Ozone layer depletion is one of the most alarming environmental issues of the present time. The chloro- and Bromo- chemical compounds released into the atmosphere from industries and domestic buildings deplete the ozone layer. The most common compounds that cause ozone layer depletion are halocarbons like Chlorofluorocarbons. Chlorine reacts with atmospheric ozone, breaking an oxygen atom from the ozone ring and leads to a chain of reactions, in which the ozone gas is decomposed into oxygen and chlorine monoxide. Also, counteracting the ozone depletion in the stratosphere, ozone layer recovery has also been observed by scientists.
The ozone layer depletion increased exponentially and the thinning of stratospheric ozone was observed more significantly in the polar regions, ever since researchers discovered the ozone hole. The gradual thinning of the ozone layer had led to the formation of an ozone hole over the Antarctic regions. Due to the depletion of the ozone layer, a greater amount of ultraviolet rays reaches the earth’s surface. There are several adverse effects of UV rays on humans as well as on the environment. Exposure to UV rays makes humans prone to cataracts, skin cancer, immune system damage, etc. However, recent climate surveys and research establish the fact that the ozone layer is recovering.
The symbol of ozone gas is O3. It is formed when oxygen absorbs photons from the ultraviolet rays and photodissociation of oxygen molecules occurs. Here, the ozone cycle refers to the continuous process of dissociation and regeneration of ozone molecules due to the effect of ultraviolet radiations. On photodissociation, a diatomic oxygen molecule (O2) dissociates into two free radicals of atomic oxygen [O]. Each of these two atomic oxygen molecules reacts with diatomic oxygen molecules forming ozone gas. Therefore, every free radical of atomic oxygen forms one molecule of ozone gas, in reaction with diatomic oxygen.
The chemical reactions involved in the formation of ozone are as follows.
O2 → [O] + [O]
[O] + O2 → O3
Ozone molecules split into diatomic oxygen and free radicals of atomic oxygen on the absorption of UV light. The resulting atomic oxygen radical again reacts with another diatomic oxygen molecule and forms an ozone molecule. This continuous process formation of ozone gas terminates when a free radical of atomic oxygen reacts with ozone and dissociates it into two molecules of diatomic oxygen.
The chemical reactions involved in the splitting of ozone into two molecules of diatomic oxygen are as follows.
O3 + UV radiations → O2 + [O]
[O] + O3 → 2O2
The ozone layer depletion has occurred due to the reaction of ozone with highly reactive free radical catalysts like Bromine (Br ), Chlorine (Cl ), Nitrous oxide (NO ), and (OH ). Each of the above free radicals has an unpaired electron, that renders them highly reactive. The hydroxyl and nitric oxide free radicals occur naturally in the atmosphere whereas the free radicals of Chlorine and Bromine have human-made sources. The most common sources of the free radicals of chlorine and bromine are Chlorofluorocarbons (CFCs). These are mostly released from cooling appliances and certain industrial equipment. The CFCs are chemical compounds that travel as high as the stratosphere without decomposing or dissociating in the troposphere, as they exhibit low reactivity.
On reaching the stratosphere, these chlorofluorocarbons absorb ultraviolet radiations and release the highly reactive free radicals of chlorine and bromine in the atmosphere.
The chemical reaction involved in the splitting of CFCs into free radicals of Chlorine and Bromine is as follows.
CFBr3 + Ultraviolet radiations → CFBr2 + Br
CFCl3 + Ultraviolet radiations → CFCl2 + Cl
These free radicals eventually react with the stratospheric ozone and degenerate it into diatomic oxygen through several types of catalytic reactions. For example, when the free radical of chlorine gas reacts with ozone, chlorine monoxide and diatomic oxygen are formed. The chlorine monoxide formed in the above step reacts with an ozone molecule and forms two molecules of diatomic oxygen. Now, another product of this reaction is a free radical of chlorine. Hence, a chain of reactions is initiated when a free radical of chlorine reacts with ozone.
