Introduction to Ozone layer and its depletion

Executive Summary:

• Ozone layer extends approximately from12 km to 45 km in the atmosphere and absorbs most of the ultraviolet radiation reaching the earth from the sun.
• The percentage of Ozone in the atmosphere is only 0.000007 %
• The equilibrium of forming and decomposing ozone in the stratosphere ensured the blocking of UV reaching the surface of earth
• The man made halogenated chemicals used for industrial purposes affected the said equilibrium which resulted the depletion of ozone layer

Ozone (O₃) is a gas composed with three atoms of oxygen. The oxygen molecules (O₂) in the air we breathe are made up of only two atoms of oxygen. Ozone molecules are created in a photochemical reaction in the presence of UV rays, which can be described in a simplified way as,

 3O₂ ⇔ 2O₂ + 2O ⇔ 2O₃

This equilibrium is very fragile and therefore, any intervention could damage the natural processes of formation and breakdown of ozone, which, in turn, would have serious consequences for life on earth.

The ozone layer

The ozone layer is a natural layer of gas in the upper atmosphere that protects humans and other living things from harmful ultraviolet (UV) radiation from the sun.

The ozone layer is a region of high ozone concentration in the stratosphere, 15 to 35 kilometres above Earth's surface. The ozone layer acts as an invisible shield and protects us from harmful ultraviolet (UV) radiation from the sun. In particular, the ozone layer protects us from the UV radiation, known as UV-B, which causes sunburn. Long-term exposure to high levels of UV-B threatens human health and damages most animals, plants and microbes, so the ozone layer protects all life on Earth.

Ozone depletion

Scientists discovered in the 1970s that the ozone layer was being depleted.

Atmospheric concentrations of ozone vary naturally depending on temperature, weather, latitude and altitude, while substances ejected by natural events such as volcanic eruptions can also affect ozone levels.

However, these natural phenomena could not explain the levels of depletion observed and scientific evidence revealed that certain man-made chemicals were the cause. These ozone-depleting substances were mostly introduced in the 1970s in a wide range of industrial and consumer applications, mainly refrigerators, air conditioners and fire extinguishers.

If ozone molecules are depleted faster than natural production, the result is an ozone deficit. The depletion of the ozone layer leads to a reduction in its shielding capacity and thus greater exposure of the earth’s surface to harmful UV-B radiation. UV radiation is categorized into three types; UV-A, UV-B and UV-C. UV-C does not reach the earth’s surface. UV-B is partially filtered by the ozone layer. And UV-A is not filtered at all by the ozone layer Ozone depletion is caused by the release of certain chemicals into the atmosphere.

Release of manmade chemicals such as halogenated hydrocarbons to atmosphere disturb the natural equilibrium of forming and destroying ozone molecules.  If the destroyed ozone molecules are not replaced quickly enough by the new ozone molecules, the equilibrium will be out of balance and the concentration of ozone molecules will be reduced. Even after one ozone molecule is destroyed, the ODS are still be available to destroy even more ozone molecules. Because the destructive lifetime of ODS may range from 100 to 400 years, depending on the type, one molecule of ODS could destroy hundreds of thousands of ozone molecules.

Figure: Illustration ozone depletion process with chlorofluorocarbons (CFCs) Source: UNEP (2013)

Ozone hole

The term ‘ozone hole’ refers to the depletion of the protective ozone layer in the upper atmosphere (stratosphere) over Earth's Polar Regions. Humans, plants, and animals living under the ozone hole are harmed by the solar radiation reaching the Earth's surface—where it causes health problems, from eye damage to skin cancer.

Stratospheric ozone is constantly produced by the action of the sun's ultraviolet radiation on oxygen molecules (known as photochemical reactions). Although ozone is created primarily at tropical latitudes, large-scale air circulation patterns in the lower stratosphere move ozone toward the poles, where its concentration builds up.

In addition to this global motion, strong winter polar vortices are also important to concentrating ozone at the poles. During the continuously dark polar winter, the air inside the polar vortices becomes extremely cold, a necessary condition for polar stratospheric cloud formation.

Polar stratospheric clouds create the conditions for drastic ozone destruction, providing a surface for chlorine to change into ozone-destroying form. They generally last until the sun comes up in the spring.

In the 1980s, scientists discovered that the ozone layer was thinning in the lower stratosphere, with particularly dramatic ozone loss—known as the "ozone hole"—in the Antarctic spring (September and October).

Scientists also discovered that the thinning in the ozone layer was caused by increasing concentrations of ozone-depleting chemicals – chlorofluorocarbons or CFCs (compounds with chlorine and/or fluorine attached to carbon) and to a lesser extent halons (similar compounds with bromine or iodine). These chemicals can remain in the atmosphere for decades to over a century.

