JOHANNESBURG, South Africa (AP) _ In the once-pristine skies over southern Africa, NASA scientists are finding massive pollution usually associated with the industrial centers of the Northern Hemisphere.

Most of the pollution is caused by peasant farmers burning huge tracts of scrub brush to clear their land, said Jack Fishman, a NASA scientist leading the six-week study in a DC-8 airliner that has been converted into a flying laboratory.

The group arrived in southern Africa in early October after studying a virtually identical problem in Brazil. Together, the pollution from the two regions is thought to be changing the atmosphere throughout the Southern Hemisphere, Fishman said Monday.

''We're finding incredibly high levels of ozone and pollutants over Africa,'' Fishman said in a telephone interview from Windhoek, Namibia, after the cloud's existence was reported Monday by The New York Times.

''The plumes of smoke we see here and over Brazil appear to have an effect on the entire hemisphere,'' he said.

The main problem is an ozone buildup caused by emissions from the fires, which are raging now as farmers prepare for spring planting.

Ozone, found naturally in the upper atmosphere, or stratosphere, is essential for blocking out the sun's ultraviolet rays.

But it also is a pollutant, and when close to the earth's surface, it causes ailments such as respiratory problems and can damage forest and agricultural land.

The Southern Hemisphere was considered relatively free of such problems until recent years, when scientists began studying the burning of the Amazon Basin. Fishman says southern Africa may be just as bad.

''The fires I saw in Zambia were worse than the ones in Brazil,'' Fishman said. ''You can't say what that means after only a couple flights, but it backs up what the satellite data seem to tell us.''

He said the cloud of pollution, stretching from the southwest coast of Africa out over the South Atlantic, has a diameter of about 2,000 miles. The ozone is found from just above the ocean's surface to an altitude of six miles.

NASA, the U.S. space agency, is the main backer of the study. But some 180 scientists from 13 countries are involved in what is considered the most detailed atmospheric study ever of the South Atlantic, said Richard Bendura, NASA's expedition manager.

The region has not been extensively studied because it was thought to be relatively free of pollution and lacked the major storms that attract scientists' attention.

Fishman said he became interested in the 1980s when satellite data revealed clouds similar to those over polluted parts of the Northern Hemisphere.

''It was puzzling,'' said Fishman. ''No one knew exactly what it was.''

Scientists gathered more information about the pollution three years ago when they launched weather balloons from the West African nation of the Congo and from Ascension Island in the South Atlantic, Fishman said.

The current project is part of a worldwide atmospheric survey NASA has been conducting over the past several years.

The team will spend 10 days flying over the Atlantic from Namibia and then complete field work with flights from Ascension Island.

Because much of the testing can be done aboard NASA's plane, the scientists hope to compile their findings within weeks.