Discovery reveals large, year-round ozone hole over tropics

Ozone (O₃) is an important component of the stratosphere, between 20 and 30 km above Earth’s surface. It absorbs much of the ultraviolet radiation that comes from the sun, thus protecting living organisms from the destructive effects of this form of radiation. 

In the mid-1970s, atmospheric research suggested the ozone layer might be depleted because of reactions between ozone and certain industrial chemicals, primarily chlorofluorocarbons (CFCs). This scenario was confirmed in 1985 when scientists discovered a large hole in the ozone layer above the Antarctic. The use of CFCs was then banned, which slowed ozone depletion. However, a Canadian scientist has now identified the presence of another large hole in the ozone layer, this time above the tropics, indicating that ozone depletion has persisted despite the reduction in CFCs

In a new research paper, published in AIP Advances, Qing-Bin Lu from the University of Waterloo in Ontario, Canada, describes a large, year-round ozone hole in the lower stratosphere over the tropics. This hole is characterized by having at least 25 percent loss of ozone when compared to unaffected atmosphere. The new hole is comparable in depth to that of the well-known springtime Antarctic hole, but is seven times larger in area. 

“The tropics constitute half the planet’s surface area and are home to about half the world’s population,” said Lu. “The existence of the tropical ozone hole may cause a great global concern.”

“The depletion of the ozone layer can lead to increased ground-level UV radiation, which can increase risk of skin cancer and cataracts in humans, as well as weaken human immune systems, decrease agricultural productivity, and negatively affect sensitive aquatic organisms and ecosystems,” he explained.

As is the case with the polar hole, ozone is not completely absent from the gap in the stratosphere above the tropics, but the concentrations are significantly reduced. At the center of both holes, ozone has been depleted by approximately 80 percent, when compared with undisturbed atmosphere. Preliminary reports show that depletion levels over equatorial regions are already endangering large populations and the associated UV radiation reaching these regions is far greater than expected.

Lu’s discovery of an ozone hole above the tropics is a surprise to the scientific community as it was not predicted by conventional models of photochemical abundance in the atmosphere. His observed data agree well with the cosmic-ray-driven electron reaction (CRE) model and strongly indicate that the same physical mechanism is involved in the formation of both the Antarctic and tropical ozone holes.

Lu said the tropical and polar holes play a major role in cooling and regulating stratospheric temperatures, mirroring the formation of “temperature holes” in the global stratosphere. He said this finding may prove crucial to better understanding global climate change.

Lu’s discovery builds on previous studies of the CRE-initiated ozone-depleting mechanism that he and his colleagues originally proposed about two decades ago.

“The present discovery calls for further careful studies of ozone depletion, UV radiation change, increased cancer risks, and other negative effects on health and ecosystems in the tropical regions,” said Lu.

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By Alison Bosman, Earth.com Staff Writer