The occurrence of ozone

Ozone is present in a very small amount in the Earth’s atmosphere, averaging about three molecules of ozone for every ten million molecules of air. It is measured in Dobson Unit (Db) which is equivalent to a 0.01 mm thickness of pure ozone if brought down to ground level pressure (1 atm) and temperature (0°C). The global average of ozone thickness is about 300Db.

Most of the ozone, around 90 per cent, resides in the upper layer of atmosphere called the stratosphere, which is 10 km above the Earth’s surface. It forms a blanket like structure around the Earth’s atmosphere called the ‘ozone layer’. The remaining 10 per cent is present in the lower region of the atmosphere, commonly known as the troposphere. The ozone layer filters out the harmful ultraviolet (UV) radiations from the sun before reaching the Earth by absorbing them. Thus, it is also known as the ‘Earth’s natural sunscreen’.

The role of ozone in the two regions of the atmosphere is different. The stratospheric ozone is called as the ‘good ozone’ due to its role in shielding the biologically damaging ultraviolet sunlight (called UV-B). Whereas, tropospheric ozone is known as ‘bad ozone’ because of its highly reactive nature with other gas molecules and it is toxic for living systems. Surface ozone is also a key component in formation of photochemical smog which is a potential air pollutant. Therefore, ozone basically has two environmental issues – the increasing concentration of ozone in the troposphere and the depletion of ozone layer in the stratosphere due to the release of several ozone-depleting substances (ODS).

Depletion of Ozone layer

The ozone molecules in the stratosphere are constantly formed and destroyed during any given time. Thus, the concentration of ozone is supposed to remain relatively stable unless the chemistry is drastically disturbed by external factors. In the beginning of the 1970s, the scientific community discovered that the ozone layer has been depleting more quickly than it is naturally formed. It was due to the human produced ozone-depleting organic compounds generally identified as halocarbons. They are combinations of elements like chlorine, bromine, fluorine, oxygen and hydrogen. Collectively they are known as Chlorofluorocarbons (CFCs) and Hydroclorofluorocarbons (HCFCs). Other ozone depleting substances are carbon tetrachloride, methyl chloroform and methyl bromide. These substances are non-reactive, non-flammable and non-toxic and can travel for a long distance reaching up to the stratosphere in a few years.

In the presence of UV light they produce free chlorine, fluorine and bromine which when in contact with ozone break down their molecules. It is estimated that, one chlorine molecule can destroy 100,000 ozone molecules before it is removed from the stratosphere (Environmental Protection Agency, US, 2016). ODSs are used in our day today appliances like refrigerators, air conditioners, fire extinguishers, foam insulators etc. The use of these substances is banned legally in most of the countries but illegal use is still widespread.

Finding a replacement for CFCs became inevitable and with effort the global scientific community found substances which are less harmful than CFCs. Hydrochlorofluorocarbons (HCFCs) are one of the temporary replacements for CFCs, while Hydroflourocarbons (HFCs) are the main long term replacements for CFCs and HCFCs. Since chlorine has the highest potential to destroy ozone, products that are entirely free of chlorine will be the ultimate replacements for CFCs and HCFCs.

The growing awareness of the serious impacts of ODSs in the ozone layer led to the groundbreaking Montreal Protocol on Substances that Deplete the Ozone Layer which was signed in 1987. It was the first joint international effort to protect the stratospheric ozone. The parties under the Protocol decided to phase out CFCs and halon productions. The measures taken up under this agreement was met with positive results as ODSs are falling and the ozone layer is expected to be fully healed near the middle of the 21st century (USEPA, 2017).

Fig: Surface and stratospheric ozone

Biological impacts of ozone depletion

Most of the biological impacts of ozone layer depletion are due to the damaging nature of UV-B radiations. The reduction of ozone concentration in the stratosphere allows more UV-B radiations to penetrate into the Earth’s atmosphere. Research says that 1 per cent decrease in ozone overhead can result in a 2 per cent increase in UV-B intensity at ground level (Baird and Cann, 2008). The DNA molecules of a living body absorb the UV radiation and can cause several genetic aberrations. It can be linked to several human conditions like skin cancers, cataracts, macular degeneration (gradual death of retinal cells) etc. Malignant melanoma is a widespread skin cancer caused by over-exposure to UV-B radiations. The impacts of UV-B radiation is confined not only to humans but plants as well. The efficiency of plant photosynthesis is reduced leading to less leaves, fruits and seeds. It also affects the production of microscopic phytoplankton which is an important base in the marine food chain.

The tropospheric ozone also has multiple effects on food production, forest growth and human health. It aggravates various respiratory diseases like asthma and emphysema, and is also linked to permanent lung damage. In plants, ozone induces early senescence and abscission of leaves, it also reduces the rate of photosynthetic carbon fixation and thereby a decrease in fruit and seed production (Wilkinson et al., 2012).

Healing of ozone hole in Antarctica

The ozone hole in Antarctica is not an actual hole but an area of exceptionally thin layer of ozone at the stratosphere that happens during the Southern Hemisphere Spring (August-October). This is due to the formation of Polar Stratospheric Clouds (PSC) particles during the winter. The inactive form of chlorine is converted into active form (Chlorine gas) in the presence of water and ice. Thus when spring comes, sunlight breaks the bond between two chlorine atoms which are in active form and undergoes a series of catalytic ozone destructions. The ozone hole grows throughout spring until the cold polar vortex vanishes and stabilizes the ozone layer. This cycle repeats each year during the spring.

On the bright side, research and monitoring have shown that the ozone hole over Antarctica has started to heal. It was reported in 2015 that the hole was around 14 million sq km smaller than it was in the year 2000, an area roughly the size of India (Solomon et al. 2016). This milestone has been credited to the international joint efforts in phasing out of ozone depleting substances. Meanwhile, researchers say that even though production of ODSs has been phased out in many countries, there is still plenty of chlorine left in the atmosphere. It is expected that complete recovery of ozone depletion would happen by around 2050-60 owing to the long lifetime and slow decay of chlorine.