The ozone layer is a blanket of naturally occurring gas that is located in the stratosphere (15 to 50 km above earth) and serves a very important role in blocking the sun’s harmful ultraviolet-B rays from reaching us on earth. However, this protective layer of gases has been undergoing a slow but steady reduction in size. Ozone depletion has been taking place since the 1960s and is now a global phenomenon that requires urgent attention.
Ozone (O3) is a gaseous compound made up of three oxygen atoms and is continually being formed and decomposed in the stratospheric zone. The major cause of this depletion is the continued use of substances containing chlorine and bromine, known as chlorofluorocarbons (CFCs).
Once the CFCs reach the stratosphere, they are broken down into constituent elements and react with the highly reactive ozone molecules, thereby reducing the size of the ozone layer. The U.S. Environmental Protection Agency estimates that a single chlorine atom can decompose more than 100,000 ozone molecules (Newman et al).
While the largest ozone depletion is recorded at the southern and northern hemispheres, the process is taking place everywhere but is minimum in the tropics. This problem has led to increased levels of exposure to UV-B radiation that will continue to have adverse effects on all people living on earth irrespective of their location or economic status.
Individuals with lightly colored skins are more vulnerable to the cancerous effects of UV-B radiation. UV-B radiation also has an impact on plant life, which in turn creates an imbalance in the ecosystem. Scientists have mentioned that depletion of the ozone layer will also reduce fish stocks in lakes, rivers, and seas hence creating a food shortage in areas that heavily depend on fish.
Formation of ozone begins when oxygen molecules decompose after absorbing UV light with a wavelength that is less than 240nm in the stratosphere, producing two oxygen atoms. One oxygen atom then reacts with an oxygen molecule to give ozone. The ozone molecule absorbs ultraviolet radiation of wavelength 310 and 200 nm, this decomposes it to oxygen molecule and an oxygen atom.
The oxygen atom then reacts with an oxygen molecule to form ozone. The process is continuous and is known as the ozone-oxygen cycle. The cycle stops when an oxygen atom reacts with ozone molecule to give two oxygen molecules, i.e.
O2 -UV radiation (<240 nm)-> 20
O + O2 -> O3
O3 -UV radiation (310-200 nm)->O + O2
O + O3 ->2O2
Generally, the level of ozone in the stratosphere is controlled by the balance between the photochemical production of oxygen atoms and the recombination reaction.
Destruction of the ozone occurs when free radicals reach the stratosphere, these radicals include the hydroxyl radical, nitric oxide radical, chlorine radical and bromine radical. Hydroxyl and nitric acid radicals reach the atmosphere through natural ways, however, chlorine and bromine radicals are due to man’s activities and are found in certain stable compounds, especially CFCs (McFarland, pp. 1207).
CFCs can reach the atmosphere without decomposing into their constituent elements since they are stable and non-reactive. When CFCs reach the stratosphere, they undergo photochemical decomposition to release chlorine or bromine atom, i.e.
CFCl3 -> CFCl2 + Cl
The liberated chlorine and/ or bromine atoms destroy ozone molecules through a series of catalytic reactions (Solomon et al, pp. 412). In an elementary example of this reaction, a chlorine atom reacts with ozone molecule forming chlorine monoxide, ClO, i.e.
Cl + O3 -> ClO + O2
The chlorine monoxide is unstable and can readily react with another ozone molecule to give two oxygen molecules as shown below:
ClO + O3 -> Cl + 2O2
This reaction reduces the number of ozone molecules in the stratosohere.
A single chlorine atom would continuously destroy ozone molecules for up to two years, however, other reactions in the stratosphere remove these elements. Bromine is more destructive than chlorine. Both of these elements are present in the stratosphere and cause considerable damage to the ozone layer. A single chlorine atom is able to destroy nearly 100,000 ozone molecules, when we consider the amount of CFCs released into the atmosphere annually, the damage done to the ozone layer becomes apparent (Storlaski et al, pp. 1015).
Trends in Ozone Layer Depletion
Studies of the ozone layer have been undertaken since the mid 20 the century, however, changes in its size became apparent between 1960 and 1970 when it was observed that it had reduced by 23 per cent between this duration. In 1985, the ozone hole was first observed in the Antarctic. By 1986, three models had been postulated to explain ozone depletion:
Solar cycle model- regular increases in the quantity of nitrogen oxides in the lower Antarctic stratosphere is due to changes in solar radiation;
Dynamical model- an alteration in the circulation pattern from downwelling of air with a high abundance of ozone from the upper stratosphere to upwelling of air deficient of ozone from the troposphere; and
Halogen model- a number of variant theories centering on the catalytic destruction of ozone layer because of CFCs and halons (Storlaski et al, pp. 1015).
A study was undertaken by McMurdo in 1986 that showed that the concentrations of nitrogen oxides were remarkably low, hence disapproved the solar cycle model. He also observed that the levels of long-lived tracers were due to complex reactions and was unlikely to occur, and that the high levels of chlorine in the stratosphere was the most probable cause of ozone depletion (Storlaski et al, pp. 1015).
The Montreal Protocol was signed in 1987, although the origin of the ozone hole was not well understood, the protocol recognized that the stratosphere had been disturbed. It was observed that the levels of chlorine in the atmosphere had increased by about 5% and that there had been a considerable loss in ozone every October in the Antarctica (Newman et al). The Montreal Protocol was signed by 31 countries and was aimed at cutting CFC emissions as shown below: