🌍 Our Environment
Causes and Effects of Ozone Depletion
Class 10 Science | CBSE + Foundation + Competitive Level
🌍 Causes and Effects of Ozone Depletion
1. Introduction – परिचय
Ozone (O₃) is a molecule made up of three oxygen atoms. A large concentration of ozone is present in the stratosphere, where it forms the ozone layer.
ओजोन (O₃) तीन ऑक्सीजन परमाणुओं से बना एक अणु है। वायुमंडल के समताप मंडल (Stratosphere) में इसकी अधिक सांद्रता पाई जाती है, जहाँ यह ओजोन परत बनाती है।
ओजोन (O₃) तीन ऑक्सीजन परमाणुओं से बना एक अणु है। वायुमंडल के समताप मंडल (Stratosphere) में इसकी अधिक सांद्रता पाई जाती है, जहाँ यह ओजोन परत बनाती है।
The ozone layer acts as a natural protective shield because it absorbs a large part of the Sun's harmful ultraviolet radiation.
ओजोन परत पृथ्वी के लिए एक प्राकृतिक सुरक्षा कवच का कार्य करती है क्योंकि यह सूर्य से आने वाली हानिकारक पराबैंगनी (UV) विकिरण का बड़ा भाग अवशोषित करती है।
ओजोन परत पृथ्वी के लिए एक प्राकृतिक सुरक्षा कवच का कार्य करती है क्योंकि यह सूर्य से आने वाली हानिकारक पराबैंगनी (UV) विकिरण का बड़ा भाग अवशोषित करती है।
2. Formation of Ozone – ओजोन का निर्माण
O₂ + UV radiation → O + O
O + O₂ → O₃
High-energy ultraviolet radiation can split oxygen molecules into individual oxygen atoms. These oxygen atoms then combine with oxygen molecules to form ozone.
उच्च ऊर्जा वाली पराबैंगनी किरणें ऑक्सीजन अणु (O₂) को अलग-अलग ऑक्सीजन परमाणुओं में तोड़ सकती हैं। ये ऑक्सीजन परमाणु O₂ से मिलकर O₃ यानी ओजोन बनाते हैं।
3. Natural Ozone–Oxygen Cycle
O₂
Oxygen Molecule
Oxygen Molecule
→
UV Radiation
Breaks O₂
Breaks O₂
→
O
Oxygen Atom
Oxygen Atom
→
O₃
Ozone
Ozone
→
UV Absorption
Ozone Breaks
Ozone Breaks
Important: Ozone formation and destruction naturally occur continuously. Under normal conditions, a dynamic balance is maintained.
महत्वपूर्ण: ओजोन का निर्माण और विघटन प्राकृतिक रूप से लगातार होता रहता है। सामान्य परिस्थितियों में इनके बीच संतुलन बना रहता है।
महत्वपूर्ण: ओजोन का निर्माण और विघटन प्राकृतिक रूप से लगातार होता रहता है। सामान्य परिस्थितियों में इनके बीच संतुलन बना रहता है।
4. Ultraviolet Radiation – पराबैंगनी विकिरण
| Radiation | Approximate Wavelength | Importance |
|---|---|---|
| UVA | 315–400 nm | Reaches Earth's surface in large amount; can contribute to skin ageing and damage. |
| UVB | 280–315 nm | Partly absorbed by ozone; excessive exposure can damage skin and eyes. |
| UVC | 100–280 nm | Very energetic; largely absorbed by atmospheric gases and ozone. |
Excessive exposure to UV radiation can damage living organisms. The ozone layer reduces the amount of harmful UV radiation reaching Earth's surface.
5. What is Ozone Depletion?
Ozone depletion means a reduction in the concentration of stratospheric ozone, especially when chemical reactions destroy ozone faster than it is naturally replenished.
