Ozone is a gas that exists both in the Earth's upper atmosphere and at ground level. Depending on where it is in the atmosphere, ozone can be "good" or "bad" for ones health and the environment.
Ozone can be found in two layers of the atmosphere. Ground level or "bad" ozone is an air pollutant that is hazardous to one's health and harms crops, trees, and other vegetation. It is a major contributor to urban smog. The stratospheric, or "good" ozone layer, extends upward from 6 to 30 miles and shields Earth's life from the sun's harmful ultraviolet (UV) rays.
Stratospheric ozone is a naturally occurring gas that filters ultraviolet (UV) radiation from the sun. When the ozone layer depletes, more UV radiation reaches the Earth's surface. Overexposure to UV rays can cause skin cancer, cataracts, and weakened immune systems in humans. Increased UV can also cause crop yield losses and disruptions in the marine food chain.
The Cause of Depletion
Ozone molecules in the stratosphere are constantly being produced and destroyed by the sun's various types of UV radiation. Normally, the production and destruction of ozone are balanced, resulting in a stable amount of ozone in the stratosphere at any given time. However, scientists have discovered that certain chemicals in the stratosphere react with UV radiation, causing them to break apart and release chlorine or bromine atoms. These atoms, in turn, deplete the ozone layer.
Ozone-depleting substances (ODS) like chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) were once widely used in refrigerants, insulating foams, solvents, and other applications. All of these substances emit chlorine atoms into the stratosphere. More than 100,000 ozone molecules can be broken apart by a single chlorine atom.
Other ozone-depleting chemicals include methyl bromide (a pesticide), halons (used in fire extinguishers), and methyl chloroform (used as a solvent in industrial processes). When methyl bromide and halons are broken down, bromine atoms are released, which are 60 times more destructive to ozone molecules than chlorine atoms.
Prior to the implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer and its subsequent revisions and amendments, atmospheric levels of these ODS rapidly increased. However, nearly all of these substances' atmospheric levels have decreased significantly over the last two decades.
Polar Stratospheric Clouds
Surface reactions of liquid and solid PSCs can significantly increase the relative abundances of the most reactive form of chlorine . These reactions change the reservoir forms of reactive chlorine gases, chlorine nitrate (ClONO2) and hydrogen chloride (HCl), to the most reactive form, ClO.
Denitrification: Gravity causes PSC(Polar stratospheric clouds )particles to fall to lower altitudes once formed. During the low-temperature winter/spring period that lasts several months in Antarctica, the largest particles can descend several kilometres or more in the stratosphere. Because PSCs frequently contain a significant fraction of available HNO3, their descent removes HNO3 from ozone layer regions. This is referred to as stratospheric denitrification. Because there is less HNO3, the highly reactive chlorine gas ClO remains chemically active for a longer period of time, increasing chemical ozone destruction.
Agreements So Far
By 1985, the world had made significant advances in scientific understanding of ozone depletion and its effects on human health and the environment. In response, the Vienna Convention for the Protection of the Ozone Layer was established. This agreement is a framework convention that lays out principles that many parties have agreed upon. However, it does not require countries to take control measures to protect the ozone layer. Later, in the form of the Montreal Protocol, this would be realised. The Vienna Convention was the first of its kind to be signed by every country involved, going into effect in 1988 and achieving universal ratification in 2009.
The Montreal Protocol on Substances that Deplete the Ozone Layer is a global agreement that aims to protect the Earth's ozone layer by phasing out depleting chemicals. The production and consumption of ozone-depleting substances are both included in this phase-out plan. The historic agreement was signed in 1987 and went into effect in 1989.
Kigali Agreement - Amendment to Montreal Protocol ( phase out HFC’s)
The Montreal Protocol was never intended to be a tool for combating climate change, but it has become one with the Kigali Amendment, which was signed in the Rwandan capital in October 2016. The amendment's phase-down of HFCs is expected to prevent the emission of up to 105 million tonnes of CO2 by 2100, thereby preventing a 0.5 degree rise in global temperatures above pre-industrial levels. HFCs don’t deplete the ozone layer, but they have a high global warming potential (GWP) of 12 to 14,000. Global warming potential is a value assigned to represent the amount of heat trapped by it, relative to the amount of heat trapped by a similar mass of carbon dioxide. The GWP of carbon dioxide, the primary greenhouse gas due to human emissions, is 1.
The Kigali Agreement, like the Montreal Protocol, has different targets for high- and low-income countries. Countries such as the United States must meet this goal by 2036, while India has until 2047 and China has until 2045.
The Kigali Amendment's contribution to mitigating global warming is especially significant in light of the Paris Agreement, another critical climate treaty.
The Paris Agreement, signed in 2015, is a legally binding policy that requires countries to reduce greenhouse gas emissions in order to slow global warming. The agreement aims to keep global temperatures "well below" a pre-determined threshold — 2°C above pre-industrial levels — in order to avoid extreme weather events and climate change.