12 Aug

Ozone LayerOzone Layer Depletion: Causes, Effects & Control Measures

What is the Ozone Layer

• Ozone is a natural gas; it is an allotrope of oxygen consisting of three atoms of oxygen bound together
• in a non-linear fashion. The chemical symbol for ozone is O3.
• It’s a pale blue gas with distinctive pungent smell.
• Pollutant at ground level – discussed with air pollution
• Ozone Layer
  • The ozone layer or ozone shield, is a region of Earth’s stratosphere that absorbs most of the sun’s ultraviolet (UV) radiation. It contains a high concentration of Ozone (O3) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere.
  • The ozone layer is mainly found in the lower stratosphere (approx. 20-30 km above Earth)
• Usefulness of the ozone layer: Prevents damaging Ultraviolet from reaching Earth, thus benefiting both plants and animals; Protects the oxygen of the lower layer, which would be broken up by the action of ultraviolet rays otherwise.

OZONE LAYER DEPLETION

• What caused Ozone Layer depletion?
  • In the 1970s, scientists discovered that Chlorofluorocarbons (CFCs), broke apart in the atmosphere and released chlorine atoms. This caused the ozone depletion. The same effect resulted when bromine atoms were released by halons. Thus, CFCs and halons are examples of ozone-depleting substances.
• What are the uses of ozone-depleting substances/ when and why are they produced?
  • Chlorofluorocarbons (CFCs): Used as refrigerants and aerosol propellants, for making plastic foam, and cleaning electronic equipment.
  • Lifetime and removal of CFCs: Unlike other chemicals, CFCs cannot be eliminated from atmosphere by the usual scavenging processes like photo dissociation, rain-out, and oxidation.
  • Escape of CFCs: The CFCs enter into atmosphere by gradual evaporation from their source, discarded refrigerators, etc.) Since the CFCs are thermally stable, they can survive in the troposphere. But in the stratosphere, they are exposed to UV radiation.

• Bromine-containing compounds: Bromine-containing compounds called halons and HBFCs, i.e., hydrobromofluorocarbons [both used in fire extinguishers] and methyl bromide (a widely used pesticide).
• Carbon Tetrachloride: It is a cheap, highly toxic solvent. Used in manufacture of synthetic rubber, the production of pesticides and pharmaceuticals.
• Methyl Chloroform: Used as cleaning solvent for clothes and metals, and a propellant in a wide range of consumer products,such as correction fluid, dry cleaning sprays,spray adhesives) and other aerosols.
• Trichloroethane: A versatile, all-purpose solvent.
• Hydrochlorofluorocarbons(HCFCs): Developed as an interim replacement for CFCs. Much less harmful then CFCs. But have high global warming potentials.
• Nitrous Oxide (N2O): It can gradually reach the middle of the stratosphere, where it is photolytically destroyed to yield nitric oxide which in turn destroys ozone.
• Sulphuric Acid Particles:
  • The most prominent acid used in various industries
  • These particles free chlorine from the molecular reservoirs and convert reactive nitrogen into inert forms, thus preventing the formation of chlorine reservoirs.

SCIENCE OF OZONE DESTRUCTION

• Through Chlorine atoms:
  • The molecules of CFCs when exposed to UV radiation break up, thus freeing chlorine atoms. A free chlorine atom reacts with an ozone molecule to form chlorine monoxide (ClO).
  • The depletion of ozone is catalytic (ClO) further combine with an atom of oxygen to form O2 and Cl. This Cl can further react with O3 and the cycle continues. Thus, a single chlorine atom can destroy thousands of ozone molecules.
• Bromine atoms:
  • Each bromine atom destroys hundred times of more ozone molecules than what a chlorine atom does.
    • Bromine + Ozone —> Bromine monoxide + Oxygen
    • Bromine monoxide + Chlorine Monoxide —> Oxygen + Bromine + Chlorine
• Nitric Oxide (NO):
  • Nitric oxide also catalytically destroys ozone
    • Nitric Oxide (NO) + Ozone (O3) -> Nitrogen dioxide (NO2) + Oxygen (O2)
    • Nitrogen dioxide (NO2) + monoxide (O) —> Nitric Oxide (NO) + oxygen (O2)

