions.

EXPERIMENTAL METHODS

Sampling Sites

The samples of airborne PM2.5 used in this study were collected at three sites: College of Chemistry of Beijing Union University (BUU), Chinese Academy of Preventive Medicine (CAPM), and Chinese Research Academy of Environmental Sciences (CRAES).

The College of Chemistry of BUU, located at Fatou town in the southeast of Beijing, is 2 kilometers from the Eastern Fourth Ring Road and 500 meters west of a chemical-industry zone. The sampling location was on the roof of the Main Teaching Building; there were no high-rise buildings nearby. The two roads, to the east and north, had little traffic.

CAPM, situated in the south downtown area of Beijing, is west of the Eastern Secondary Ring Road and 300 meters from Guangming Bridge. Sampling



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