Beijing tackles its environmental problems with a new field experiment. (Guest Editorials).

As an extremely large metropolitan area and the host city of the 2008 summer Olympics, Beijing, China, faces a great challenge to improve its air quality as it strives to showcase a "green Olympics" in a few years. What can be done to alleviate the impact of dust storms and reduce air pollution? The Beijing City Air Pollution Observation Field Experiment (BECAPEX) is one step toward improving air quality in the capital city.

The experiment, which was sponsored by the Ministry of Science and Technology of China, is a subprogram of the Beijing Urban Environment Project. The focus of the field experiment carried out in the Beijing metropolitan area in spring 2001 was on the dynamics of air pollution, a key theoretical problem with applications in the formation mechanism, control, and alleviation of air pollution. A three-dimensional observing network using ground-based observations and space-based satellite remote sensing was set up to monitor the urban environment in and around Beijing. It was the first broad attempt at observation in Beijing using advanced observation instruments such as atmospheric profilers, tethersondes, ultrasonic anemometers, and sonic detection and ranging (SODAR).

The results from BECAPEX provided a detailed three-dimensional description of the dynamic and thermal structure of Beijing's urban atmospheric environment, the physical and chemical characteristics of air pollutants and their variations, and the mechanisms of transportation, diffusion, and transformation of air pollution. The data revealed the existence of an air dome (a dome-shape boundary layer) around urban Beijing and provided a comprehensive set of parameters that defined the characteristics of the air dome. The variation of these parameters determines the severity of air pollution in Beijing. Thus, warning signals related to severe pollution events can be detected by monitoring these parameters.

Important findings of this study include the synchronous characteristics of the life cycle of pollutants within the urban atmospheric boundary layer and obvious diurnal variation in the vertical transport of pollutants in the boundary layer or under the air dome. In fact, the data revealed that the air dome acted like a large canopy covering the city of Beijing. This phenomenon was closely associated with the temperature inversion and the vertical structure of the boundary layer. Moreover, the diffusion and distribution of air pollutants were related to the multiscale interaction among large-scale circulation, mountain and valley wind, and urban heat island circulation. As a result, air pollution in Beijing usually comes from local sources, but the sources in the vicinity also have prominent influence.

The primary goal of the experiment was to determine the effect of dust storms on Beijing's urban environment. Each spring, dust storms cause serious weather conditions in Beijing, which affects the entire population. BECAPEX studied the dust storm events that occurred in 2000 and identified the sources of these events. The data showed that most of the dust storms in Beijing during 2000 originated in a large dry area north of the city. High-speed winds caused friction on the ground that allowed the dust-laden winds to penetrate Beijing's urban air dome, resulting in thick, dust-laden air near the ground. These findings laid a solid foundation for developing strategies to improve air quality and for the city's Blue Sky, Clear Water project.

I thank Lian Xie (North Carolina State University, Raleigh, NC) for valuable assistance in translating the editorial into English.

Xiangde Xu is a chief scientist of the Beijing Urban Environment Project and Deputy Director of the Chinese Academy of Meteorological Sciences.

Xiangde Xu
Beijing Urban Environment Project
Beijing, People's Republic of China
E-mail: [e-mail]