Mediterranean endures emissions from afar. (Air-Pollution Pileup).

Most Mediterranean countries aren't big polluters. A new survey suggests, however, that the area is a crossroads for pollution-carrying air currents from Europe, Asia, and North America.

According to an international team of 31 atmospheric chemists, climatologists, and others, these currents converge over the Mediterranean Sea, creating pollutant concentrations that routinely surpass the European Union's air-quality standards. The team reports its findings in the Oct. 25 Science.

Previous measurements of unusually high ozone concentrations in the region prompted Jos Lelieveld of the Max-Planck Institute for Chemistry in Mainz, Germany, and his colleagues to attempt a more detailed analysis. In the summer of 2001, they used aircraft as well as atmosphere-measuring stations on Crete and Malta to monitor pollutants and climatic activity.

The researchers found that trace gases, such as carbon monoxide and nitric oxide, and atmospheric aerosol particles were between 2 and 10 times as concentrated over the Mediterranean Sea as in areas above the northern Pacific. "It was rather astonishing," says Lelieveld, "to find similar levels above [the Mediterranean Sea] as you'd expect to find above a ... city."

Computer models suggest that as little as 20 percent of the carbon monoxide found in the lowest atmospheric layers originated in Greece, Yugoslavia, and other Mediterranean nations, the researchers report. Instead, 60 to 80 percent of the gas appears to be coming from Russia, Poland, France, Germany, and other European nations. Northerly summertime winds in the lower atmosphere are to blame for transporting the pollution, says Lelieveld.

The scientists also found that pollution at greater altitudes--4 to 13 kilometers above Earth's surface--had been carded to the Mediterranean from as far away as Asia via westerly winds that pass over the Pacific, North America, and the Atlantic.

It's astounding that pollution can travel across two continents and remain so concentrated, comments Daniel Jaffe, an environmental chemist at the University of Washington in Bothell.

The poor air quality resulting from these transported pollutants has substantial implications not only for health and the environment but also for climate, says Lelieveld.

He and his team speculate that aerosol particles blanketing the Mediterranean area may inhibit cloud formation and thus rainfall, even in places far removed from the Mediterranean Sea. Aerosol particles absorb solar energy that would usually cause evaporation at the sea's surface, says Lelieveld. "High European pollution levels in the 1980s may have been co-responsible for droughts" in countries such as Ethiopia, he says.

Joseph M. Prospero, an atmospheric scientist at the University of Miami, commends the scope of the study but says that more research is required to establish that pollution is responsible for regional droughts. Desert dust can have a similar effect, he adds.

Mediterranean countries themselves have no control over these emissions, notes Jaffe. "Countries need to work together to reduce pollution problems," he says.