Saturday, 27 November, 2021


Ozone Depletion by Nitrogen Fertilizer Should be Tackled Soon

Dr Kanan Purkayastha

Ozone is a molecule with three oxygen atoms. Ozone constitutes a very small part of our atmosphere, but its presence is vital to human well-being. Most ozone resides in the atmosphere, between 10 and 40 km above the Earth's surface. This region is called the stratosphere and it contains about 90% of all the ozone in the atmosphere. The solar ultraviolet (UV) radiation plays an important part in the production of ozone in the atmosphere. The oxygen molecules by photolysis yield two oxygen atoms. Oxygen atoms combined with oxygen molecule generate ozone and these two are continuously inter-converting each other. In addition to this process, ozone in the stratosphere can be destroyed by a number of radicals such as hydroxyl, nitric oxide, chlorine, bromine and iodine radicals. Different industrial sectors are involved in generating ozone depleting substances. For examples, aerosol products, cancer therapy, foam, fire extinguisher, refrigerant, fumigant in agriculture and different coatings and adhesives use ozone depleting substances.

It is clear that since the ozone layer absorbs UV radiation from the sun, ozone layer depletion can increase surface UV level, leading to the increase of human skin cancer, cell carcinomas, cortical cataracts, malignant melanoma and various other biological effects. The scientific confirmation of the depletion of the ozone layer prompted the international cooperation to take action to protect the ozone layer. For the Protection of the ozone Layer, this was formalised in the Vienna Convention, which was adopted and signed by 28 countries on 22 March 1985. In September 1987, this led to the drafting of the Montreal Protocol on Substances that deplete the ozone layer.

In 1994, the United Nations General Assembly proclaimed 16 September the International Day to celebrate the preservation of the ozone layer or ‘World Ozone Day’. This is the day when in 1987 the Montreal protocol was signed. The Parties to the Montreal Protocol on substances that deplete the ozone layer reached an agreement at their 28th Meeting of the Parties on 15 October 2016 in Kigali, Rwanda to phase-down hydrofluorocarbons (HFCs).

Though the principal aim of the Montreal Protocol was to protect the ozone layer by taking measures to control total global production and consumption of substances that deplete it, halocarbons such as chemicals in which one or more carbon atoms are linked to one or more halogen atoms (fluorine, chlorine, bromine or iodine) were get more prominence in the Protocol. Less attention has been given to date about the affect of nitrous oxide on ozone layer, the chemical that relates to nitrogen fertilizer. This essay examines the impact of nitrous oxide on ozone layer and climate change.

About one-third of global nitrous oxide emissions are from human activities. In 1970, Paul Crutzen pointed out that emissions of nitrous oxide from the Earth's surface could affect the amount of nitric oxide in the stratosphere. Crutzen showed that nitrous oxide lives long enough to reach the stratosphere, where it is converted into nitric oxide. Crutzen then noted that the increasing use of fertilizers might have led to an increase in nitrous oxide emissions over the natural background, which would in turn result in an increase in the amount of nitric oxide in the stratosphere.

It has been now established that nitrous oxide is emitted to atmosphere during the denitrification of anaerobic bacteria in soil or by bacterial decomposition of fertilizer, biomass burning, livestock manure, sewage treatment, combustion and certain other industrial processes. Atomic oxygen generated by photolysis in stratosphere reacts with nitrous oxide to produce nitric oxide, which then reacts with ozone in the upper stratosphere and depleted the ozone level. Thus human activity such as the use of nitrogen based fertilizer could affect the stratospheric ozone layer.

National Oceanic and Atmospheric Administration (NOAA) scientists say in a new study that nitrous oxide has now become the largest ozone-depleting substance emitted through human activities, and is expected to remain the largest throughout the 21st century. Another issue of the presence of nitrous oxide in atmosphere is its’ global warming potential. It has been observed that if we compare one molecule of carbon dioxide with one molecule of nitrous oxide, we will see that nitrous oxide can cause 265 times more warming than carbon dioxide.

It has not yet been possible to develop a zero carbon fertilizer. We need significant innovation in this area, not only for the protection of ozone layer in the stratosphere but also for the production of food that can feed the increasing level of world population without compromising the global warming and climate change. Technically it is possible to get crops to absorb nitrogen more efficiently, but the cost of technology used is higher than the cost of the excess use of fertiliser. Some companies have developed additives for the maximisation of nitrogen uptake by plants, so that less to wash into groundwater or evaporate into the atmosphere. But the additives are not consistently effective. Some researchers are doing genetic work on new varieties of crops that can use bacteria to fix nitrogen. In this technique instead of adding nitrogen fertilizer, bacteria are added to the soil that can produce nitrogen. This is still in an experimental stage. If works, it will reduce the need for fertilizer and all the emission responsible for depleting ozone and global warming.

Secretary General of United Nations António Guterres on ‘World Ozone Day’ mentioned that “the Montreal Protocol is more than just an example of how multilateralism can and should work. It is an active tool to help meet our global vision for sustainable development. Under the Kigali Amendment to the Protocol, nations have committed to phase down hydro fluorocarbons (HFCs), powerful greenhouse gases used as coolants. When fully implemented, the Kigali amendment could prevent 0.4 degrees Celsius of global warming this century.”  However, this is not enough to protect the Earth’s precious ozone layer. Ozone hole is not just above the sources of chlorofluorocarbons or happening above Antarctica, it is happening above other part of the Earth as well. For example in 2006, a 2.5 million square kilometre ozone hole was detected over Tibet. In 2011, an ozone hole appeared over mountainous regions of the Hindu Kush, along with an unprecedented hole over the Arctic. In fact, there are findings of significant and localised ozone holes above other parts of the Earth.

However, if different nations act together anything is possible. So, let us act now to find out alternative technique for increasing food production without depleting the earth precious ozone layer and also reducing global warming and climate change.


The writer is a UK based academic,chartered scientist and environmentalist, columnist and author