Researchers studying the effects of a recent traffic ban in China have made a startling discovery: Curbing the use of motor vehicles caused a substantial and immediate drop in the production of a particular class of smog-causing chemicals--a far bigger reduction than anyone had expected. The finding could help authorities craft strategies for cleaning up one of the world's smoggiest cities during next year's Olympics, and it could guide efforts to reduce smog in other cities around the globe.
Last November, Beijing became a test case for the Chinese government's attempts at pollution control. Both Olympic and health officials are concerned about the effects of smog on world-class athletes during next year's summer games. A major component of smog is NOx, a mixture of nitrous oxide (NO) from vehicle exhaust and nitrogen oxide (NO2), which is formed when NO is released into the atmosphere. NOx can cause a wide array of environmental damage--including smog, acid rain, and even degradation of water quality--as well as endangering the health of human lung tissue. Because vehicle emissions contribute an estimated 70% of atmospheric NOx rising over Beijing, officials enacted restrictions that kept about 800,000 vehicles--nearly 30% of Beijing's total--off of the streets for 3 days last November. Officials immobilized government and private vehicles and lowered bus fares.
The ban had dramatic effects. Scientists using the data from Dutch-Finnish Ozone Monitoring Instrument aboard NASA's polar-orbiting Aura satellite, plus computer models, found that atmospheric concentrations of NOx immediately dropped by 40%, compared with levels a week before and 2 weeks afterward. The Chinese action "clearly demonstrated the effectiveness of traffic restrictions on reducing major air pollutants in urban areas," says atmospheric scientist and lead researcher Yuxuan Wang of Harvard University, whose team reports its findings in the 28 April issue of Geophysical Research Letters. "Controlling vehicular emissions would be an important step in achieving better air quality in the urban environment," she says.
The findings make sense, says atmospheric chemist Christopher Cantrell of the National Center for Atmospheric Research in Boulder, Colorado. "In urban areas, motor vehicles are the main source of NO2," he notes. "If you cut back on emissions, you would expect a similar result." The more impressive aspect of the data, says Cantrell, is the high resolution of the Ozone Monitoring Instrument, which allows measurements from space of areas as small as cities.