EXTENT OF MAXIMUM DAMAGE TO THE OZONE LAYER

In 2000, the areas of Antarctic Ozone hole reached a record of 29 million sq km

POLAR STRATOSPHERIC CLOUDS AND OZONE DEPLETION

• What is Polar Stratospheric Cloud?
  • PSCs, also known as nacreous clouds (or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitude of 15 – 25 kms. They contain water, nitric acid and/or sulfuric acid.
• Role in ozone depletion:
  • Situation without PSCs
    • Chlorine released by the breakdown of CFCs exists initially as pure chlorine or as chlorine monoxide but these two react further to form compounds Chlorine nitrate and HCL that are stable (inactive chlorine)
    • The stable compounds HCL and CLONO2 reservoirs of chlorine, and therefore for chlorine to take part in reactions of any sort, it has to be freed.
• Role of PSCs: Activating chlorine and absorbing nitrogen:
  • Ice particles of the PSC provides substrates for chemical reaction which frees chlorine from its reservoirs. Usually, the reaction between HCL and CLONO2 (Chlorine Nitrate) is very slow, but this reaction occurs at a faster rate in the presence of suitable substrate which is provided by the stratospheric clouds at the poles.
  • HCL + Chlorine Nitrate —> Cl2 (Molecular chlorine) + HNO3 (Nitric Acid)
  • PSCs not only activate chlorine, but they also absorb reactive nitrogen. If nitrogen oxides were present, they would combine with chlorine monoxide to form a reservoir of chlorine nitrate (ClONO2).

WHY IS OZONE DEPLETION PREDOMINANT OVER ANTARCTIC (AND NOT ARCTIC) AND OTHER AREAS WHICH PRODUCE MORE OZONE?

• Antarctic is colder than the Arctic: The Antarctic stratosphere is much colder. The low temperature enables the formation of PSCs below 20 km.
• Stability of Vortex is longer here:
  • The vortex is a ring of rapidly circulating air that confines the ozone depletion in the Antarctic region.
  • The longevity of the Antarctic vortex is another factor, enhancing favorable conditions for the depletion of ozone.
  • The vortex in Antarctic remains, in fact, throughout the polar winter, well into midspring, whereas the vortex in the Arctic disintegrates by the time of polar spring (March-April)

ENVIRONMENTAL IMPACT OF OZONE DEPLETION: IMPACT OF UV B RADIATION ON LIVING AND NON-LIVING THINGS ON EARTH

Decrease in the quantity of total-column ozone tend to cause increased penetration of solar UV-B
radiation (290-315 nm) to the earth’s surface. It has profound effect on human health, animal plants,
microorganisms, material, and air quality.

• Effect on Human and Animal Health:
  • Eye disease, skin cancer and infectious morbidity
  • In susceptible (light skinned colored) population UV-B radiations is the key risk factor for development of non-melanoma skin cancer (NMSC).
• Effects on terrestrial plants and Aquatic Ecosystem:
  • Physiological and developmental process are affected.
• Effects on biogeochemical cycles:
  • Alternates both source and sinks of greenhouse and chemically important trace gases
• Effects on air quality:
  • Higher photo dissociation rates of key trace gases that controls the chemical reactivity of the troposphere.
  • Increase both production and destruction of ozone (O3) and related oxidants such as hydrogen peroxide (H2O2), which are known to have adverse effects on human health, terrestrial plants, and outdoor materials.
  • Can lead to increased production of particulates such as cloud condensation nuclei.
• Effects on Materials:
  • Synthetically occurring polymers and naturally occurring biopolymers, as well as other materials, are adversely affected by solar UV radiation.
  • It increases the photodegradation of these materials, limiting their life outdoors.
    

