KEEP an eye on the sky this autumn.Arlin Krueger at the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center in Greenbelt, Md., says that "people can expect ... rosy sunsets" over much of the Northern Hemisphere as the volcanic plume spreads from Mt. Pinatubo's eruption in the Philippines. But atmospheric scientists such as Dr. Krueger are interested in more than sunsets. Pinatubo's eruption has given them an unplanned opportunity to study the effect that such a massive stratospheric injection of volcanic material has on weather and atmospheric chemistry. Krueger has explained it is "possible that the cloud will reflect back into space some of the sunlight that would have reached the ground, resulting in a small change in the heat balance of the Earth." Many scientists will be watching for a possible cooling over the Northern Hemisphere, or even globally, of a few tenths of a degree. Michael Coffey is a member of a team at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., that is analyzing the cloud's composition. He uses spectroscopic data gathered from an aircraft to identify chemical compounds. By establishing early conditions in the cloud and watching how these change, he and his colleagues hope to learn more about the complex chemistry that converts volcanic gases into climate-driving aerosol particles. Dr. Coffey also notes that the unexpected eruption "is going to test" the prediction of his NCAR colleague Guy P. Brasseur that these particles will provide reaction sites for chemistry that will enhance destruction of stratospheric ozone. Until now, the biggest eruption of this century, in terms of material shot to stratospheric heights, was that of El Chichon in Mexico in 1982. Plume-tracker Lamont Poole, assistant head of the aerosol-research branch at the NASA Langley Research Center in Hampton, Va., estimates that Pinatubo's cloud "certainly is at least in the same class as El Chichs, if not larger." Not all eruptions have major atmospheric implications. Speaking recently at his agency's regional office in Menlo Park, Calif., United States Geological Survey volcanologist Robert I. Tilling explained the difference, saying: "Volcanoes are similar in that they all offer channels for molten rock [magma] from deep below to travel to the surface. Volcanoes differ, however, in that the type of rock that forms the magma and the amount of gas in the rock determines whether the material will flow as a heavy molten liquid [as in Hawaii] when it reaches the surface, or whether it will explode from the volcano as ash and other bits of volcanic material." He added, "Some of this explosive material can travel at high speeds down the sides of a volcano in destructive pyroclastic flows, and other material rises high into the atmosphere and later falls to earth as volcanic ash." This spring's eruption of Pinatubo and of Mount Unzen in Japan, which involved pyroclastic flows, reflect the relative movement of great pieces (plates) of Earth's crust along the western Pacific rim. Here, an oceanic plate slides beneath the plate carrying the Philippines and Japan. Their interaction generates molten rock that rises and eventually erupts explosively. Even when such an eruption shoots a massive cloud into the stratosphere, it won't have much of an atmospheric impact unless that cloud is rich in gases that form aerosol particles. An ash cloud may look bad locally, but it falls out too quickly to have a global effect. El Chichon and Pinatubo injected the stratosphere with such gases as hydrogen sulphide and sulfur dioxide. These two gases react chemically with other compounds to produce sulphate particles and droplets of sulfuric acid. The acid particles, especially, remain in the stratosphere for several years and spread around the world. Many scientists believe these aerosols do cool our planet slightly. But the effect hasn't shown up clearly in climatic data, including data for the year following El Chichs eruption. It appears that the cooling may have been masked by a recurring phenomenon called El Nino that involves warming in the equatorial Pacific Ocean. This, in turn, affects global weather. In 1989, Clifford F. Mass and David A. Portman at the University of Washington in Seattle found five cases over the past century in which El Ninos followed large eruptions. Subtracting the El Nino influence, they found probable Northern Hemisphere cooling of from 0.1 to 0.5 degrees C. Their analysis of the El Chichon event, together with an independent study by climatologist James Angell with the National Atmospheric and Oceanic Administration in Silver Spring, Md., suggests that El Chichon did cool the globe by several tenths of a degree. The compensating warming by the 1983 El Nino hid that effect. Once again, an El Nino seems to be starting [see story on Page 12]. This may obscure any cooling by the Pinatubo cloud. Ozone destruction should be more obvious. Last December, Dr. Brasseur, together with his NCAR colleagues Stacy Walters and visiting scientist Claire Granier of the University of Paris, suggested in a paper in the journal Nature that particles in volcanic clouds could enhance the ozone-destroying effect of chlorine compounds already in the stratosphere. They warned that a new eruption on the scale of El Chichon could trigger widespread ozone destruction. They warned it might offset the good effect of the internationally planned curbs on use of ozone-destroying chlorofluorocarbon (CFC) chemicals. This is the prediction that Micheal Coffey expects the Pinatubo cloud to test. Meanwhile, volcanologist Michael Sheridan of the State University of New York at Buffalo is on Siberia's Kamchatka peninsula trying to test his admittedly novel hypothesis that volcanoes themselves may sometimes put CFCs into the stratosphere. He is a leader of a joint American/Soviet expedition to measure the composition of volcanic fluids, which are abundant on the peninsula. Dr. Sheridan notes that there have been suggestions of CFCs in two or three volcanoes, but no one has actually measured them coming out of a volcano. He explains that, if he finds traces of CFCs coming out of the Kamchatka fumaroles, it will point to an unsuspected natural source for what are considered strictly man-made chemicals. "It's a new line of research for me," he says.