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Environmental Chemistry PYQs – Chemistry, SSC & Railway

89 Environmental Chemistry (Chemistry) questions asked in recent SSC and Railway exams, newest first, with exam, date & shift, answers and solutions.

📚 89 questionsSSC 41 · Railway 48
SSC Stenographer 2026 – 15 September 2026 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q1. The AQI of Delhi during winter often exceeds 400. What is the primary scientific and policy-based reason for such critical levels?

  1. (a) Lack of monsoon showers
  2. (b) Concentrated rural energy use
  3. (c) Thermal inversion and transboundary stubble burning from Punjab and Haryana
  4. (d) Industrial shutdowns during festivals
Answer & solution

Answer: (c) Thermal inversion and transboundary stubble burning from Punjab and Haryana

Answer: Option 3 — Thermal inversion and transboundary stubble burning from Punjab and Haryana
In winter a layer of warm air sits above the cold air near the ground (a temperature inversion), so smoke cannot rise and disperse; winds are also weak.
On top of this comes smoke from paddy-stubble (parali) burning in Punjab and Haryana in October–November and Delhi's own vehicle, dust and firecracker emissions — pushing the AQI into the "Severe" band (401–500).
Revision note — Air Quality Index and Delhi's winter smog
• The National AQI was developed by the CPCB with IIT Kanpur and launched in 2015 as part of the Swachh Bharat mission.
• It covers 8 pollutants — PM10, PM2.5, NO2, SO2, CO, O3, NH3 and lead; the overall AQI is the worst sub-index among them.
• Six bands: Good 0–50, Satisfactory 51–100, Moderate 101–200, Poor 201–300, Very Poor 301–400, Severe 401–500.
• PM2.5 (particles below 2.5 micrometres) is the dominant pollutant in Delhi; it enters deep into the lungs and bloodstream.
• Normally temperature falls about 6.5 degree C per km of height; in an inversion it rises with height, trapping pollutants under a lid — the same reason smog forms in valleys.
• GRAP (Graded Response Action Plan), notified in 2017, is enforced in Delhi-NCR in four stages by the Commission for Air Quality Management (CAQM), a statutory body since 2021.
Do not mix these up
• Temperature inversion vs urban heat island — the inversion is a cold ground layer under warm air that traps smoke; the heat island is a warmer city than its surroundings caused by concrete and traffic.
• AQI "Severe" vs "Very Poor" — Severe is above 400; Very Poor is 301–400.
Also asked from this topic: the AQI value classified as "Severe" (above 400); the number of pollutants measured under India's AQI (8); the agency that monitors real-time AQI in cities (CPCB); the dominant pollutant in Delhi's AQI (PM2.5).
SSC Stenographer 2026 – 12 September 2026 Shift 2 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q2. Which Indian city had the cleanest air (lowest AQI) as per CPCB data in early 2025?

  1. (a) Delhi
  2. (b) Tirunelveli
  3. (c) Ghaziabad
  4. (d) Chandigarh
Answer & solution

Answer: (b) Tirunelveli

Answer: Option 2 — Tirunelveli
In the CPCB's daily AQI bulletins of early 2025, Tirunelveli in Tamil Nadu repeatedly recorded the lowest AQI in the country, in the 'Good' band (0–50), thanks to coastal winds and little heavy industry.
Delhi and Ghaziabad sit at the opposite end of the table every winter; Chandigarh's air is moderate, not the cleanest.
Revision note — Air Quality Index in India
• The National AQI was launched by the CPCB in September 2014 under the slogan "One Number – One Colour – One Description"; live values are on the Sameer app.
• It is computed from 8 pollutants: PM2.5, PM10, NO₂, SO₂, CO, O₃, NH₃ and lead.
• Six bands: Good 0–50, Satisfactory 51–100, Moderate 101–200, Poor 201–300, Very Poor 301–400, Severe 401–500.
• The CPCB is a statutory body set up in September 1974 under the Water Act, 1974, working under the Ministry of Environment, Forest and Climate Change.
• The National Clean Air Programme (NCAP, 2019) covers 131 non-attainment cities and targets a 40 per cent cut in particulate matter by 2026 (from 2017 levels).
• In the IQAir World Air Quality Report 2024, Byrnihat (Assam–Meghalaya border) was the world's most polluted city and Delhi the most polluted capital.
Do not mix these up
• AQI vs PM2.5 concentration — the AQI is a 0–500 index combining eight pollutants; PM2.5 is one pollutant measured in µg/m³.
• CPCB vs NGT — the CPCB monitors and reports; the National Green Tribunal (2010) is a court that adjudicates environmental cases.
Also asked from this topic: the agency that monitors the AQI in Indian cities (Central Pollution Control Board); the number of pollutants in India's AQI (8); the AQI range for 'Good' air (0–50).
SSC Stenographer 2026 – 11 September 2026 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q3. How many pollutants are used in India's official AQI calculation since its launch?

  1. (a) 3
  2. (b) 5
  3. (c) 8
  4. (d) 10
Answer & solution

Answer: (c) 8

Answer: Option 3 — 8
India's National Air Quality Index, launched on 17 September 2014, is computed from eight pollutants.
They are PM10, PM2.5, NO2, SO2, CO, O3, NH3 and lead (Pb).
Revision note — National Air Quality Index (AQI)
• Launched at Delhi on 17 September 2014 by Prime Minister Narendra Modi under the theme "One Number, One Colour, One Description".
• Published by the Central Pollution Control Board (CPCB) under the Ministry of Environment, Forest and Climate Change; live data is on the Sameer app.
• Eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, Pb. At least three must be monitored, and one of them must be PM10 or PM2.5.
• Six categories: Good 0-50, Satisfactory 51-100, Moderate 101-200, Poor 201-300, Very Poor 301-400, Severe 401-500.
• The AQI reported is the worst (highest) sub-index among the pollutants measured, never their average.
• Methane and carbon dioxide are not in the AQI — they are greenhouse gases, not criteria pollutants.
Do not mix these up
• AQI vs NAAQS — the AQI is a public communication index; the National Ambient Air Quality Standards, 2009 set legal limits for 12 pollutants.
• PM10 vs PM2.5 — PM10 is up to 10 micrometres across, PM2.5 up to 2.5 micrometres, which reaches deeper into the lungs.
Also asked from this topic: how many pollutants the AQI measures (8); the band for "Good" air (0-50); the body that releases the AQI (CPCB); the pollutant NOT covered by it (methane).
SSC Stenographer 2026 – 11 September 2026 Shift 2 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q4. What public health advisory applies when AQI in Delhi exceeds 300 ('Very Poor')?

  1. (a) Reduced outdoor activity
  2. (b) Encourage jogging outside
  3. (c) No restrictions
  4. (d) Encourage open windows
Answer & solution

Answer: (a) Reduced outdoor activity

Answer: Option 1 — Reduced outdoor activity
An AQI of 301 to 400 is "Very Poor": the CPCB warns it may cause respiratory illness on prolonged exposure, so people are advised to cut down outdoor exertion.
In Delhi-NCR this level triggers Stage II of GRAP.
Jogging outside or opening windows would increase exposure.
Revision note — National Air Quality Index (AQI)
• Launched by the CPCB in 2014; runs on a scale of 0 to 500 with six categories.
• Good 0–50, Satisfactory 51–100, Moderate 101–200, Poor 201–300, Very Poor 301–400, Severe 401–500.
• Based on eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3 and lead (Pb); the worst sub-index becomes the AQI.
• Health note for Very Poor: respiratory illness on prolonged exposure; for Severe: affects even healthy people and seriously hits those with existing disease.
• GRAP (Graded Response Action Plan) for Delhi-NCR is enforced by the Commission for Air Quality Management (CAQM).
• GRAP stages: Stage I Poor, Stage II Very Poor, Stage III Severe (401–450), Stage IV Severe Plus (above 450).
• Real-time AQI data come from the CPCB's SAMEER app and the National Air Quality Monitoring Programme.
Do not mix these up
• Poor vs Very Poor — Poor is 201–300 (breathing discomfort on prolonged exposure); Very Poor is 301–400 (respiratory illness on prolonged exposure).
• AQI vs NAAQS — the AQI is a daily public index; the National Ambient Air Quality Standards are the legal limits for each pollutant.
Also asked from this topic: the number of pollutants in India's AQI (8); the AQI range for "Good" (0–50); the AQI value above which air is "Severe" (400); the agency that monitors AQI in cities (CPCB).
RRB Group D 2025-26 – 19 August 2026 Shift 3 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q5. Which organisation had invoked Stage-I of the Graded Response Action Plan across Delhi-NCR in May 2026 after the Air Quality Index entered the ‘Poor’ category?

  1. (a) Central Pollution Control Board
  2. (b) Ministry of Environment and Forests
  3. (c) Commission for Air Quality Management
  4. (d) National Green Tribunal
Answer & solution

Answer: (c) Commission for Air Quality Management

Correct Answer:
Correct Answer: Option 3 – Commission for Air Quality Management
Explanation:
The Commission for Air Quality Management (CAQM) invoked Stage-I of the Graded Response Action Plan (GRAP) across Delhi-NCR in May 2026 after the AQI entered the 'Poor' category.
Important Facts:
• CAQM is the statutory body responsible for air quality management in the NCR and adjoining areas.
• GRAP has four stages based on AQI severity (Poor, Very Poor, Severe, Severe+).
• CAQM later revoked/lifted Stage-I restrictions once air quality improved.
RRB Group D 2025-26 – 19 August 2026 Shift 2 · Ozone layer and its depletion

Q6. The primary harmful effect of UV radiation on humans, if not blocked by the ozone layer, is:

  1. (a) hypertension
  2. (b) asthma
  3. (c) anaemia
  4. (d) skin cancer
Answer & solution

Answer: (d) skin cancer

Correct Answer:
Correct Answer: Option 4 – Skin cancer
Explanation:
The ozone layer absorbs most of the Sun's harmful ultraviolet (UV) radiation. If it is depleted, increased UV exposure reaching the Earth's surface can damage skin cell DNA, significantly raising the risk ofskin cancer, along with cataracts and other skin damage.
Important Facts:
• UV radiation is classified as UV-A, UV-B, and UV-C; UV-B is most linked to skin cancer.
• Ozone layer depletion is caused mainly by CFCs (chlorofluorocarbons).
RRB Group D 2025-26 – 18 August 2026 Shift 3 · Ozone layer and its depletion

Q7. If the ozone layer becomes thinner, which of the following could happen on Earth?

  1. (a) Reduction in oxygen levels
  2. (b) Cooling of the Earth’s atmosphere only
  3. (c) Increase in ultraviolet radiation reaching Earth
  4. (d) Decrease in sunlight reaching plants
Answer & solution

Answer: (c) Increase in ultraviolet radiation reaching Earth

Concept: Ozone Layer Depletion
The ozone layer in the stratosphere absorbs most of the Sun's harmful ultraviolet (UV) radiation before it reaches the Earth's surface. If the ozone layer becomes thinner, more UV radiation penetrates to the surface, which can cause skin cancer, cataracts, and damage to plants and marine ecosystems.
Correct Answer: Increase in ultraviolet radiation reaching Earth
RRB Group D 2025-26 – 18 August 2026 Shift 3 · Ozone layer and its depletion

Q8. Why is depletion of the ozone layer by chlorofluorocarbons (CFCs) considered dangerous for organisms?

  1. (a) It leads to an increase in atmospheric oxygen levels
  2. (b) It leads to an decrease in rainfall
  3. (c) It leads to the generation of infrared radiation
  4. (d) It leads to penetration of UV rays into the atmosphere
Answer & solution

Answer: (d) It leads to penetration of UV rays into the atmosphere

Concept: Ozone Depletion by CFCs
Chlorofluorocarbons (CFCs) released into the atmosphere break down ozone (O₃) molecules in the stratosphere, thinning the protective ozone layer. Since ozone normally absorbs harmful ultraviolet (UV) radiation from the sun, its depletion allows increasedpenetration of UV raysto reach the Earth's surface. This increased UV exposure is dangerous for organisms as it can cause skin cancer, cataracts, weakened immunity, and damage to plant life and aquatic ecosystems.
Correct Answer: It leads to penetration of UV rays into the atmosphere
RRB Group D 2025-26 – 17 August 2026 Shift 3 · Greenhouse effect and greenhouse gases

Q9. As per the 2026 NITI Aayog report, which sector was identified as the largest contributor to India’s greenhouse gas emissions, prompting targeted emission reduction strategies?

  1. (a) Transportation systems and logistics
  2. (b) Agriculture and allied activities
  3. (c) Energy production including coal-based power plants
  4. (d) Industrial manufacturing and processing
Answer & solution

Answer: (c) Energy production including coal-based power plants

NITI Aayog 2026 - GHG emissions
As per NITI Aayog's 2026 report ('Scenarios Towards Viksit Bharat and Net Zero - Power'), theenergy/power sector, including coal-based power plants, is the largest contributor to India's greenhouse gas emissions - accounting for roughly 39.4% of total national GHG emissions in 2020.
Confirmed by NITI Aayog's official sectoral insights report on the power sector.
Correct Answer: Energy production including coal-based power plants
RRB Group D 2025-26 – 17 August 2026 Shift 2 · Greenhouse effect and greenhouse gases

Q10. Which wavelength range is primarily responsible for heating the Earth's surface and contributing to the greenhouse effect?

  1. (a) Visible light
  2. (b) Ultraviolet radiation
  3. (c) Radio waves
  4. (d) Infrared radiation
Answer & solution

Answer: (d) Infrared radiation

Greenhouse Effect and Wavelength Range
The Sun's visible and UV radiation reaches and warms the Earth's surface, which then re-emits this energy as long-wavelengthinfrared radiation. Greenhouse gases (CO₂, methane, water vapour, etc.) absorb and re-radiate this outgoing infrared radiation, trapping heat in the atmosphere — this is the mechanism of the greenhouse effect.
• Visible light and UV mostly pass through the atmosphere to reach the surface but are not the radiation trapped by greenhouse gases.
• Radio waves have no significant role in surface heating.
Correct Answer: Infrared radiation
RRB Group D 2025-26 – 11 August 2026 Shift 1 · Ozone layer and its depletion

Q11. What is the main function of the ozone layer in the upper part of the atmosphere?

  1. (a) Increasing atmospheric carbon dioxide levels
  2. (b) Absorbing ultraviolet radiation coming from the sun
  3. (c) Causing rainfall
  4. (d) Producing oxygen to support aerobic life
Answer & solution

Answer: (b) Absorbing ultraviolet radiation coming from the sun

Answer: Option 2 – Absorbing ultraviolet radiation coming from the Sun.
• The stratosphericozone layerabsorbs harmfulUV radiation, protecting life on Earth.
RRB Group D 2025-26 – 10 August 2026 Shift 2 · Ozone layer and its depletion

Q12. Which of the following environmental problems is related to the penetration of harmful UV rays into our atmosphere?

  1. (a) Ozone hole
  2. (b) Global warming
  3. (c) Greenhouse effect
  4. (d) Acid rain
Answer & solution

Answer: (a) Ozone hole

Answer: Option 1 – Ozone hole.
• Theozone layerin the stratosphere absorbs harmfulUV rays. Its depletion (ozone hole), mainly due to CFCs, lets more UV reach the Earth.
• Global warming, greenhouse effect and acid rain are pollution problems not linked to UV penetration.
RRB Group D 2025-26 – 5 August 2026 Shift 2 · Ozone layer and its depletion

Q13. Which of the following devices earlier used CFCs extensively?

  1. (a) Refrigerator
  2. (b) Water purifier
  3. (c) Bicycle
  4. (d) Solar cooker
Answer & solution

Answer: (a) Refrigerator

Answer: Option 1 — Refrigerator
Refrigerators(and ACs) earlier used CFCs as refrigerants → Option 1.
RRB Group D 2025-26 – 4 August 2026 Shift 1 · Ozone layer and its depletion

Q14. Which of the following chemical compounds is mainly responsible for ozone depletion in the upper atmosphere?

  1. (a) Sulphur dioxide
  2. (b) Methane
  3. (c) Oxides of nitrogen
  4. (d) Chlorofluorocarbons
Answer & solution

Answer: (d) Chlorofluorocarbons

Answer: Option 4 — Chlorofluorocarbons
CFCsrelease chlorine radicals that destroy ozone in the stratosphere → Option 4.
RRB Group D 2025-26 – 4 August 2026 Shift 1 · Ocean acidification

Q15. Excessive atmospheric CO₂ increases its dissolution in ocean water. This makes seawater more ________.

  1. (a) Basic
  2. (b) Oxygenated
  3. (c) Neutral
  4. (d) Acidic
Answer & solution

Answer: (d) Acidic

Answer: Option 4 — Acidic
Dissolved CO₂ forms carbonic acid, making seawater moreacidic(ocean acidification) → Option 4.
RRB Group D 2025-26 – 3 August 2026 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q16. Which organization revoked all actions under Stage-I of GRAP (Graded Response Action Plan) in the National Capital Region (NCR) in March 2026?