VARIOUS INITIATIVES TO CONTAIN OZONE DEPLETION

• VIENNA CONVENTION
  • Background: Signed in 1985 and came into force in 1988
  • Convention:
    • The objective of the convention was for countries to promote cooperation by means of systematic observations, research and information exchange on the effects of human activities on the ozone layer and to adopt legislative and administrative measures.
    • However, it set an important precedent. For the first time, nations agreed in principle to tackle a global environmental problem before its effects were felt or conclusively proven by science.
    • In 2009, the Vienna Convention became the first convention of any kind to achieve universal ratification.
• MONTREAL PROTOCOL:
  • Once the scientific observation confirmed the ozone hole, governments recognized the need for stronger measures to reduce the production and consumption of several CFCs and halons.
  • The Montreal Protocol was signed in September 1987. It is an international treaty designed to protect the ozone layer through the reduction of the production and consumption of ODS. It came into force in 1989.
  • Key features:
    • It required all parties to eliminate the production and import of nearly 100 substances that deplete the ozone layer, in accordance with agreed timelines.
    • Special provisions for developing countries -> grace period of 10-15 years.
    • Multilateral funds – a financial mechanism to help qualifying developing countries phase out their consumption of ozone-depleting substances.
    • It required parties to report annually on the production, import, and export of ODSs.
    • Precludes parties from trading ozone-depleting substances with non-parties.
    • Requires regular assessments to enable parties to make informed decisions with the most up-to-date information.
  • Chemicals covered:
    • The Montreal protocol controls nearly 100 chemicals, grouped in the following categories:
      • CFCs
      • Halons
      • Carbon tetrachloride (CTC)
      • HCFC
      • Methyl Chloroform
      • Methyl Bromide
  • It has been ratified by 197 parties making it first and only universally ratified protocol in UN history.
  • Impact of Montreal Protocol:
    • It has also been a highly successful international arrangement, as it has phased-out more than 98% of the ODS which was part of its main mandate by 2021. The remaining ODS are HCFCs, which are in the process of being phased out.
  • What has India done under the Montreal Protocol:
    • India has already phased out CFCs, and CTC.
    • In Jan 2020, India also achieved complete phaseout of Hydrochlorofluorocarbon (HCFC)-141 b, which is a chemical used by foam manufacturing enterprise and is one of the most potent ODS after CFCs.
      • It is mainly used as a blowing agent in the production of rigid polyurethane (PU) foams.
    • Currently India is engaged in the phase-out of production and consumption of other Hydrochlorofluorocarbons (HCFCs) with an accelerated phase out schedule as per the Montrea Protocol.
      • India’s current plan will result in 60 percent phase out of HCFCs by Jan 1, 2023.
• KIGALI AMENDMENT TO MONTREAL PROTOCOL
  • About Kigali Agreement to Montreal Protocol.
    • During the 28th Meeting of Parties (MoP) of the Montreal Protocol in 2016, Kigali Agreement was finalized.
    • Kigali agreement refers to an amendment to the 1989 Montreal Protocol to eliminate planet-warming HFC gases.
      • It calls for phasing-out of HFCs, a set of 19 gases in Hydrofluorocarbon family that are used extensively in air-conditioning and refrigerant industry.
      • Currently, they may have a small contribution in global warming, but with the increase in the use of Air-Conditioning and Refrigeration, its contribution will be huge. Some estimates show that if the growth in the use of HFCs continues at the current rate, their contribution to global warming may reach 19% by 2050.
    • Why put the target in Montreal Protocol and not UNFCCC?
      • Montreal Protocol is much more successful than the UNFCCC and have fairly good track record in controlling various kinds of emissions.
    • Legally binding commitments:
      • Rich and industrialized countries bring down their HFC production and consumption by at least 85 percent by 2036 compared to their annual average values in the period 2011-13, starting from 2019.
      • A group of developing countries (more than 100) including China, Brazil and South Africa are mandated to reduce their HFC use by 80 percent of their average value in 2020-22 by the year 2045 starting from 2024.