Polluted air chokes northern Indian Ocean.

A sooty haze hovered over the northern Indian Ocean this spring, outstripping the worst expectations of an international team of scientists that spent 6 weeks in the area studying the pollution.

The haze stretched from the ocean surface to an altitude of more than 2 kilometers. Surveying the atmosphere from a research aircraft, James A. Coakley of Oregon State University in Corvallis was astounded by the scope of the pollution. "We had a devil of a time finding clean air," he says.

Scientists believe that every year, cooking fires, vegetation burns, and industrial fuel combustion in urban areas of south Asia spawn the buildup of airborne sooty particles, or aerosols. Although summer monsoons probably wash away the aerosols, they quickly mount again, say researchers.

The work is part of an ongoing international effort called the Indian Ocean Experiment, or INDOEX. In February and March, more than 150 scientists from the United States, Europe, and India gathered data from weather satellites, ships, aircraft, and island observatories.

The work found that the aerosols reached as far south as the Intertropical Convergence Zone, close to the equator, where trade winds swirl together and shoot hot air to high altitudes. These winds can sweep pollution to far-flung areas.

"It's very disturbing," comments Anthony S. Wexler of the University of Delaware in Newark. The data suggest that human activity is doing major damage to the environment, he asserts. "The more large-scale perturbations that we see, the more chance that we're doing larger damage," he says.

On one ship, Russell R. Dickerson of the University of Maryland in College Park used paper filters to collect particles. "You didn't need any analytical instruments to know that there was a greasy, black soot all over the filters," he says.

Dickerson was surprised to find that despite the pervasiveness of the haze, it contained very little ozone, a prime component of Los Angeles-type smog. "It suggests that there's some fairly major aspect of atmospheric chemistry that we don't understand yet," he says.

The INDOEX project is the first to experimentally study the effects of the Indian Ocean region's persistent urban pollution on climate, rather than relying on theoretical models of the influence of aerosols, says Coakley.

In this area, sooty, dark aerosols absorb sunlight and contribute to climate warming. Brighter aerosols such as sulfates and nitrates reflect light back into space, exerting a cooling effect. Additionally, the particles seed large and long-lasting clouds, which also scatter light and promote cooling. The researchers are eager to learn which influence dominates.

If the pollution proves to have a net cooling effect, could it be an environmental blessing in disguise?

"Not by a long shot," says V. Ramanathan of the Scripps Institution of Oceanography in La Jolla, Calif., one of the project's chief scientists. He notes that the pollution alters the cycle of rain and evaporation from the ocean. Moreover, he observes, the aerosols don't remain aloft indefinitely. They return to the surface as acid rain.