ओजोन क्षरण का अर्थ समताप मंडल में ओजोन की सांद्रता में कमी होना है, विशेषकर तब जब रासायनिक अभिक्रियाएँ ओजोन को उसके प्राकृतिक निर्माण की तुलना में अधिक तेजी से नष्ट करती हैं।
ओजोन क्षरण का अर्थ समताप मंडल में ओजोन की सांद्रता में कमी होना है, विशेषकर तब जब रासायनिक अभिक्रियाएँ ओजोन को उसके प्राकृतिक निर्माण की तुलना में अधिक तेजी से नष्ट करती हैं।
6. Major Causes of Ozone Depletion – प्रमुख कारण
1. Chlorofluorocarbons (CFCs)
CFCs were widely used in refrigeration, air-conditioning, aerosol propellants and some foam manufacturing processes.
क्लोरोफ्लोरोकार्बन (CFCs) का उपयोग रेफ्रिजरेशन, एयर-कंडीशनिंग, एयरोसोल प्रोपेलेंट और कुछ फोम निर्माण प्रक्रियाओं में किया जाता था।
CFCs were widely used in refrigeration, air-conditioning, aerosol propellants and some foam manufacturing processes.
क्लोरोफ्लोरोकार्बन (CFCs) का उपयोग रेफ्रिजरेशन, एयर-कंडीशनिंग, एयरोसोल प्रोपेलेंट और कुछ फोम निर्माण प्रक्रियाओं में किया जाता था।
2. Halons
Halons containing bromine were used in certain fire-extinguishing systems. Bromine can participate in catalytic ozone destruction.
Halons containing bromine were used in certain fire-extinguishing systems. Bromine can participate in catalytic ozone destruction.
3. Carbon Tetrachloride (CCl₄)
It was historically used as a solvent and in industrial applications and can release ozone-depleting chlorine-containing compounds.
It was historically used as a solvent and in industrial applications and can release ozone-depleting chlorine-containing compounds.
4. Methyl Chloroform
Also known as 1,1,1-trichloroethane, it was historically used as an industrial solvent.
Also known as 1,1,1-trichloroethane, it was historically used as an industrial solvent.
5. Methyl Bromide
A bromine-containing substance historically used as a fumigant and recognised as an ozone-depleting substance.
A bromine-containing substance historically used as a fumigant and recognised as an ozone-depleting substance.
6. Nitrogen Oxides and other reactive compounds
Certain reactive nitrogen compounds can participate in atmospheric chemistry affecting ozone, although the major anthropogenic ozone-depletion problem addressed by the Montreal Protocol has been associated with chlorine- and bromine-containing substances.
Certain reactive nitrogen compounds can participate in atmospheric chemistry affecting ozone, although the major anthropogenic ozone-depletion problem addressed by the Montreal Protocol has been associated with chlorine- and bromine-containing substances.
7. CFCs – A Major Cause
CFC molecules are relatively stable in the lower atmosphere. They can eventually reach the stratosphere, where intense ultraviolet radiation breaks them apart and releases chlorine atoms.
CFC अणु निचले वायुमंडल में काफी स्थिर होते हैं। वे धीरे-धीरे समताप मंडल तक पहुँच सकते हैं, जहाँ तीव्र UV radiation उन्हें तोड़कर chlorine atoms मुक्त कर सकती है।
8. CFC → UV → Chlorine → Ozone Destruction
9. Chlorine-Catalysed Ozone Destruction
Cl• + O₃ → ClO• + O₂
ClO• + O → Cl• + O₂
Overall: O₃ + O → 2O₂
The chlorine atom is regenerated at the end of the catalytic cycle. Therefore, one chlorine atom can participate in many ozone-destruction reactions.
Chlorine atom अभिक्रिया के अंत में फिर से regenerate हो जाता है। इसलिए एक chlorine atom कई ozone molecules के विनाश में भाग ले सकता है।
Chlorine atom अभिक्रिया के अंत में फिर से regenerate हो जाता है। इसलिए एक chlorine atom कई ozone molecules के विनाश में भाग ले सकता है।
10. Role of Bromine and Halons
Bromine-containing compounds such as halons can release reactive bromine in the stratosphere. Bromine participates in catalytic cycles that destroy ozone and can be highly effective per atom in ozone-destroying chemistry.
