By Meenambika Menon
Recently, there was news that “scientists recorded the smallest ozone hole since 1982 above Antarctica in September 2019.” The questions that come up are what the ozone hole is, where it is, why its size is changing and how it affects us, etc.
Let us start with, what is ozone?
Ozone is an allotrope of oxygen. When a chemical element exists in more than one form due to the arrangement or number of atoms, they are called allotropes. Ozone (O3) is an extremely reactive gas composed of three oxygen atoms. The oxygen (O2) that we breathe is a gas composed of two oxygen atoms.
Ozone naturally occurs in the earth’s upper atmosphere, the stratosphere. The stratospheric ozone layer is formed due to the reaction between solar ultraviolet (UV) radiation with molecular oxygen (O2). This “ozone layer,” which is the second layer in the earth’s atmosphere, reduces the amount of harmful UV radiation reaching the earth’s surface from the sun.
The “ozone hole” is the term commonly used for areas where the ozone layer gets damaged. Ozone layer damage is more like a lean patch than a hole. This damage is visible in the stratosphere above Antarctica and now the Arctic.
Ozone depletion, a gradual thinning of the earth’s ozone layer, is caused by the reaction of ozone with chemical compounds containing gaseous chlorine or bromine. All the coolants (like the ones used in refrigerators, air-conditioners) and aerosols in products like deodorants and aircraft fuel, release chloro-fluoro carbon (CFC) gases. These CFC atoms break the O3 molecules into O2 molecules causing a gap in the ozone blanket above our earth.
Why is this damage occurring over the poles when the gases causing this depletion is produced in the main continents where almost all the population lives?
The Antarctic ozone hole forms during late winters in the Southern Hemisphere. These reactions involve chemically active forms of chlorine and bromine on the surfaces of cloud particles that form in cold stratospheric layers, leading to the destruction of ozone molecules. In warmer temperatures, fewer polar stratospheric clouds form and don’t persist as long, slowing or limiting the ozone-depletion process.
Now, how is this hole decreasing or the patch mending itself? There are two reasons:
Reduction in the production of chloro-fluoro carbon gases and other ozone depleting gases: The recognition of the dangers presented by CFCs to the ozone layer resulted in an international effort to stop the production and restrict the use of CFCs and other compounds. The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer (the only universally ratified treaty in United Nations’ history), began the phase out of CFCs in 1993. By 2005 the consumption of ozone-depleting chemicals controlled by the agreement had fallen by 90–95 percent in the 197 countries that were parties to the protocol.
Warmer Antarctic lower stratosphere: This minimised the formation and persistence of the polar stratospheric clouds that are the main ingredient in the ozone-destroying process. Along with this the strong weather system brought ozone-rich air from higher latitudes elsewhere in the Southern Hemisphere to the area above the Antarctic ozone hole. These two effects led to higher ozone concentration over Antarctica compared to the usual levels since the mid-1980s.
What is this news about a new ozone hole or patches having been found?
While the ozone hole over the South Pole is a regular feature, the conditions that allow these holes to form are much rarer in the Northern Hemisphere. The Arctic ozone hole happened this year because the cold air was concentrated in the area for much longer than is typical.
The annual Antarctic ozone hole, which is roughly four decades old, will remain a seasonal reality for years to come. An assessment done by the World Meteorological Organization in 2018 found that the southern ozone hole has been shrinking by about 1% to 3% per decade since 2000. Scientists are optimistic that the hole may be starting to close, however, it likely won’t heal completely until at least till 2050.
(The writer is Lead, Curriculum – Science & Math at Shiv Nadar School.)
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