WASHINGTON — After studying some 25 years worth of data collected by balloons, ground-based instruments and satellites, a group of scientists has determined that ozone within a key layer of the atmosphere is recovering from past declines, at least in part due to an international accord that limits the release of ozone-depleting substances into the atmosphere.

Preservation of the ozone layer is critical for keeping harmful ultraviolet radiation from seeping through to the Earth’s surface, posing health hazards such as skin cancer. For their study, the scientists zeroed in on two distinct yet adjacent regions of the stratosphere : one between 11 and 18 kilometers in altitude, and the other between 18 and 25 kilometers , said Mike Newchurch, a professor of atmospheric sciences at the University of Alabama in Huntsville .

In the higher region, the scientists observed that declines in ozone levels that began in the 1980s have come to a halt , Newchurch said in a telephone interview Sept. 5. In the lower region, observations indicate that the tide has reversed itself and ozone levels are actually increasing , he added.

The latter observation caught scientists off guard , according to Joseph Zawodny, a senior research scientist at NASA’s Langley Research Center in Hampton, Va.

“The surprise was that levels were not only flattening, but there was a tendency towards an increasing of the ozone layer,” Zawodny said. “That was rather startling, and we brought in other data sets in order to confirm that figure.”

Newchurch and Zawodny participated in the study as part of a team that included representatives from NASA’s Jet Propulsion Laboratory in Pasadena, Calif., the Georgia Institute of Technology in Atlanta, Hampton University of Hampton, Va., and the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo.

The results of the NASA-funded study were published Sept. 9 in the Journal of Geophysical Research, according to Eun-Su Yang of the Georgia Institute of Technology , who was the lead author on the paper.

The study team attributed the trends observed in the two regions of the stratosphere in large part to a 1987 international accord called the Montreal Protocol on Substances that Deplete the Ozone Layer and succeeding amendments , according to an Aug. 30 press release from the Georgia Institute of Technology . The protocol requires signatories to decrease ozone-depleting emissions, such as aerosols from spray cans.

“That’s a great success for what we consider one of the most significant geopolitical decisions,” Newchurch said.

If the current trends continue , Newchurch said, ozone in the vertical region between 11 and 18 kilometers could return to pre-1980s levels . He predicted that it will take approximately 50 years for this to occur, give or take a decade .

Newchurch said the changes being observed are far more dramatic than predicted based on the emission limits imposed by the Montreal Protocol, and suggested that other factors may be at work.

In an e-mail Sept. 6, Yang said changes in atmospheric transport patterns — the movement and distribution of chemicals in the atmosphere — also could be playing a role in ozone recovery.

Phenomena such as global warming or El Nio might be driving the latest atmospheric transport patterns, Zawodny said.

The team used data from five different satellite instruments for the research , including two versions of NASA’s Stratospheric Aerosol and Gas Experiment satellite sensor and the Halogen Occultation Experiment sensor, Newchurch said.

“These sensors used two different techniques to measure ozone amounts,” Newchurch said. “It turns out they had very, very similar results, and we have a high level of confidence that they are both right.”

The scientists also measured ozone levels throughout the atmosphere using Total Ozone Mapping Spectrometer and the Solar Backscatter Ultraviolet satellite instruments.

The satellite data was combined with direct measurements taken by atmospheric balloons , as well as with readings by more than 100 ground-based spectrometers that measure the intensity of the sunlight that penetrates through the ozone layer to reach Earth.

Scientists specializing in atmospheric dynamics likely will build off the NASA team’s work and try to figure out what is causing the stratosphere improvement to occur even more quickly than anticipated, he added.

Zawodny said the work represents an important conclusion in the overall scheme of ozone-related study.

“I feel self-fulfilled just to be part of the initial assessment work on the ozone layer, and to see the governments come forward and take action, and to finally see that have an affect on the atmosphere,” he said. “I would have never thought that the politicians would get together, listen to what the scientists were saying, take action, and that we’d have closure.”