11. Ozone Hole – ओजोन छिद्र
The term ozone hole refers to a severe seasonal depletion of stratospheric ozone, most famously observed over Antarctica.
ओजोन छिद्र से तात्पर्य समताप मंडल में ओजोन की अत्यधिक मौसमी कमी से है, जिसका सबसे प्रसिद्ध उदाहरण अंटार्कटिका के ऊपर देखा जाता है।
ओजोन छिद्र से तात्पर्य समताप मंडल में ओजोन की अत्यधिक मौसमी कमी से है, जिसका सबसे प्रसिद्ध उदाहरण अंटार्कटिका के ऊपर देखा जाता है।
During the Antarctic winter, extremely low temperatures help form polar stratospheric clouds (PSCs). Chemical reactions on PSC particles convert relatively inactive chlorine compounds into forms that can release reactive chlorine when sunlight returns in spring.
Very Cold
Antarctic Stratosphere
Antarctic Stratosphere
→
Polar Stratospheric
Clouds
Clouds
→
Active Chlorine
Forms
Forms
→
Spring Sunlight
Activates Chemistry
Activates Chemistry
→
Rapid Ozone
Loss
Loss
12. Causes at a Glance
| Cause | How it contributes | Result |
|---|---|---|
| CFCs | Release reactive chlorine in stratosphere | Ozone destruction |
| Halons | Release reactive bromine | Ozone destruction |
| Carbon tetrachloride | Can release chlorine-containing reactive species | Ozone depletion |
| Methyl chloroform | Releases chlorine after atmospheric processing | Ozone depletion |
| Methyl bromide | Releases bromine | Ozone depletion |
| Cold polar conditions | Promote PSC chemistry | Enhance seasonal Antarctic ozone loss |
13. Effects on Human Health – मानव स्वास्थ्य पर प्रभाव
1. Skin Damage: Excess UV exposure can cause sunburn and contribute to premature skin ageing and skin cancers.
2. Eye Damage: Excessive UV exposure can increase the risk of cataract and other eye damage.
3. Immune System: Excessive UV-B exposure can affect immune responses.
4. DNA Damage: UV radiation can damage DNA and contribute to mutations and abnormal cell growth.
14. Effects on Plants – पौधों पर प्रभाव
Excessive UV-B exposure can damage plant tissues, interfere with photosynthesis, affect growth and development, and alter plant physiology.
- Photosynthetic efficiency may be affected.
- Leaf tissues can suffer damage.
- Growth patterns may change.
- Crop productivity can be affected in sensitive species.
15. Effects on Aquatic Ecosystems – जलीय पारितंत्र
Increased UV-B radiation can affect phytoplankton and other microscopic aquatic organisms near the surface.
Phytoplankton are important primary producers in aquatic food webs. Damage to these organisms can have effects that move through aquatic ecosystems.
Increased UV-B
→
Phytoplankton
Stress
Stress
→
Reduced Primary
Production
Production
→
Food Web
Effects
Effects
16. Effects on Materials – पदार्थों पर प्रभाव
Increased UV radiation can accelerate degradation of some polymers, plastics, paints, rubber and other exposed materials.
17. Ozone Depletion vs Global Warming
| Feature | Ozone Depletion | Global Warming |
|---|---|---|
| Main concern | Reduction of stratospheric ozone | Increase in Earth's average temperature |
| Important substances | CFCs, halons and other ODS | CO₂, CH₄, N₂O and other greenhouse gases |
| Major consequence | More harmful UV radiation can reach surface | Climate and Earth-system changes |
| Important international response | Montreal Protocol | UNFCCC, Kyoto Protocol, Paris Agreement and other climate actions |
Some ozone-depleting substances are also greenhouse gases, so ozone protection and climate change are connected in atmospheric chemistry, but the two problems are not identical.