  1. (a) Ministry of Environment, Forest and Climate Change
  2. (b) Commission for Air Quality Management
  3. (c) Delhi Pollution Control Committee
  4. (d) Central Pollution Control Board
Answer & solution

Answer: (b) Commission for Air Quality Management

Answer: Option 2 — Commission for Air Quality Management
TheCommission for Air Quality Management (CAQM)in the NCR revoked GRAP Stage-I actions in March 2026 → Option 2. (CAQM is the statutory body that invokes/revokes GRAP in NCR.)
RRB NTPC UG CBT-1 2025-26 – 20 June 2026 Shift 1 · Acid rain

Q17. Which gas is the main contributor to acid rain formation?

  1. (a) Methane
  2. (b) Sulphur dioxide
  3. (c) Hydrogen
  4. (d) Neon
Answer & solution

Answer: (b) Sulphur dioxide

Topic:Acid Rain Formation
Main Contributors:
SO₂ (Sulphur dioxide) + H₂O → H₂SO₃ (Sulphurous acid)
SO₂ oxidizes to SO₃ → H₂SO₄ (Sulphuric acid)
Sources:Coal burning, industrial emissions, volcanic activity
NO₂ also contributes but SO₂ is the primary cause
Answer: Option 2 → Sulphur dioxide
RRB NTPC UG CBT-1 2025-26 – 19 June 2026 Shift 2 · Air pollutants and their sources

Q18. Which of the following is a method specifically used to control particulate air pollution?

  1. (a) Catalytic combustion in open furnaces
  2. (b) Absorption of gases using liquid scrubbers
  3. (c) Increasing chimney height without any treatment
  4. (d) Use of fabric filters to trap dust and fly ash
Answer & solution

Answer: (d) Use of fabric filters to trap dust and fly ash

Topic:Particulate Air Pollution Control Methods
Particulate matter = solid/liquid particles suspended in air (dust, smoke, fly ash)
Analysis of options:
Option 1: Catalytic combustion → controls gaseous pollutants, not particulates
Option 2: Liquid scrubbers → primarily for gas absorption, not particulate removal
Option 3: Increasing chimney height → disperses pollutants, doesn't remove them
Option 4: Fabric filters (baghouses) → specifically trap particulate matter
Fabric filtersuse woven/felted cloth to capture dust and fly ash particles
Widely used in industries like cement, power plants, steel manufacturing
Answer: Option 4 → Use of fabric filters to trap dust and fly ash
RRB NTPC UG CBT-1 2025-26 – 18 June 2026 Shift 2 · Water pollution and hardness of water

Q19. Which of the following is a non-point source of water pollution?

  1. (a) Industrial discharge pipe
  2. (b) Urban stormwater drain
  3. (c) Sewage outlet
  4. (d) Agricultural runoff
Answer & solution

Answer: (d) Agricultural runoff

Topic: Water Pollution Sources
Point Source:Pollution from single identifiable location (pipe, outlet, drain).
Non-Point Source:Pollution from diffused/scattered areas without specific discharge point.
Options 1, 2, 3 → All have specific discharge points (pipes/outlets).
Option 4 → Agricultural runoff spreads over large area through rain/irrigation, no single point.
Answer: Option 4 → Agricultural runoff
RRB NTPC UG CBT-1 2025-26 – 17 June 2026 Shift 2 · Air pollutants and their sources

Q20. Which of the following statements about carbon monoxide in the air is correct?

  1. (a) It is harmless because it is quickly converted into oxygen
  2. (b) The incomplete combustion of fossil fuels produces it and replaces oxygen in haemoglobin
  3. (c) It is released only from natural forest fires and volcanoes
  4. (d) It is less dangerous than carbon dioxide for human health
Answer & solution

Answer: (b) The incomplete combustion of fossil fuels produces it and replaces oxygen in haemoglobin

Topic:Carbon Monoxide (CO) - Air Pollutant
Source:Incomplete combustion of fossil fuels (vehicles, industries)
Health Effect:CO binds with haemoglobin forming carboxyhaemoglobin
This prevents oxygen transport in blood → suffocation
Why other options wrong:
Option 1: CO is NOT converted to oxygen, it's toxic
Option 3: Also released from vehicles, industries (major sources)
Option 4: CO is MORE dangerous than CO₂ for humans
Answer: Option 2 → The incomplete combustion of fossil fuels produces it and replaces oxygen in haemoglobin
SSC GD 2026 – 25 May 2026 Shift 1 · Ozone layer and its depletion

Q21. Which of the following ozone-depleting substances (ODS) releases bromine atoms when exposed to ultraviolet (UV) radiation in the stratosphere?

  1. (a) HCFCs
  2. (b) Halon
  3. (c) CFCs
  4. (d) Carbon Tetrachloride
Answer & solution

Answer: (b) Halon

Correct Answer: Halon — Option (b)
Halons are ozone-depleting substances (ODS) that release bromine atoms when exposed to UV radiation in the stratosphere. About Halons: Halons are fully halogenated compounds containing bromine (and usually fluorine and carbon); Common halons: Halon-1211 (bromochlorodifluoromethane), Halon-1301 ( bromotrifluoromethane ), Halon-2402; When halons are released into the atmosphere and reach the stratosphere, UV radiation breaks them apart, releasing bromine atoms; Bromine is approximately 40-50 times more effective than chlorine at destroying ozone molecules; Used as fire extinguishing agents; Halons are regulated under the Montreal Protocol (1987); ODS and the halogen they release: HCFCs (Hydrochlorofluorocarbons): Release CHLORINE atoms; CFCs (Chlorofluorocarbons): Release CHLORINE atoms; Carbon Tetrachloride (CCl4): Releases CHLORINE atoms; Methyl Bromide (CH3Br): Releases BROMINE atoms; Halons: Release BROMINE atoms; About the Montreal Protocol: International treaty to phase out ozone-depleting substances; Signed in 1987; Considered the most successful environmental agreement; The ozone layer is gradually recovering as a result.
Option (a) HCFCs — Incorrect: HCFCs (Hydrochlorofluorocarbons) release CHLORINE atoms (not bromine) when broken down by UV radiation in the stratosphere; HCFCs were introduced as transitional replacements for CFCs but are themselves ozone-depleting. The candidate selected this incorrect option.
Option (c) CFCs — Incorrect: CFCs (Chlorofluorocarbons) release CHLORINE atoms (not bromine) when exposed to UV radiation; CFCs are the primary cause of ozone depletion but release chlorine, not bromine.
Option (d) Carbon Tetrachloride — Incorrect: Carbon Tetrachloride (CCl4) releases CHLORINE atoms (not bromine) when broken down; it is an ODS but releases chlorine, not bromine atoms.
SSC GD 2026 – 25 May 2026 Shift 1 · Ozone layer and its depletion

Q22. Which of the following is the correct pair of ozone-depleting substance (ODS) and the halogen released upon its exposure to UV radiation?

  1. (a) Methyl Bromide – Iodine
  2. (b) Carbon Tetrachloride – Bromine
  3. (c) HCFCs – Chlorine
  4. (d) Halon – Nitrogen
Answer & solution

Answer: (c) HCFCs – Chlorine

Correct Answer: HCFCs – Chlorine — Option (c)
The correct pair is HCFCs (Hydrochlorofluorocarbons) and Chlorine — HCFCs release chlorine atoms when exposed to UV radiation in the stratosphere. About ODS and Halogens Released: HCFCs (Hydrochlorofluorocarbons) → release CHLORINE (Cl): HCFCs contain chlorine atoms in their chemical structure; UV radiation in the stratosphere breaks the C-Cl bond, releasing free chlorine radicals; These chlorine atoms catalytically destroy ozone (O3) in the stratosphere; CFCs (Chlorofluorocarbons) → release CHLORINE (Cl); Carbon Tetrachloride (CCl4) → release CHLORINE (Cl); Halons → release BROMINE (Br) — not nitrogen; Methyl Bromide (CH3Br) → release BROMINE (Br) — not iodine; Key distinction: Chlorine-releasing ODS: CFCs, HCFCs, Carbon Tetrachloride, Methyl Chloroform; Bromine-releasing ODS: Halons, Methyl Bromide; Montreal Protocol controls all these substances; Role of Chlorine in ozone depletion: One chlorine atom can destroy up to 100,000 ozone molecules through a catalytic chain reaction: Cl + O3 → ClO + O2; ClO + O → Cl + O2 (Cl is regenerated and the cycle continues).
Option (a) Methyl Bromide – Iodine — Incorrect: Methyl Bromide (CH3Br) releases BROMINE atoms (not iodine) when broken down by UV radiation; iodine is not a halogen released by common ODS in significant quantities; this pairing is completely incorrect.
Option (b) Carbon Tetrachloride – Bromine — Incorrect: Carbon Tetrachloride (CCl4) releases CHLORINE atoms (not bromine) when exposed to UV radiation — the chemical formula shows 4 chlorine atoms and no bromine; pairing it with bromine is incorrect.
Option (d) Halon – Nitrogen — Incorrect: Halons release BROMINE atoms (not nitrogen) when exposed to UV radiation in the stratosphere; nitrogen is not released by halons; this is a completely incorrect pairing.
SSC GD 2026 – 22 May 2026 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q23. The Comprehensive Environmental Pollution Index (CEPI) in India was published by which of the following?

  1. (a) CPCB
  2. (b) NGT
  3. (c) CGWB
  4. (d) NITI Aayog
Answer & solution

Answer: (a) CPCB

Correct Answer: CPCB — Option (a)
The Comprehensive Environmental Pollution Index (CEPI) in India is published by the Central Pollution Control Board (CPCB). About CEPI: Full form: Comprehensive Environmental Pollution Index; The CEPI is a tool developed to assess the pollution load of industrial clusters across India; It considers environmental pollution across air, water, and land (soil) dimensions; Developed by: CPCB in collaboration with IIT Delhi (National Environmental Engineering Research Institute — NEERI also involved); Used to identify: Critically Polluted Areas (CPAs) — industrial clusters scoring above 70 on CEPI; Severely Polluted Areas — clusters scoring between 60 and 70; CEPI scoring: Scored on a scale of 0-100; higher score = more polluted; a score above 70 triggers a Moratorium (ban on new industries); Critically Polluted Areas (CPAs): Tarapur (Maharashtra), Vapi (Gujarat), Angul-Talcher (Odisha), Ludhiana (Punjab), etc.; About CPCB: Central Pollution Control Board is a statutory organisation constituted in September 1974 under the Water (Prevention and Control of Pollution) Act, 1974; It is under the Ministry of Environment, Forest and Climate Change ( MoEFCC ); Headquarters: New Delhi; Main functions: Promote cleanliness of streams and wells, improve quality of air, provide technical assistance and guidance to SPCBs (State Pollution Control Boards).
Option (b) NGT — Incorrect: The National Green Tribunal (NGT) is a judicial body established in 2010 to handle environmental cases — it adjudicates environmental disputes and can impose penalties; it does NOT publish the CEPI; the CEPI is published by CPCB.
Option (c) CGWB — Incorrect: CGWB (Central Ground Water Board) is a national apex agency under the Ministry of Jal Shakti that monitors and manages groundwater resources in India — it does NOT publish the CEPI; the CEPI is a CPCB publication. The candidate selected this incorrect option.
Option (d) NITI Aayog — Incorrect: NITI Aayog is the government's policy think-tank that publishes reports on various developmental topics including environment, but the Comprehensive Environmental Pollution Index (CEPI) specifically is published by the Central Pollution Control Board (CPCB), not NITI Aayog.
SSC GD 2026 – 19 May 2026 Shift 2 · Air pollutants and their sources

Q24. The government has notified the National Ambient Air Quality Standards (NAAQS) for 12 pollutants in _____.

  1. (a) 1999
  2. (b) 2001
  3. (c) 2009
  4. (d) 2015
Answer & solution

Answer: (c) 2009

Correct Answer: 2009 — Option (c)
The Government of India notified the revised National Ambient Air Quality Standards (NAAQS) for 12 pollutants in 2009 under the Air (Prevention and Control of Pollution) Act, 1981. The NAAQS 2009 set standards for 12 pollutants: PM10, PM2.5, Sulphur Dioxide (SO2), Nitrogen Dioxide (NO2), Ozone (O3), Lead (Pb), Carbon Monoxide (CO), Ammonia (NH3), Benzene (C6H6), Benzo(a)pyrene, Arsenic, and Nickel. The standards define the maximum permissible concentrations of these pollutants in ambient (outdoor) air to protect public health and the environment. PM2.5 standards were introduced for the first time in India through this 2009 revision.
Option (a) 1999 — Incorrect: The NAAQS was revised in 2009, not 1999; earlier versions of NAAQS existed but the comprehensive 12-pollutant notification was in 2009.
Option (b) 2001 — Incorrect: 2001 is not the year when NAAQS for 12 pollutants was notified; the correct year for the comprehensive NAAQS notification is 2009.
Option (d) 2015 — Incorrect: 2015 was not the year of NAAQS notification for 12 pollutants; the comprehensive NAAQS including PM2.5 standards was notified in 2009.
SSC GD 2026 – 8 May 2026 Shift 1 · Air pollutants and their sources

Q25. How many pollutants are measured under India's Air Quality Index (AQI)?

  1. (a) 4
  2. (b) 6
  3. (c) 8
  4. (d) 10
Answer & solution

Answer: (c) 8

Correct Answer: 8 — Option (c)
India's Air Quality Index (AQI), launched by the Central Pollution Control Board (CPCB) in 2014, measures 8 pollutants: (1) PM2.5 (Particulate Matter < 2.5 micrometres), (2) PM10 (Particulate Matter < 10 micrometres), (3) NO₂ (Nitrogen Dioxide), (4) SO₂ (Sulfur Dioxide), (5) CO (Carbon Monoxide), (6) O₃ (Ozone), (7) NH₃ (Ammonia), (8) Pb (Lead). The AQI categorizes air quality into six categories from 'Good' (0–50) to 'Severe' (401–500). The highest AQI among all measured pollutants determines the overall AQI.
Option (a) 4 — Incorrect: India's AQI measures 8 pollutants, not 4.
Option (b) 6 — Incorrect: Six is not the correct number; India's AQI covers 8 pollutants.
Option (d) 10 — Incorrect: The AQI measures 8 specific pollutants, not 10.
SSC GD 2026 – 8 May 2026 Shift 3 · Air pollutants and their sources

Q26. Which AQI (Air Quality Index) category represents 'Good' air quality in India?

  1. (a) 0–50
  2. (b) 51–100
  3. (c) 101–200
  4. (d) 201–300
Answer & solution

Answer: (a) 0–50

Correct Answer: 0–50 — Option (a)
India's Air Quality Index (AQI) has six categories: Good (0–50) — Minimal impact; Satisfactory (51–100) — Minor breathing discomfort to sensitive people; Moderate (101–200) — Breathing discomfort to people with lung/heart disease; Poor (201–300) — Breathing discomfort to most people on prolonged exposure; Very Poor (301–400) — Respiratory illness on prolonged exposure; Severe (401–500) — Affects healthy people and seriously impacts those with existing diseases. The 'Good' category (0–50) indicates clean air with minimal health effects on the general population.
Option (b) 51–100 — Incorrect: The AQI range 51–100 represents 'Satisfactory' air quality, not 'Good'.
Option (c) 101–200 — Incorrect: AQI 101–200 represents 'Moderate' air quality, indicating some health concerns.
Option (d) 201–300 — Incorrect: AQI 201–300 represents 'Poor' air quality, causing breathing discomfort to most people.
RRB NTPC UG CBT-1 2025-26 – 8 May 2026 Shift 3 · Acid rain

Q27. Which pollutant from the following is mainly responsible for the formation of acid rain?

  1. (a) Carbon monoxide
  2. (b) Ammonia
  3. (c) Sulphur dioxide
  4. (d) Ozone
Answer & solution

Answer: (c) Sulphur dioxide

Topic: Acid Rain Formation
Sulphur dioxide (SO₂)is the primary pollutant causing acid rain.
Process: SO₂ + H₂O → H₂SO₃ (sulphurous acid)
Further oxidation: H₂SO₃ → H₂SO₄ (sulphuric acid)
Sources: Coal burning, industrial emissions, vehicle exhaust.
Note: Nitrogen oxides (NOₓ) also contribute to acid rain.
Answer: Option 3 → Sulphur dioxide
SSC GD 2026 – 7 May 2026 Shift 2 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q28. Which Indian agency is responsible for real-time Air Quality Index (AQI) monitoring in cities?

  1. (a) National Institution for Transforming India (NITI Aayog)
  2. (b) Council of Scientific and Industrial Research (CSIR)
  3. (c) State Reference Organisation
  4. (d) Central Pollution Control Board (CPCB)
Answer & solution

Answer: (d) Central Pollution Control Board (CPCB)

Correct Answer: Central Pollution Control Board (CPCB) — Option (d)
The Central Pollution Control Board (CPCB), under the Ministry of Environment, Forest and Climate Change, is the apex body responsible for real-time Air Quality Index (AQI) monitoring across Indian cities. CPCB operates the National Air Quality Monitoring Programme (NAMP) and maintains the Sameer app and AQI portal (https://aqicn.org/india/) that provides real-time AQI data from hundreds of monitoring stations across India. CPCB also coordinates with State Pollution Control Boards (SPCBs) for air quality data collection and management.
Option (a) NITI Aayog — Incorrect: NITI Aayog is a policy think tank; it does not conduct real-time AQI monitoring.
Option (b) CSIR — Incorrect: The Council of Scientific and Industrial Research conducts scientific research; AQI monitoring is the mandate of CPCB.
Option (c) State Reference Organisation — Incorrect: This is not the primary agency for national real-time AQI monitoring; CPCB is the central authority.
SSC GD 2026 – 7 May 2026 Shift 2 · Air pollutants and their sources

Q29. The Air Quality Early Warning System is primarily designed for which purpose?

  1. (a) Reducing soil pollution
  2. (b) Monitoring water pollution
  3. (c) Alerting citizens about upcoming days with high pollution
  4. (d) Improving air filtration in industries
Answer & solution

Answer: (c) Alerting citizens about upcoming days with high pollution

Correct Answer: Alerting citizens about upcoming days with high pollution — Option (c)
The Air Quality Early Warning System (AQEWS) was developed by the Ministry of Earth Sciences ( MoES ) and IITM Pune, and is primarily designed to forecast and alert citizens and authorities about upcoming days with dangerously high air pollution levels — especially in Delhi-NCR during winter months (October–February). It provides 5-day forecasts of particulate matter (PM2.5, PM10) and air quality indices, enabling authorities to take preemptive measures (like the Graded Response Action Plan/GRAP) and allowing citizens to plan outdoor activities accordingly.
Option (a) Reducing soil pollution — Incorrect: AQEWS is focused on AIR quality; it is not designed to address soil pollution.
Option (b) Monitoring water pollution — Incorrect: Water quality monitoring is a separate domain handled by CPCB and National Water Monitoring Programme ; AQEWS is specifically for air quality forecasting.
Option (d) Improving air filtration in industries — Incorrect: Industrial air filtration is regulated through emission standards and compliance; AQEWS is a forecasting/early warning tool for public benefit, not an industrial intervention.
SSC GD 2026 – 6 May 2026 Shift 1 · Air pollutants and their sources

Q30. Which of the following meteorological factors can reduce Air Quality Index (AQI) temporarily?

  1. (a) High humidity without rain
  2. (b) Calm wind
  3. (c) Temperature inversion
  4. (d) Rain
Answer & solution

Answer: (d) Rain

Correct Answer: Rain — Option (d)
Rain is a natural air purifier that temporarily reduces the Air Quality Index (AQI) by washing pollutants — including PM2.5, PM10, dust, and gases — out of the atmosphere through wet deposition. Raindrops capture particulate matter and bring them to the ground, leading to a significant but temporary improvement in air quality. This is why AQI values often drop sharply during and immediately after rainfall episodes.
Option (a) High humidity without rain — Incorrect: High humidity without rain can actually WORSEN AQI by helping particles absorb moisture and grow larger, increasing haze and particulate concentration.
Option (b) Calm wind — Incorrect: Calm (low) wind allows pollutants to accumulate near the source without dispersal, which WORSENS AQI rather than improving it.
Option (c) Temperature inversion — Incorrect: Temperature inversion (warm air above cold air) traps pollutants near the ground, severely WORSENING AQI — it is one of the main causes of smog episodes.
SSC GD 2026 – 5 May 2026 Shift 1 · Air pollutants and their sources

Q31. What does the Air Quality Index (AQI) 'Moderate' category indicate?

  1. (a) Air quality may adversely affect health of sensitive groups
  2. (b) Air quality is excellent
  3. (c) Air quality is satisfactory
  4. (d) Air quality is dangerous for everyone
Answer & solution

Answer: (a) Air quality may adversely affect health of sensitive groups

Correct Answer: Air quality may adversely affect health of sensitive groups — Option (a)
The AQI 'Moderate' category (AQI range 101–200 in India's scale) indicates that the air quality may cause discomfort to people who are unusually sensitive to air pollution, such as those with respiratory conditions (asthma, COPD), heart diseases, the elderly, and children. While the general public is less likely to be affected at this level, sensitive groups are advised to limit prolonged outdoor activities.
Option (b) Air quality is excellent — Incorrect: 'Excellent' or 'Good' (AQI 0–50) indicates air quality posing little or no risk to health; this is not 'Moderate'.
Option (c) Air quality is satisfactory — Incorrect: 'Satisfactory' (AQI 51–100) means air quality is acceptable; it is one step below 'Moderate'.
Option (d) Air quality is dangerous for everyone — Incorrect: 'Severe' or 'Hazardous' categories (AQI 401–500) indicate danger for everyone, not the 'Moderate' category.
SSC GD 2026 – 5 May 2026 Shift 3 · Ozone layer and its depletion

Q32. The ozone hole was first discovered over ___.

  1. (a) Arctic
  2. (b) Antarctica
  3. (c) Indian Ocean
  4. (d) Pacific Ocean
Answer & solution

Answer: (b) Antarctica

Correct Answer: Antarctica — Option (b)
The ozone hole was first discovered over Antarctica in 1985 by British scientists Joe Farman, Brian Gardiner, and Jonathan Shanklin of the British Antarctic Survey. They observed a dramatic seasonal thinning of the stratospheric ozone layer over the South Pole each spring (September–November). The discovery shocked the scientific community and led to the Montreal Protocol (1987), the international treaty to phase out ozone-depleting substances like CFCs.
Option (a) Arctic — Incorrect: While ozone depletion also occurs over the Arctic, the ozone hole was first discovered over Antarctica, not the Arctic.
Option (c) Indian Ocean — Incorrect: The ozone hole phenomenon is specifically associated with the polar regions (Antarctica), not the Indian Ocean.
Option (d) Pacific Ocean — Incorrect: The Pacific Ocean is not associated with ozone hole discovery; the ozone hole is a polar phenomenon first found over Antarctica.
RRB NTPC GL CBT-1 2025-26 – 27 March 2026 Shift 1 · Greenhouse effect and greenhouse gases

Q33. Which gases act as the main greenhouse gases with rising atmospheric concentrations?