At the poles, CFCs attach to ice particles in clouds. When the sun comes out again in the polar spring, the ice particles melt, releasing the ozone-depleting molecules from the ice particle surfaces.

Once released, these ozone-destroying molecules do their dirty work, breaking apart the molecular bonds in UV radiation-absorbing ozone.

Most of the ozone-depleting substances emitted by human activities remain in the stratosphere for decades, meaning that ozone layer recovery is a very slow, long process.

The chart below shows the development of the (annual maximum) size of the ozone hole over the Antarctic. The hole grew in the years following ratification of the Montreal Protocol, due to the lag caused by the fact that ozone-depleting substances remain in the stratosphere for a long time. The maximum size of the ozone hole is now decreasing.

 

For the status of the currently ongoing ozone hole, you can visit the Copernicus web site 

https://ozonewatch.gsfc.nasa.gov/  Link to NASA Ozone Watch

Maximum ozone hole extent over the southern hemisphere, from 1979 to 2019.

The images below show analyses of total ozone over the Antarctic by Copernicus. The blue colours indicate the lowest ozone amounts, while yellow and red indicate higher ozone amounts.

Source: European Environment Agency

Actions required globally to continue the recovery of the ozone layer are:

• Ensuring that existing restrictions on ozone-depleting substances are properly implemented and global use of ozone-depleting substances continue to be reduced.
• Ensuring that banks of ozone-depleting substances (both in storage and contained in existing equipment) are dealt with in an environmentally-friendly manner and are replaced with climate-friendly alternatives.
• Ensuring that permitted uses of ozone-depleting substances are not diverted to illegal uses.
• Reducing use of ozone-depleting substances in applications that are not considered as consumption under the Montreal Protocol.
• Ensuring that no new chemicals or technologies emerge that could pose new threats to the ozone layer (e.g. very short-lived substances).

Effects of ozone depletion for humans and the environment

Ozone layer depletion causes increased UV radiation levels at the Earth's surface, which is damaging to human health.

Negative effects include increases in certain types of skin cancers, eye cataracts and immune deficiency disorders. UV radiation also affects terrestrial and aquatic ecosystems, altering growth, food chains and biochemical cycles. Aquatic life just below the water’s surface, the basis of the food chain, is particularly adversely affected by high UV levels. UV rays also affect plant growth, reducing agricultural productivity.

It is the UV-B radiation that is harmful and mainly responsible for damaging human health and the environment.

Human health

Increased exposure to UV-B radiation can suppress the immune system by damaging DNA. The UV-B radiation also causes skin cancers - both non-melanoma (the less dangerous) and the virulent cutaneous malignant melanoma, quick ageing and eye cataracts. Similar effects may occur to the other animal population.

Flora

Plants are highly sensitive to UV-B radiation, and small increases in UV-B exposure can have significant biological effects to plant metabolic processes such as photosynthesis, respiration, reproduction, growth and development, and yield formation. It reduces the resilience of plants for biotic stress (Pests and diseases) conditions. Decline in plant productivity would in turn affect soil erosion and the carbon cycle.

Aquatic organisms

UV-B radiation damages aquatic organisms (plankton, aquatic plants and fish larvae, shrimp and larvae of crabs) and can cause damage to early developmental stages of marine life. The most severe effects are decreased reproductive capacity and impaired larval development.

Materials

Common building materials such as paint, rubber, wood and plastics are degraded by UV-B radiation, particularly the plastics and rubbers used outdoors. Damage can be severe in tropical regions, where the effects of UV-B radiation are high.  

Ground-level smog

UV-B radiation increases ground-level smog, especially in cities where vehicle and industry emissions provide the basis for photochemical reactions. These reactions have their own adverse effects on human health and the environment.

The Montreal Protocol

In 1987, to address the destruction of the ozone layer, the international community established the Montreal Protocol on ozone-depleting substances. It was the first international treaty to be signed by all countries of the world and is considered the greatest environmental success story in the history of the United Nations.

The Montreal Protocol’s objective is to cut down the production and consumption of ozone-depleting substances, in order to reduce their presence in the atmosphere and thus protect the Earth's ozone layer.

The chart below shows the decreasing consumption of ozone-depleting substances covered by the Montreal Protocol, both globally and by the EEA-33 (the 28 EU Member States plus Iceland, Liechtenstein, Norway, Switzerland and Turkey).

Relationship between ozone-depleting substances & climate change

ODS are chemical substances—basically chlorinated, fluorinated or brominated hydrocarbons—that have the potential to react with ozone molecules in the stratosphere. If a substance is only fluorinated (does not contain chlorine and/or bromine), it is not an ozone-depleting substance.