18. Stratospheric Ozone vs Ground-Level Ozone
| Stratospheric Ozone | Ground-Level Ozone |
|---|---|
| Generally protective | Air pollutant |
| Absorbs harmful UV radiation | Can irritate lungs and damage vegetation |
| Located mainly in stratosphere | Occurs near Earth's surface |
| Often called the protective ozone layer | Forms mainly through photochemical reactions involving NOx and VOCs |
19. Montreal Protocol
The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in 1987.
It established international controls to phase out the production and consumption of many ozone-depleting substances.
Exam Point:
Montreal Protocol → Protection of ozone layer → Control/phase-out of ozone-depleting substances.
Montreal Protocol → Protection of ozone layer → Control/phase-out of ozone-depleting substances.
20. Kigali Amendment
The Kigali Amendment was adopted in 2016 under the Montreal Protocol. It targets the phase-down of hydrofluorocarbons (HFCs), which generally do not deplete ozone but can have high global warming potential.
21. Prevention and Control
1. Reduce and phase out ozone-depleting substances.
2. Use ozone-friendly alternatives and technologies.
3. Properly recover and dispose of refrigerants.
4. Avoid releasing old refrigerants into the atmosphere.
5. Follow environmental regulations.
6. Promote energy-efficient and environmentally responsible technologies.
7. Increase public awareness.
2. Use ozone-friendly alternatives and technologies.
3. Properly recover and dispose of refrigerants.
4. Avoid releasing old refrigerants into the atmosphere.
5. Follow environmental regulations.
6. Promote energy-efficient and environmentally responsible technologies.
7. Increase public awareness.
22. Important Definitions
Ozone: A triatomic form of oxygen, O₃.
Ozone Layer: Region of the stratosphere containing relatively high concentrations of ozone.
Ozone Depletion: Reduction in stratospheric ozone concentration due to enhanced destruction.
Ozone Hole: Region and period of severe seasonal ozone depletion, especially over Antarctica.
CFC: Chlorofluorocarbon; a class of chlorine- and fluorine-containing compounds historically used in refrigeration and other applications.
UV Radiation: Electromagnetic radiation with wavelengths shorter than visible violet light.
ODS: Ozone-Depleting Substances.
23. Important Difference: Ozone Hole ≠ Complete Absence of Ozone
The term ozone hole does not mean that ozone completely disappears from the atmosphere. It refers to a region where ozone concentrations become much lower than normal during a particular season.
24. Golden Memory Trick
🧠 Remember: “CFC → UV → Cl → O₃ ↓ → UV ↑”
CFC reaches stratosphere → UV breaks CFC → Cl becomes reactive → O₃ is destroyed → more harmful UV can reach Earth's surface.
