Ozone hole over Antarctica
We have read and keep reading a lot about the depletion of the Ozone layer and the formation of the Ozone hole over Antarctica. Most of the blogs and posts you find on the internet generally discuss only the causes and consequences of the depletion of the Ozone layer and formation of the Ozone hole. But you won’t really find a post with concrete material regarding various interesting and relevant questions that explain crucial aspects about the same, like:
- Does the structure of the Earth have anything to do with the formation of the Ozone hole?
- Why does the Ozone hole form only over Antarctica?
- Why does Antarctica experience 6 months of day and 6 months of night?
- Why does the Ozone hole form only during the month of October?
- From where does Chlorofluorocarbons (CFCs) in Antarctica come from if it is a frozen continent and no AC or refrigerators are used there?
The answer to these and many more questions can be found in this post. So before jumping directly on the formation of Ozone hole, let me brush up some basic aspects about the Earth’s structure and atmosphere to maintain a systematic progression of the topic.
The Earth’s Atmosphere
Earth’s atmosphere has a series of layers, each with its own specific traits. Each layer is located at a specific height and possesses distinct characteristics and properties from one another due to varying altitude.
There are 6 major layers of the atmosphere moving upward from ground level:
The lowest layer of our atmosphere starting at ground level and extends upward to about 10 km (6.2 miles or about 33,000 feet) above sea level.
It extends from the top of the troposphere to about 50 km (31 miles) above the ground. The infamous Ozone Layer is found within the stratosphere. Unlike the troposphere, the stratosphere actually gets warmer the higher you go.
It extends upward to a height of about 85 km (53 miles) above our planet. Most meteors burn up in the mesosphere. Unlike the stratosphere, temperatures once again grow colder as you rise up through the mesosphere.
The layer of very rare air above the mesosphere is called the thermosphere. High-energy X-rays and UV radiation from the Sun are absorbed in the thermosphere, raising its temperature to hundreds or at times thousands of degrees. However, the air in this layer is so thin that it would feel freezing cold to us.
Some experts consider the exosphere to be the actual “final frontier” of Earth’s gaseous envelope. The “air” in the exosphere is extremely thin, making this layer even more space-like than the thermosphere.
Though is not a distinct layer like the others mentioned above, the ionosphere is a series of regions in parts of the mesosphere and thermosphere where high-energy radiation from the Sun has knocked electrons lose from their parent atoms and molecules.
The Ozone Layer
The Ozone Layer is a thin film or envelope of Ozone molecules covering the Earth’s surface. Located in the Stratosphere at an altitude of roughly 35 km, it consists of Ozone molecules that absorb high-energy ultraviolet (UV) light from the Sun, converting the UV energy into heat. It absorbs 97 to 99 percent of the Sun’s medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which otherwise would potentially damage exposed life forms near the surface.
The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth’s atmosphere as a whole is about 0.3 parts per million. About 90 percent of the ozone in the atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 km (66,000 and 131,000 ft.) where they range from about 2 to 8 parts per million.
Ozone in the Earth’s stratosphere is created by ultraviolet light striking ordinary oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create ozone, O3.
The ozone molecule is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an individual atom of oxygen, a continuing process called the ozone-oxygen cycle.
O2 + ℎνuv → 2 O
O + O2 ↔ O3
Depletion of the Ozone layer
Numerous human activities over the past five decades have led to release of huge amounts of free radical catalysts including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br) into the atmosphere which can degrade the Ozone layer. While there are natural sources for all of these species, the concentrations of chlorine increased markedly in recent decades because of the release of large quantities of man-made organo-halogen compounds, especially Chlorofluorocarbons (CFCs).
The primary source of CFCs being released into the Earth’s atmosphere is the extensive use of Air conditioners and refrigerators. When sunlight reacts with this CFC, Chlorine free radicals Cl ˚ are eliminated from the CFC molecule which then reacts with the Ozone molecules and starts disintegrating them and causing a breakdown of the Ozone layer.
The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation is able to reach the Earth’s surface at a higher intensity.
Formation of the Ozone hole over Antarctica
We have frequently read in several NEWS articles and environmental blogs that the constant depletion of the Ozone layer is gradually leading to the formation of a hole in the Ozone layer above Antarctica, or in other words the South Pole. Approximately 5 percent of the Earth’s surface, around the north and south poles, has seen much larger seasonal declines in the amount of Ozone in the atmosphere as compared to other geographic locations, and are described as “Ozone holes”.
We know that the primary gas that leads to the disintegration and depletion of Ozone is Chlorofluorocarbons (CFCs), leading to the formation of an Ozone hole. We also know that the biggest source of CFCs being released into the atmosphere is from ACs and refrigerators. If that is the case, then
- Why is the hole in Ozone layer forming only over Antarctica which is a frozen content?
- Who would and why would someone use ACs and refrigerators on a frozen continent?
- Where do CFCs come from on a frozen continent like Antarctica where there is no use of ACs and refrigerators?
Before understanding the process of formation of the hole in the ozone layer, we need to understand some key points regarding the structure of the earth which plays a crucial role in this aspect.
- The Earth is in the shape of a spheroid, meaning it is curved or bulged at the equator and flatter at the Poles.
- Since the earth is flatter at the poles, the troposphere is nearer to the Earth surface at the pole, especially the South Pole since the North Pole is covered by the Arctic Ocean and the South Pole is covered by the landmass of Antarctica.
- Antarctica is a frozen continent located at the South Pole.
- The Ozone hole forms over Antarctica during the month of October and slowly starts subsiding from November onwards.
- The Earth rotates at a slightly tilted angle and revolves around the sun at this angle due to which both the poles experience 6 months of day and 6 months of night throughout the year.
- When the North Pole experiences 6 months of day, the South Pole (Antarctica) experience 6 months of night and vice versa.
Here is the most logically proven reason that will answer the questions mentioned earlier in the most sensible manner.
- Antarctica is a frozen continent and has a minimal population and absolutely NO USE OF AC AND REFRIGERATORS.
- The CFCs released into the atmosphere from the nearby countries and continents get settled on the Polar Stratospheric Clouds (PSCs) and travel across the seas and oceans over Antarctica.
- These PSCs keep accumulating over Antarctica over a course of 6 months of night. 6 months is a long period and a lot of CFCs reach the atmosphere above Antarctica by then.
- As soon as the Earth completes half a revolution (6 months) around the Sun and the South Pole faces the Sun, Antarctica begins witnessing daylight in the month of October.
- The sunlight reacts with the CFCs on the clouds and eliminates countless Chlorine free radicals Cl ˚
- These Chlorine free radicals then react with the Ozone molecules and start disintegrating them. One Chlorine free radical can disintegrate countless Ozone molecules.
- After the entire CFCs from the PSC have reacted with the Ozone, the ozone hole slowly starts subsiding from November onwards since the Ozone from nearby start flowing into the hole, filling it up. This shows how nature has a tendency of balancing the unevenness it experiences!
The above-mentioned steps and the chain reaction to show the disintegration of Ozone molecules by Cl ˚ can be roughly represented by the diagrams shown below: