An international team of scientists, alarmed by the threat to the Earth's ozone layer, is gathering in California Sunday to prepare for the largest study ever done of the Arctic atmosphere.

During the next two weeks, they will fine-tune scientific instruments in preparation for a $30 million expedition involving 200 scientists from seven nations that will be centered in Stavanger, Norway, from late December through mid-February.

The scientific team _ which includes William Brune of the Pennsylvania State University _ will use high-altitude aircraft, balloons, satellites and sophisticated computers to seek to understand how the ozone layer is being eroded by manmade chemical pollutants in the Northern Hemisphere.

The expedition will also include experiments in the northern regions of Greenland, Canada and the Soviet Union.

"If we can understand what is going on in the Arctic, it will help us understand what is happening with the ozone layer over Philadelphia and other American communities," said Charles Redmond, a spokesman for NASA, which is coordinating the expedition.

In May, scientists from Harvard University and the National Aeronautics and Space Administration said they had found "important and alarming" indications that chemicals were attacking the ozone layer, the gaseous shield 12 to 30 miles above the planet, in northern Canada and Greenland. This fragile layer of gases protects life on the planet from deadly ultraviolet rays given off by the sun.

This marked the first time scientists had direct chemical evidence to show that the same sort of erosion of the ozone layer that has been documented in Antarctica was also occurring in the Northern Hemisphere. Scientists suspect that manmade chemicals such as chlorofluorocarbons (CFCs), which are used in refrigerants and insulation, are damaging the ozone layer worldwide.

In March, an international team of 100 scientists reported that the levels of ozone in the upper atmosphere had declined by as much as 3 percent from 1969 to 1986 over the northern United States, Europe and the Soviet Union. That same report said that, farther north, the ozone had declined by more than 6 percent in Alaska, Scandinavia and other northern latitudes.

Scientists estimate that for each percentage-point drop in the ozone layer there may be a 5 to 7 percent increase in the number of cases of skin cancer. And they say that manmade pollutants such as CFCs may be warming up the Earth's atmosphere, changing wind and rainfall patterns and damaging plants and marine animals.

International concern about the threat to the Earth's ozone layer led to a treaty in Montreal in 1987 to dramatically reduce the production of CFCs. It has been signed by more than 30 countries, including the United States. However, many scientists say that recent findings show the Montreal treaty does not go far enough and that more stringent measures are now needed.

Until recently, most of the concern about the ozone layer has been centered on Antarctica, where British scientists announced in 1985 that they had discovered over the South Pole a "hole" in the protective shield the size of the United States. This hole forms each year during the Antarctic spring, which occurs from September to November.

Scientists say the Arctic "hole" occurring over Norway and other Arctic areas each year is considerably smaller and more erratic in shape than the hole found over the South Pole. But the potential impact of the Arctic hole is much greater than the Antarctica hole because far more people live in the heavily populated Northern Hemisphere.

Redmond, of NASA, said scientists involved in the research effort, known as the Airborne Arctic Stratospheric Expedition, would use a Nimbus-7 research satellite to determine where clouds known as polar stratospheric clouds were located. Ice crystals needed for the chemical reaction that causes ozone depletion to occur are found inside these clouds.

Researchers will then fly two research airplanes _ a DC-8 and a NASA ER-2, a modified U-2 spy plane _ through these regions to measure temperature, winds, pressures, concentrations of chlorine, bromine, nitrogen, water, ozone, CFCs and other chemicals.

As data are collected, they will be continuously fed into a sophisticated computer to determine what chemical reactions are occurring. During the 45-day expedition scientists expect about a dozen flights of each of the planes will occur.

Redmond said the research expedition would use the same aircraft and much of the same equipment that was used to study the ozone layer over Antarctica in August and September 1987.

"The Antarctica experiment was relatively easy because we knew there was a definite ozone hole," he said. "The Arctic is a more complicated situation. The conditions are not as well defined."

During the same period that researchers zigzag airplanes through the ozone above Norway, scientists in the Soviet Union will use aircraft and ground instruments to conduct measurements and observations in northern Siberia and Franz Josef Land, a cluster of islands in the Arctic Ocean north of Novaya Zemlya.

Danish, British, West German and Canadian scientists will also collect information by launching weather balloons in Canada, Greenland, Norway and the northern British Isles, said Michael Kurylo, a NASA scientist and one of the expedition's directors.

Brune, of Pennsylvania State University, will use instruments attached to the left wing of the ER-2 aircraft to measure chlorine and bromine, two of the chemicals implicated in the erosion of the ozone layer.