Effects = S-E-P-A-M
- S – Skin damage
- E – Eye damage
- P – Plant damage
- A – Aquatic ecosystem effects
- M – Material degradation
25. 30 MCQs – Multiple Choice Questions
1. Ozone is represented by which formula?
A. O₂
B. O₃
C. CO₂
D. O₄
Answer: B
2. The protective ozone layer is mainly present in the:
A. Troposphere
B. Mesosphere
C. Stratosphere
D. Thermosphere
Answer: C
3. Which radiation is most strongly absorbed by the ozone layer?
A. UVC
B. Radio waves
C. Microwaves
D. Visible light
Answer: A
4. A major class of ozone-depleting substances is:
A. CFCs
B. Oxygen
C. Nitrogen
D. Water vapour
Answer: A
5. CFCs can release which reactive atom in the stratosphere?
A. Sodium
B. Chlorine
C. Calcium
D. Iron
Answer: B
6. Halons are particularly important because they can release:
A. Bromine
B. Sodium
C. Silicon
D. Calcium
Answer: A
7. Ozone depletion can result in greater exposure to:
A. Infrared radiation
B. Radio waves
C. Harmful UV radiation
D. Sound waves
Answer: C
8. The ozone hole is most famously associated with:
A. Sahara
B. Antarctica
C. Amazon
D. Himalayas
Answer: B
9. Polar stratospheric clouds are especially important in:
A. Antarctic ozone depletion
B. Soil formation
C. Ocean tides
D. Photosynthesis only
Answer: A
10. The chlorine atom in catalytic ozone destruction is:
A. Permanently consumed in one reaction
B. Regenerated
C. Converted into sodium
D. Converted into nitrogen
Answer: B
11. Ozone depletion may increase the risk of:
A. Skin damage
B. Sound pollution
C. Earthquakes
D. Tides
Answer: A
12. Excess UV exposure can damage:
A. Eyes
B. Rocks only
C. Mountains only
D. Metals only
Answer: A
13. Phytoplankton are important because they are:
A. Primary producers
B. Decomposers only
C. Carnivores only
D. Parasites only
Answer: A
14. The Montreal Protocol was adopted in:
A. 1972
B. 1987
C. 1997
D. 2016
Answer: B
15. Montreal Protocol primarily addresses:
A. Ozone-depleting substances
B. Earthquakes
C. Ocean currents
D. Noise pollution
Answer: A
16. Kigali Amendment primarily concerns the phase-down of:
A. Oxygen
B. HFCs
C. Nitrogen
D. Water
Answer: B
17. Which is a bromine-containing ozone-depleting substance?
A. Halon
B. Oxygen
C. Methane
D. Nitrogen
Answer: A
18. Which one is NOT an ozone-depleting substance?
A. CFC
B. Halon
C. Oxygen
D. Methyl bromide
Answer: C
19. Increased UV-B radiation can affect:
A. Phytoplankton
B. Gravity
C. Earth's rotation
D. Magnetic poles directly
Answer: A
20. Ozone is a form of:
A. Carbon
B. Nitrogen
C. Oxygen
D. Hydrogen
Answer: C
21. Ozone depletion is mainly a problem involving:
A. Stratospheric ozone
B. Ocean water
C. Soil minerals
D. Rocks
Answer: A
22. Which organ can be affected by excessive UV exposure?
A. Eye
B. Bone only
C. Tooth enamel only
D. Hair only
Answer: A
23. Ozone layer mainly protects Earth from:
A. Harmful UV radiation
B. Gravity
C. Sound
D. Ocean waves
Answer: A
24. Which compound historically served as a solvent and is an ozone-depleting substance?
A. Carbon tetrachloride
B. Water
C. Oxygen
D. Sodium chloride
Answer: A
25. Ozone depletion is dangerous because it can:
A. Increase harmful UV reaching the surface
B. Stop gravity
C. Stop rainfall everywhere
D. Remove oxygen completely
Answer: A
26. Which process is involved in natural ozone formation?
A. O₂ splitting by UV radiation
B. NaCl electrolysis
C. Water freezing
D. Carbon combustion only
Answer: A
27. Ground-level ozone is generally:
A. A protective shield
B. An air pollutant
C. A nutrient
D. A mineral
Answer: B
28. The ozone hole does NOT mean:
A. Complete disappearance of all atmospheric ozone
B. Severe seasonal ozone depletion
C. Lower ozone concentration
D. Antarctic ozone loss
Answer: A
29. Which international agreement is strongly associated with ozone protection?
A. Montreal Protocol
B. Ramsar Convention
C. Basel Convention only
D. Antarctic Treaty only
Answer: A
30. The best long-term approach to ozone protection is:
A. Increase CFC release
B. Use ozone-depleting chemicals freely
C. Phase out ozone-depleting substances
D. Increase aerosol emissions
Answer: C
26. 30 Subjective Questions with Answers
2 Marks
Q1. What is ozone?
Ozone is a triatomic form of oxygen represented by O₃. It is concentrated mainly in the stratosphere and absorbs harmful ultraviolet radiation.
2 Marks
Q2. What is ozone depletion?
Ozone depletion is the reduction in stratospheric ozone concentration due to enhanced chemical destruction of ozone.
2 Marks
Q3. Name two ozone-depleting substances.
CFCs and halons are important ozone-depleting substances.
2 Marks
Q4. Why is the ozone layer important?
It absorbs a significant fraction of harmful UV radiation and protects living organisms from excessive UV exposure.
2 Marks
Q5. What is an ozone hole?