ODS include:

• Chlorofluorocarbons (CFCs)
• Hydrochlorofluorocarbons (HCFCs)
• Halons
• Hydrobromofluorocarbons (HBFCs)
• Bromochloromethane (CH2BrCl)
• 1,1,1-trichloroethane (methyl chloroform)
• Carbon tetrachloride (CCl4)
• Methyl bromide (MeBr)

The ability of the ODS to deplete Ozone Layer is referred as Ozone Depleting Potential (ODP). Each substance is assigned an ODP relative to CFC-11 (trichlorofluoromethane) whose ODP is defined as 1.0. An ODP higher than 1.0 means that the chemical has a greater ability than CFC-11 to destroy the ozone layer; an ODP lower than 1.0 means that the chemical’s ability to destroy the ozone layer is less than that of CFC-11. These ODS also have Global Warming Potential (GWP). Therefore, phasing out ODS not only protects the ozone layer, but also contributes to combat climate change. ODP and GWP of various ODS are tabulated in Annexure 1 with reference to the Montreal Protocol, its amendments and IPCC assessment reports (4^th and 5^th).

What are the common uses of ODS?

In most developing countries, the largest sector in which ODS are still used is refrigeration and air-conditioning. CFCs and HCFCs are used as refrigerants for the cooling circuits. However, the production and consumption of CFCs has been phased out since 2010 and perhaps the use of CFCs is almost zero in the world. HCFCs which are transitional substances, are in the process of being phased out worldwide under the Montreal Protocol. ODSs were widely used as blowing agents for foam manufacturing, as cleaning solvents in the electronics industry and in dry-cleaning, as propellant in aerosol applications and in metered dose inhalers (MDIs) used for treating pulmonary diseases, as sterilant in hospitals, as firefighting agents until 2010. MeBr is still use as fumigants for controlling pests and for quarantine and pre-shipment. 56 ODSs have been already phased-out and HCFC -22 out of remaining 40 ODSs (HCFCs) is still being used in refrigeration and air conditioning sector.

What are ODS substitutes?

ODS are gradually being phased out from all applications except for a few specific areas considered essential. The following are the main substitutes for CFCs and HCFCs which are presently used in large quantities in almost all applications:

• Hydrofluorocarbons (HFCs): HFC-134a (R-134a), HFC-152a (R -152a), HFC-32 (R-32) and the R -410 A, R 404 A, R 407 C and R-507 (mixtures of HFCs) are the most popular HFCs. Most HFCs are also potent greenhouse gases.
• Natural Refrigerant
  • Hydrocarbons (HCs): R-290 (propane), n-pentane and R-600a (Isobutane) are the most popular ODS substitutes. However, HCs are flammable substances.
  • Ammonia (NH3): Widely used in refrigeration and is being introduced for air conditioning chillers.
  • Carbon dioxide (CO₂): Industrial and commercial refrigeration
• Unsaturated HFCs:also known as hydrofluoroolefins (HFOs), which have much lower GWPs than HFCs. The most popular HFOs are: HFC-1234yf (used in refrigeration and air conditioning) and HFC-1234ze (used in foam blowing).

How are ODS released into the stratosphere?

ODS are released to the atmosphere in a variety of ways, including through;

• Venting and purging during servicing of refrigeration and air-conditioning systems
• Use of methyl bromide for quarantine and pre-shipment applications
• Disposal of ODS-containing products and equipment such as foams and refrigerators without prior recovery of the ODS
• Leaks in equipment and products that contain ODS.

Once released into the atmosphere, ODS are diluted into the ambient air. They can reach the stratosphere through air currents, thermodynamic effects and diffusion. 

What are the linkages between ozone depletion and global warming?

Most ODS are also powerful greenhouse gases, which means they contribute to climate change when released. Such gases trap the outgoing heat from the earth, causing the atmosphere to become warmer. The impacts of global climate change are extremely serious and may include a rise in sea level, intensified weather patterns, unpredictable effects on agriculture ecosystems and natural disasters.

Interaction between ozone depletion and climate change

Therefore, the global phase-out of ozone depleting substances such as hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) has also made a significant positive contribution to the fight against climate change.

On the other hand, the global phase-out has led to a large increase in the use of other types of gases, to replace ozone depleting substances in various applications. These fluorinated gases (‘F-gases’) do not damage the ozone layer, but do have a significant global warming effect. Therefore, in 2016, Parties to the Montreal Protocol agreed to add the most common type of F-gas, hydrofluorocarbons (HFCs), to the list of controlled substances.