New data gathered from satellites and ground-based stations support the idea that much of the destruction of Antarctic ozone involves the action of cosmic rays, says a physicist in Canada. This goes against the widely-accepted notion that the ozone layer — which shields Earth from harmful ultraviolet radiation — is depleted via the action of direct sunlight.

Qing-Bin Lu of the University of Waterloo also predicts, given the timing of the 11–year cosmic-ray cycle, that the ozone hole will be particularly large in 2008–09 and 2019–2020 (Phys Rev Lett 102 118501 ).

Broken up by light or electrons?

Over the Antarctic, ozone concentration has dropped to as low as one third of its pre–1975 levels, with this ozone “hole” occurring during the southern polar spring. The conventional understanding of the depletion process is that chlorofluorocarbon (CFC) pollutants are broken down by the Sun’s ultraviolet light. This occurs at high altitudes (around 40 km), and the CFC fragments are then transported to lower altitudes (below 20 km) via air circulation. In the Antarctic winter these fragments settle on ice particles, where a number of chemical reactions convert them into molecular chlorine. The arrival of sunlight in the Antarctic spring then releases atomic chlorine, which destroys ozone.

There is no need for this extra mechanism because the chlorine can be produced by direct sunlight Neil Harris, European Ozone Research Coordinating Unit

Lu, however, believes that cosmic rays break up the CFCs. He says that when cosmic rays ionize atmospheric molecules the liberated electrons can be stored on the surface of the ice particles and that these electrons, rather than the sunlight, break up the CFCs and convert the fragments into molecular chlorine.

In 1999 and 2001, Lu and colleagues provided evidence to back up this theory by carrying out experiments at low temperatures that showed the breakup of CFCs by electrons is greatly enhanced when the CFCs are placed on a surface along with polar molecular ice. In 2001 Lu also used satellite data to show a correlation between cosmic ray intensity and ozone loss at latitudes between 0 and 65 degrees S. This variation occurred within the cosmic ray cycle taking place between 1981 and 1992.

New data from ground and space

Lu has now improved the case for his model by drawing on more extensive climate data. Using measurements of ozone concentrations taken from NASA’s TOMS and OMI satellites and cosmic-ray data from several ground stations, he has shown that cosmic ray intensity and annual mean total ozone were correlated, at latitudes between 0 and 60 degrees S, between 1980 and 2007 — a period covering two cosmic ray cycles.

He also found a correlation between cosmic ray intensity and the fluctuation in ozone in the Antarctic (between latitudes of 60 and 90 degrees S) from one October to the next between 1990 and 2007.

Any non-cosmic-ray related mechanism, if it exists, must be a very minor or negligible effect Qing-Bin Lu, University of Waterloo

“These correlations mean that nearly 100% of the ozone loss over Antarctica must be driven by cosmic rays,” he says, pointing out that the degree of variation of cosmic ray intensity and Antarctic ozone are very similar (both about 10%). “In other words, any non-cosmic-ray related mechanism, if it exists, must be a very minor or negligible effect.”

2008 prediction

As well as analysing past data, Lu also made a prediction at the time of writing his paper in August last year. He said that the amount of ozone over Antarctica in October 2008 would be about 14.5% lower than it was in October 1992 (his reference point), and that there would also be another significant minimum in 2019–2020. He says the latest satellite data agree with his 2008 prediction to within 5%, and also points out that the Antarctic ozone concentrations in November and December last year were almost record lows.

However, Neil Harris of the European Ozone Research Coordinating Unit in Cambridge, UK, is not convinced. He told physicsworld.com that showing a statistical correlation is not enough to prove the validity of the cosmic-ray mechanism since there could be other causal factors varying throughout the solar cycle. In any case, he says, Lu is wrong to compare cosmic ray intensity against total ozone because measurements of the latter depend on the movement of ozone around the atmosphere as well as the actual disappearance of ozone.

"He has put forward an additional mechanism to explain the creation of atomic chlorine," adds Harris. "But there is no need for this extra mechanism because the chlorine can be produced by direct sunlight."