It is a region and period of severe seasonal depletion of stratospheric ozone, especially observed over Antarctica.
2 Marks
Q6. What are CFCs?
CFCs are chlorofluorocarbons, a group of chlorine- and fluorine-containing compounds formerly used widely in refrigeration, air-conditioning and other applications.
2 Marks
Q7. Name two effects of excessive UV radiation.
Skin damage and eye damage are two important effects.
2 Marks
Q8. What is the role of chlorine in ozone depletion?
Reactive chlorine participates in catalytic reactions that convert ozone into oxygen, while chlorine can be regenerated and continue the cycle.
3 Marks
Q9. Explain the formation of ozone.
UV radiation splits O₂ into oxygen atoms. The oxygen atoms then combine with O₂ molecules to form O₃. Thus, sunlight drives the natural ozone-oxygen cycle.
3 Marks
Q10. Explain how CFCs cause ozone depletion.
CFCs can reach the stratosphere. UV radiation breaks them and releases chlorine radicals. These reactive chlorine species participate in catalytic reactions that destroy ozone and regenerate chlorine.
3 Marks
Q11. Write three effects of ozone depletion on humans.
It can increase UV exposure, contributing to skin damage, eye damage such as cataract risk, and effects on immune responses.
3 Marks
Q12. How can ozone depletion affect plants?
Excess UV-B can damage plant tissues, affect photosynthesis, alter growth and development, and reduce productivity in sensitive species.
3 Marks
Q13. Why are phytoplankton important in the context of ozone depletion?
Phytoplankton are primary producers in aquatic food webs. Increased UV-B exposure can damage them and potentially affect aquatic food chains.
3 Marks
Q14. What are polar stratospheric clouds?
They are clouds that form under extremely cold conditions in the polar stratosphere. Their surfaces promote chemical reactions that convert chlorine compounds into forms capable of releasing reactive chlorine.
3 Marks
Q15. Distinguish between ozone depletion and ozone hole.
Ozone depletion is the general reduction of stratospheric ozone. An ozone hole refers to a particularly severe seasonal region of depletion, especially over Antarctica.
4 Marks
Q16. Explain the chlorine catalytic cycle of ozone destruction.
First, chlorine reacts with ozone:
Cl• + O₃ → ClO• + O₂
Then ClO reacts with an oxygen atom:
ClO• + O → Cl• + O₂
Chlorine is regenerated, allowing it to participate in repeated cycles.
Cl• + O₃ → ClO• + O₂
Then ClO reacts with an oxygen atom:
ClO• + O → Cl• + O₂
Chlorine is regenerated, allowing it to participate in repeated cycles.
4 Marks
Q17. Describe four harmful effects of increased UV radiation.
1. Skin damage and increased skin-cancer risk.
2. Eye damage including increased cataract risk.
3. Effects on immune responses.
4. Damage to plants and aquatic organisms.
2. Eye damage including increased cataract risk.
3. Effects on immune responses.
4. Damage to plants and aquatic organisms.
4 Marks
Q18. Explain why ozone depletion can affect aquatic ecosystems.
Increased UV-B can penetrate surface waters and affect phytoplankton and other sensitive organisms. Since phytoplankton are primary producers, changes in their abundance or productivity can influence aquatic food webs.
4 Marks
Q19. Explain the importance of the Montreal Protocol.
The Montreal Protocol, adopted in 1987, established international controls to phase out many ozone-depleting substances. It is a major global framework for protecting the ozone layer.
4 Marks
Q20. Differentiate between stratospheric and ground-level ozone.
Stratospheric ozone protects Earth by absorbing UV radiation. Ground-level ozone is a pollutant that can harm the respiratory system and vegetation.
5 Marks
Q21. Explain the complete mechanism of CFC-induced ozone depletion.
CFCs are stable in the lower atmosphere and can reach the stratosphere. UV radiation breaks CFC molecules and releases reactive chlorine. Chlorine reacts with ozone to form chlorine monoxide and oxygen. Chlorine monoxide can react further and regenerate chlorine. Thus chlorine acts catalytically and repeated cycles can destroy many ozone molecules.