The chemical reactions involved in this chain reaction are as follows.
O3 + Cl → O2 + ClO
O3 + ClO → 2O2 + Cl
The free radical of chlorine produced in the above reaction will react with another ozone molecule forming diatomic oxygen and chlorine monoxide.
Ozone Hole Over the Antarctic
The ozone column over the Antarctic regions had depleted by about 60 percent with respect to the average global ozone layer depletion. This phenomenon was recorded in 1985 by Joseph C. Farman, Jonathan D. Shanklin, and Brian G. Gardiner, in the British Antarctic Survey. In comparison with the previous records, this decrease in the ozone layer was found to be even greater than 50 percent. Ozone layer depletion is categorised into two phenomena, one is the depletion of ozone in the troposphere and the other is the formation of the ozone hole in the stratosphere at the polar regions.
Why is the Hole in the Ozone Layer Recovering?
The ozone hole recovery began in the late 1990s and the size of the hole has been decreasing ever since. In 1982, the size of the ozone hole was recorded to be 16.3 million km2. There are several international treaties such as NASA ozone layer recovery that are being religiously followed by various countries across the globe, to control the production and consumption of ozone-depleting substances. The recorded reductions are quite promising to anticipate that the ozone hole will be mostly recovered in the near future.
Ozone Layer Recovery
The ozone layer recovery has been one of the most significant agendas for the conservation of the environment. As per the recent records, the ozone layer is recovering and it is expected that the concentration of ozone in the atmosphere is likely to be restored like that of the 1970s by 2060s. The objective of the Montreal Protocol on Substances That Deplete The Ozone Layer was to control the production and use of CFCs and similar halocarbons that are potent threats to the ozone layer. Ideally, by 1998, the levels of global consumption of CFCs and similar halocarbons were to be reduced by 50 percent from the levels of 1986. By 2005, after several amendments, the consumption of CFCs and other ozone layer depleting substances was reduced by about 95 percent.
When is the Ozone Layer Expected to Recover?
It had been anticipated that the ozone holes over Antarctica will be of smaller sizes, if the consumption of CFCs and halocarbons is controlled, after 2040. Also, scientists have recorded that the stratospheric ozone levels will be restored to that of 1980, by 2060s. The concentration of ozone in the stratosphere and troposphere are affected by the levels of gases like carbon dioxide, nitrous oxide, methane, in the atmosphere. These gases can release reactive free radicals that deplete the ozone layer. The ozone level is expected to go up steadily but slowly since the halocarbons like CFCs stay longer in the atmosphere.
An increase in stratospheric ozone levels was discovered in 2014 as a result of the phase-out of CFCs and other ozone-depleting substances across the globe. The practices as per the international treaties were to be accredited for this hike in the stratospheric ozone levels. Researchers observed that the ozone concentration in the upper stratosphere has been increasing since 2000, while the ozone hole over Antarctica has been reducing in size. The smallest ozone hole was recorded in 2019, this was a milestone in the ozone hole recovery and bolsters the fact that reduced consumption of substances that deplete ozone has the ozone recovered. This evidently shows how much has the ozone layer recovered with the international treaties to control the consumption of substances potent to deplete the ozone layer.
Ozone is one of the greenhouse gases and any changes in its concentration in the troposphere and stratosphere, have an impact on the earth’s temperature. The ozone hole over Antarctica is an opening for the harmful UV rays to be incident directly on its inhabitants. Also, the decrease in the concentration of stratospheric ozone has a cooling effect on the climate of our planet. Nevertheless, the increasing carbon dioxide levels in the atmosphere produces a counteractive heating effect as well. As the ozone layer is recovering gradually, researchers are anticipating that the ozone concentration in the stratosphere will be restored within a few decades. Hence, the earth ozone layer recovering will also reduce the cooling effect at the poles due to ozone depletion.