NASA announced that measurements taken by satellite show that the decline in chlorine, resulting from an international ban on chlorine-containing chemicals called chlorofluorocarbons (CFCs), has resulted in about 20% less ozone depletion during the Antarctic winter than there was 12 years ago when the first measurements of chlorine and ozone during the Antarctic winter were made by NASA's Aura satellite.
Ozone is a gas made up of three oxygen atoms (O3). It occurs naturally in small (trace) amounts in the upper atmosphere (the stratosphere). Ozone protects life on Earth from the Sun’s ultraviolet (UV) radiation. In the lower atmosphere (the troposphere) near the Earth’s surface, ozone is created by chemical reactions between air pollutants from vehicle exhaust, gasoline vapors, and other emissions. At ground level, high concentrations of ozone are toxic to people and plants. However, 90% of the ozone in the atmosphere sits in the stratosphere, the layer of atmosphere between about 10 and 50 kilometers above ground. The natural level of ozone in the stratosphere is a result of a balance between sunlight that creates ozone and chemical reactions that destroy it.
CFCs are long-lived chemical compounds that eventually rise into the stratosphere, where they are broken apart by the Sun's ultraviolet radiation, releasing chlorine atoms that go on to destroy ozone molecules. In 1986 and 1987 Dr. Susan Solomon of MIT led the National Ozone Expedition where the team gathered the evidence to confirm the accelerated reactions. The team, Susan Solomon, Rolando R. Garcia, F. Sherwood Rowland & Donald J. Wuebbles published, “On the depletion of Antarctic ozone.” In the 1980’s this was groundbreaking at the time.
Two years later, the nations of the world signed the Montreal Protocol on Substances that Deplete the Ozone Layer, which regulated ozone-depleting compounds. Later amendments to the Montreal Protocol completely phased out production of CFCs. So the news of the healing of the ozone hole over Antarctica was heralded as the demonstrated success of the Montreal Protocol the prototype for all environmental treaties.
Now, a team led by Joanna Haigh of the Grantham Institute at Imperial College London, UK, has discovered that while ozone density is indeed improving at the poles, it is not doing so at lower latitudes, roughly between 60 degrees north and 60 degrees south. In a new study published in Atmospheric Chemistry and Physics, the scientists report that the section of the ozone layer above the most densely populated areas on earth still seems to be declining – very slowly but steadily.
In their paper: “Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery” the scientists report that since 1998 ozone in the upper stratosphere is rising again. However, their measurements of the total column of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes outside the polar regions.
The scientists found evidence from multiple satellite measurements that ozone in the lower stratosphere at moderate latitues has indeed continued to decline since 1998. Even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric ozone in the moderate latitueds of 60 degrees north and 60 degrees south.
The scientists found that total column ozone between 60° S and 60° N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and so the scientists can only speculate and guess at this time.
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