Ozone Layer: Importance, Depletion, Causes, and Global Measures | Home Academy
The ozone layer is a thin protective layer of ozone (O₃) gas in the stratosphere, located approximately 15–35 km above Earth’s surface, which absorbs most of the sun’s harmful ultraviolet (UV) radiation. It is vital for life on Earth as it prevents DNA damage in living organisms and protects ecosystems.
Regions of the Ozone Layer
The ozone layer mainly lies in the stratosphere at an altitude of 15–35 km, where the maximum concentration of ozone is found. Above this, in the mesosphere, ozone concentration decreases, offering minimal protection. The tropopause region, located at 8–18 km in the tropics and 10–25 km in temperate regions, contains moderate ozone levels.
Exam Highlight: The ozone layer is mainly in the stratosphere.
Units of Measurement: Ozone thickness is measured in Dobson Units (DU). 1 DU equals the thickness of 0.01 mm if compressed to standard temperature and pressure. Normal global ozone levels are about 300 DU.
Causes of Ozone Depletion
The ozone layer is destroyed when chemical reactions break down ozone molecules faster than they form. The primary cause is the emission of Chlorofluorocarbons (CFCs) from refrigerants, aerosols, and foam products. Halons, used in fire extinguishers, release bromine, which is more destructive than chlorine. Other contributors include Nitrous Oxide (N₂O) from fertilizers and methyl bromide (CH₃Br) from fumigants.
Natural factors like volcanic eruptions and solar activity also affect ozone but are less significant.
Exam Highlight: CFCs and halons are the main ozone-depleting substances.
of Depletion: Increased UV radiation leads to skin cancer, cataracts, immune system suppression, and damage to crops and marine life.
Discovery of the Ozone Hole
The ozone hole is a region of severely depleted ozone over the Antarctic, discovered in 1985 by Joseph Farman, Brian Gardiner, and Jonathan Shanklin of the British Antarctic Survey. Ozone levels during the Antarctic spring (September–October) dropped below 220 DU, far below normal levels. A smaller ozone hole has also been identified over the Arctic.
Exam Highlight: Ozone hole discovered in 1985 over Antarctica by Farman, Gardiner, and Shanklin.
International Conventions and Measures
The Vienna Convention (1985) provided a framework for global cooperation on ozone protection, promoting research, monitoring, and information exchange. The Montreal Protocol (1987) aimed to phase out ozone-depleting substances (ODS) like CFCs, halons, and methyl bromide, and is considered one of the most successful environmental treaties. The Kigali Amendment (2016) focuses on reducing hydrofluorocarbons (HFCs), which do not harm ozone but contribute to global warming.
Exam Highlight: Montreal Protocol and Vienna Convention are key international efforts to protect the ozone layer.
Key Facts and Highlights for Exams
The ozone layer absorbs 97–99% of UV-B radiation, preventing severe harm to life on Earth. Normal thickness is ~300 DU, while the Antarctic ozone hole can reduce it to 100 DU or less. Full recovery is expected by 2060–2070 if the reduction of ozone-depleting substances continues. Monitoring agencies include NASA, NOAA, UNEP, and WMO.
Exam Highlight: Dobson Unit (DU) measures ozone thickness; O₃ absorbs UV-B; full recovery expected by 2060–2070.
Conclusion
The ozone layer is a critical shield that protects life from harmful UV radiation. Human activities, especially the use of CFCs, halons, and other ODS, have caused significant depletion, resulting in the Antarctic ozone hole. International agreements like the Vienna Convention and Montreal Protocol have successfully reduced emissions and promoted global cooperation. Continued vigilance is essential for full recovery and the protection of life on Earth.
MCQs for Exam
What is the main cause of ozone depletion? Answer: Chlorofluorocarbons (CFCs)
Who discovered the ozone hole over Antarctica? Answer: Joseph Farman, Brian Gardiner, Jonathan Shanklin
Ozone layer thickness is measured in Dobson Units
Advanced-Level MCQs on Ozone Layer
The primary mechanism of ozone depletion by Chlorofluorocarbons (CFCs) involves:
A) Direct absorption of UV-B by CFCs
B) Release of chlorine radicals catalyzing ozone destruction
C) Conversion of O₂ into O₃ by CFCs
D) Formation of greenhouse gases increasing ozone breakdown
Answer: B) Release of chlorine radicals catalyzing ozone destruction
Which reaction cycle is responsible for the largest proportion of stratospheric ozone loss in the Antarctic ozone hole?
