China's air pollution chokes crop growth.

With China's economy and population expanding at breakneck speed, its infamous air-pollution problems have started to claim new victims. Data collected at rural Chinese sites indicate that ozone pollution has already reached the point where it's harming agriculture--raising questions about whether the world's fastest developing country will be able to feed itself in the next century.

"There has been a lot of discussion of whether China can meet its future food demands, but there has been very little discussion of how regional environmental degradation is affecting agriculture there," says William L. Chameides of the Georgia Institute of Technology in Atlanta, who led the study.

When ozone, a highly reactive gas, builds up in the lowest layer of the atmosphere, it harms plants and animals. The pollutant forms when hydrocarbons and nitrogen oxides mingle in the air and get cooked by sunlight. Both these ingredients come from the incomplete combustion of fossil fuels. Hydrocarbons also are released by vegetation.

In a collaboration with Chinese researchers, Chameides and his U.S. team analyzed ozone measurements made at four rural sites in China during 1994 and 1995. These are the first systematic ozone readings taken outside cities in China, according to the researchers.

They found the highest pollutant concentrations at two places: an agricultural town called Linan, near the coastal city of Hangzhou, and a part of Hong Kong Island upwind of the city. At both spots, ozone often exceeded 60 parts per billion, an approximate threshold for harmful effects. Experiments in the United States and elsewhere have shown that the range of ozone exposure at these two Chinese sites is enough to reduce crop yields by 10 percent or more, the scientists report in the April 1 Geophysical Research Letters.

The scientists were surprised to find the highest ozone readings in fall, winter, and spring--the reverse of the U.S. pattern. In China, they say, summer storms are strong enough to clear out polluted air from the coastal sites where the measurements were made.

The highest ozone readings in the study occurred in Linan, which sits in the Yangtze River Delta, one of China's most important agricultural regions. The timing of the air-pollution problems threatens the delta's winter wheat crop, which supplies 20 percent of China's total wheat harvest, say the researchers.

Chameides' team used a computer model to see how far the ozone problem might extend beyond the four measured spots. In the simulation, harmful concentrations of ozone blanketed much of eastern China.

The Chinese pollution measured is "what you'd see at a rural U.S. site in summer, maybe a little bit dirtier," says Jennifer A. Logan, an atmospheric chemist at Harvard University.

Agricultural studies in the United States suggest that ozone pollution reduces U.S. crop yields by $5 billion to $10 billion annually, says Ellis B. Cowling of North Carolina State University in Raleigh.