5 Marks
Q22. Explain the major causes of ozone depletion.
Major causes include release of CFCs, halons, carbon tetrachloride, methyl chloroform and methyl bromide. These substances can reach the stratosphere and release reactive chlorine or bromine. Very cold polar conditions can further enhance seasonal ozone loss through polar stratospheric cloud chemistry.
5 Marks
Q23. Explain the effects of ozone depletion on humans, plants and ecosystems.
Humans may experience greater UV exposure, causing skin and eye damage. Plants can experience physiological and photosynthetic damage. Phytoplankton and other aquatic organisms may also be affected, potentially influencing aquatic food webs.
5 Marks
Q24. Explain the formation and significance of the Antarctic ozone hole.
During the Antarctic winter, extremely low temperatures produce polar stratospheric clouds. Chemical reactions on these clouds transform chlorine compounds into reactive forms. When sunlight returns in spring, rapid catalytic ozone destruction occurs, producing severe seasonal ozone depletion.
5 Marks
Q25. Suggest measures for protecting the ozone layer.
Use ozone-friendly technologies, phase out ozone-depleting substances, properly recover refrigerants, prevent release of old CFC-containing equipment, follow environmental regulations and support international agreements for ozone protection.
6 Marks
Q26. Explain ozone depletion with a suitable flow chart.
The sequence can be represented as:
CFC release → Stratosphere → UV breaks CFC → Reactive Cl released → Ozone destruction → More UV reaches surface → Effects on living organisms.
The chlorine is regenerated during catalytic reactions, allowing repeated ozone destruction.
CFC release → Stratosphere → UV breaks CFC → Reactive Cl released → Ozone destruction → More UV reaches surface → Effects on living organisms.
The chlorine is regenerated during catalytic reactions, allowing repeated ozone destruction.
6 Marks
Q27. Discuss the environmental importance of the ozone layer.
The ozone layer acts as a natural UV shield. By absorbing much of the harmful UV radiation, it protects humans, animals, plants and aquatic organisms. Reduction of this protective layer can increase UV exposure and cause biological and ecological damage.
6 Marks
Q28. Explain why one chlorine atom can destroy many ozone molecules.
Chlorine participates in catalytic reactions. It reacts with ozone to form ClO, and subsequent reactions regenerate Cl. Since chlorine is regenerated rather than permanently consumed in each cycle, the same chlorine atom can participate repeatedly in ozone-destruction reactions.
6 Marks
Q29. Compare ozone depletion and global warming.
Ozone depletion concerns reduction of stratospheric ozone and increased UV exposure. Global warming concerns long-term changes in Earth's climate system due largely to increased concentrations of greenhouse gases. CFCs are relevant to both atmospheric chemistry and climate, but the two environmental problems are distinct.
6 Marks
Q30. Explain the role of international cooperation in protecting the ozone layer.
Ozone-depleting substances are released globally and can circulate through the atmosphere. Therefore, international cooperation is necessary. The Montreal Protocol created coordinated controls on ozone-depleting substances, while subsequent amendments strengthened and updated these controls.
27. Assertion–Reason Questions
Q1. Assertion: The ozone layer protects living organisms from harmful UV radiation.
Reason: Ozone absorbs significant amounts of ultraviolet radiation.
Reason: Ozone absorbs significant amounts of ultraviolet radiation.
Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Q2. Assertion: CFCs can contribute to ozone depletion.
Reason: UV radiation can break CFC molecules in the stratosphere and release reactive chlorine.
Reason: UV radiation can break CFC molecules in the stratosphere and release reactive chlorine.
Answer: Both are true, and the Reason correctly explains the Assertion.
Q3. Assertion: The ozone hole means that all ozone has disappeared from the atmosphere.
Reason: The Antarctic ozone hole represents severe seasonal ozone depletion.
Reason: The Antarctic ozone hole represents severe seasonal ozone depletion.
Answer: Assertion is false, Reason is true.
Q4. Assertion: Chlorine can participate in catalytic ozone destruction.