A) HOx catalytic cycle
B) NOx catalytic cycle
C) ClOx catalytic cycle
D) BrOx catalytic cycle
Answer: C) ClOx catalytic cycle
Ozone in the stratosphere is mainly formed by:
A) Direct combination of three oxygen atoms
B) Photodissociation of O₂ by UV-C radiation followed by O + O₂ → O₃
C) Oxidation of nitrogen compounds
D) Reaction of CO₂ with UV light
Answer: B) Photodissociation of O₂ by UV-C radiation followed by O + O₂ → O₃
The Antarctic ozone hole appears primarily during spring (September–October) because:
A) Temperatures are highest, favoring ozone breakdown
B) Polar stratospheric clouds (PSCs) form, releasing chlorine and bromine radicals
C) Sunlight is absent, allowing ozone accumulation
D) Nitrogen oxides are at maximum, destroying ozone
Answer: B) Polar stratospheric clouds (PSCs) form, releasing chlorine and bromine radicals
Which of the following gases is most effective on a per-atom basis in destroying ozone?
A) Chlorine (Cl)
B) Bromine (Br)
C) Nitrous oxide (N₂O)
D) Carbon dioxide (CO₂)
Answer: B) Bromine (Br)
Dobson Unit (DU) is defined as:
A) The number of ozone molecules per cubic centimeter of air
B) Thickness in mm of ozone if compressed to standard temperature and pressure
C) Fraction of UV-B absorbed by ozone in stratosphere
D) Mass of ozone in the entire stratosphere
Answer: B) Thickness in mm of ozone if compressed to standard temperature and pressure
Which amendment or protocol addresses hydrofluorocarbons (HFCs) that do not deplete ozone but contribute to global warming?
A) Vienna Convention
B) Montreal Protocol
C) Kigali Amendment
D) Kyoto Protocol
Answer: C) Kigali Amendment
Which of the following statements about ozone in the troposphere is correct?
A) Tropospheric ozone is protective like stratospheric ozone
B) It is a greenhouse gas and a pollutant contributing to smog
C) It is the primary source of UV absorption
D) Tropospheric ozone levels are measured in Dobson Units
Answer: B) It is a greenhouse gas and a pollutant contributing to smog
The phenomenon called “ozone hole” is not observed over the Arctic as severely as in Antarctica because:
A) Arctic winters are colder than Antarctic winters
B) Polar vortex is weaker in the Arctic, limiting PSC formation
C) Arctic receives more UV radiation than Antarctica
D) Arctic has higher CFC concentrations than Antarctic
Answer: B) Polar vortex is weaker in the Arctic, limiting PSC formation
Which of the following statements is correct regarding ozone layer recovery?
A) Full recovery is expected by 2030 if current trends continue
B) Recovery depends solely on the natural breakdown of ozone
C) Montreal Protocol compliance is crucial for recovery
D) Ozone cannot recover due to permanent stratospheric damage
Answer: C) Montreal Protocol compliance is crucial for recovery
World Ozone Day (International Day for the Preservation of the Ozone Layer)
Date: 16th September every year
Purpose: To raise global awareness about the importance of the ozone layer, ozone depletion, and international efforts to protect it.
Background
World Ozone Day was established by the United Nations in 1994.
It commemorates the signing of the Montreal Protocol on 16th September 1987, a landmark international treaty aimed at phasing out ozone-depleting substances (ODS).The day highlights global efforts to protect the stratospheric ozone layer, which shields life on Earth from harmful ultraviolet (UV) radiation.
Annual Themes
Each year, the United Nations Environment Programme (UNEP) announces a theme to focus on a specific aspect of ozone protection and climate action. Some recent and important themes include:
2026: “Ozone Layer Protection: Global Responsibility for Future Generations”
2025: “Montreal Protocol @ 38: Global Cooperation Protecting Life on Earth”2024: “Caring for All Life under the Sun: Celebrating 37 Years of the Montreal Protocol”
2023: “Ozone for Life: 36 Years of Global Action”
2022: “Montreal Protocol – Keeping us, our food and vaccines cool”
2021: “Montreal Protocol – Keeping us, our food and vaccines cool”
2020: “Ozone layer protection: The link between climate change and ozone”