Reason: Chlorine can be regenerated during the reaction cycle.
Reason: Chlorine can be regenerated during the reaction cycle.
Answer: Both are true, and the Reason correctly explains the Assertion.
Q5. Assertion: Ground-level ozone and stratospheric ozone have the same environmental role.
Reason: Both forms are chemically O₃.
Reason: Both forms are chemically O₃.
Answer: Assertion is false, Reason is true.
28. HOTS – Higher Order Thinking Questions
HOTS 1: Why is ozone called both a useful and harmful gas depending on its location?
Stratospheric ozone is useful because it absorbs harmful UV radiation. Ground-level ozone is harmful because it is an air pollutant that can damage lungs and vegetation.
HOTS 2: Why does ozone depletion not mean that oxygen will disappear?
Ozone is only a small fraction of atmospheric oxygen. Ozone depletion refers to reduction of O₃ in the stratosphere, not disappearance of atmospheric O₂.
HOTS 3: Why can a small amount of reactive chlorine have a large effect?
Because chlorine participates in catalytic cycles and can be regenerated, allowing repeated reactions with ozone.
HOTS 4: Why is Antarctica especially important in discussions of ozone depletion?
The extremely cold Antarctic stratosphere promotes formation of polar stratospheric clouds, which facilitate chemical conversion of chlorine compounds into highly reactive forms. Spring sunlight then drives rapid ozone destruction.
HOTS 5: Why is protection of phytoplankton important when studying ozone depletion?
Phytoplankton are primary producers and form the base of many aquatic food webs. Their damage can have wider ecological effects.
29. Quick Revision Chart
| Topic | Key Point |
|---|---|
| Ozone formula | O₃ |
| Location | Stratosphere |
| Main function | Absorbs harmful UV radiation |
| Major ODS | CFCs, halons, CCl₄, methyl chloroform, methyl bromide |
| Main reactive atoms | Chlorine and bromine |
| Famous ozone hole | Antarctica |
| Important clouds | Polar stratospheric clouds |
| Major human effects | Skin and eye damage |
| Ecological effects | Plant and aquatic organism damage |
| International agreement | Montreal Protocol, 1987 |
| Kigali Amendment | HFC phase-down |
30. One-Minute Revision
🌍 Ozone = O₃
☀️ Ozone layer = Stratospheric UV shield
🧪 CFCs → UV → Reactive Chlorine
⚠️ Reactive Chlorine → Ozone destruction
🧊 Antarctic cold → PSCs → Enhanced ozone loss
👨⚕️ More UV → Skin/Eye/DNA damage
🌱 More UV → Plant and aquatic effects
🌐 Montreal Protocol → Ozone protection
31. Final Summary – निष्कर्ष
Ozone depletion is an important environmental issue because the stratospheric ozone layer protects life from excessive ultraviolet radiation. Human-made ozone-depleting substances such as CFCs and halons can release reactive chlorine and bromine in the stratosphere. These substances participate in catalytic reactions that destroy ozone.
The effects of increased UV exposure can include damage to human skin and eyes, effects on plants, and impacts on phytoplankton and aquatic ecosystems. The Antarctic ozone hole is a major example of severe seasonal ozone depletion.
International cooperation, particularly through the Montreal Protocol, has played a major role in controlling ozone-depleting substances.
Exam Formula:
CFC → UV → Cl/Br → O₃ ↓ → UV exposure ↑ → Biological effects
The effects of increased UV exposure can include damage to human skin and eyes, effects on plants, and impacts on phytoplankton and aquatic ecosystems. The Antarctic ozone hole is a major example of severe seasonal ozone depletion.
International cooperation, particularly through the Montreal Protocol, has played a major role in controlling ozone-depleting substances.
Exam Formula:
CFC → UV → Cl/Br → O₃ ↓ → UV exposure ↑ → Biological effects
Our Environment | Causes and Effects of Ozone Depletion
Class 10 Science • CBSE • Foundation • Competitive Preparation
Class 10 Science • CBSE • Foundation • Competitive Preparation