  1. (a) Hydrochlorofluorocarbons, argon, oxygen
  2. (b) Carbon dioxide, methane, nitrous oxide
  3. (c) Argon, oxygen, nitrogen
  4. (d) Hydrofluorocarbons, oxygen, nitrogen
Answer & solution

Answer: (b) Carbon dioxide, methane, nitrous oxide

Greenhouse Gases
Main greenhouse gases with rising concentrations:
•Carbon dioxide (CO₂)- from fossil fuels, deforestation
•Methane (CH₄)- from agriculture, livestock, wetlands
•Nitrous oxide (N₂O)- from fertilizers, industrial processes
Note: Argon, oxygen, nitrogen are abundant but NOT greenhouse gases.
HCFCs and HFCs are greenhouse gases but less significant.
Answer: Option 2 → Carbon dioxide, methane, nitrous oxide
RRB NTPC GL CBT-1 2025-26 – 25 March 2026 Shift 2 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q34. India’s National Clean Air Programme (NCAP) originally aimed to reduce particulate pollution by what percentage by 2024–25 compared to the 2017–18 baseline?

  1. (a) 40–50%
  2. (b) 20–30%
  3. (c) 10–15%
  4. (d) 60–70%
Answer & solution

Answer: (b) 20–30%

National Clean Air Programme (NCAP)
Launched: 2019 by Ministry of Environment
Original Target:Reduce PM10 and PM2.5 concentrations by20–30%by 2024 compared to 2017 baseline
Covers: 132 non-attainment cities across India
Later revised to 40% reduction target by 2025–26
Answer: Option 2 → 20–30%
RRB NTPC GL CBT-1 2025-26 – 24 March 2026 Shift 2 · Acid rain

Q35. Which of the following pollutants is most closely associated with the formation of acid rain?

  1. (a) Ammonia
  2. (b) Sulphur dioxide
  3. (c) Ozone
  4. (d) Methane
Answer & solution

Answer: (b) Sulphur dioxide

Topic: Acid Rain Formation
Key Facts:
1. Acid rain forms when SO₂ and NOₓ react with water vapor in atmosphere
2. SO₂ (Sulphur dioxide) from coal burning and industries is primary cause
3. SO₂ + H₂O → H₂SO₃ (sulphurous acid) → H₂SO₄ (sulphuric acid)
Other options: Ammonia is alkaline, Ozone is oxidant, Methane is greenhouse gas
Answer: Option 2 → Sulphur dioxide
RRB NTPC GL CBT-1 2025-26 – 22 March 2026 Shift 1 · Water pollution and hardness of water

Q36. Thermal pollution primarily results from which action related to industrial processes?

  1. (a) Use of water as a coolant
  2. (b) Release of heavy metals
  3. (c) Suspended solid accumulation
  4. (d) Addition of detergents
Answer & solution

Answer: (a) Use of water as a coolant

Thermal Pollution
Definition: Increase in water temperature due to human activities.
Primary Cause:Industries use water from rivers/lakes as coolant in power plants and factories.
Process: Cold water absorbs heat from machinery → Hot water discharged back → Raises water body temperature.
Effect: Reduces dissolved oxygen, harms aquatic life.
Other options cause chemical/physical pollution, not thermal.
Answer: Option 1 → Use of water as a coolant
RRB NTPC GL CBT-1 2025-26 – 19 March 2026 Shift 2 · Air pollutants and their sources

Q37. Which of the following is NOT a primary air pollutant?

  1. (a) Nitrogen oxide
  2. (b) Sulfur dioxide
  3. (c) Carbon monoxide
  4. (d) Ozone
Answer & solution

Answer: (d) Ozone

Primary vs Secondary Air Pollutants
Primary pollutants:Emitted directly from sources
• Nitrogen oxide (NO) - from vehicles, industries
• Sulfur dioxide (SO₂) - from coal burning
• Carbon monoxide (CO) - from incomplete combustion
Secondary pollutants:Formed by chemical reactions in atmosphere
•Ozone (O₃)- formed when NO₂ reacts with sunlight
Answer: Option 4 → Ozone
RRB JE CBT-1 2025-26 – 25 February 2026 Shift 3 · Ozone layer and its depletion

Q38. Ozone shields the surface of the Earth from which type of radiation coming from the Sun?

  1. (a) White light
  2. (b) X rays
  3. (c) Infra-red
  4. (d) Ultra violet
Answer & solution

Answer: (d) Ultra violet

Ozone layer is located in the Earth's stratosphere.
It absorbs the majority of the Sun's harmful ultraviolet (UV) radiation.
This protection prevents most UV rays from reaching the Earth's surface.
UV radiation can cause skin cancer and other health issues.
Hence, the ozone layer acts as a shield against ultraviolet radiation from the Sun.
Correct answer is Option 4: Ultra violet.
RRB JE CBT-1 2025-26 – 19 February 2026 · Pollution control devices

Q39. Which measure has been made mandatory for brick kilns in India to reduce emissions, as per the 2024 government report on pollution control?

  1. (a) Use of electrostatic filters in stacks
  2. (b) Conversion to solar thermal drying systems
  3. (c) Transition to zig-zag kiln technology
  4. (d) Retrofitting of biomass combustion units
Answer & solution

Answer: (c) Transition to zig-zag kiln technology

The 2024 government report mandates zig-zag kiln technology for brick kilns in India.
This technology reduces emissions significantly compared to traditional methods.
It improves combustion efficiency and cuts down air pollutants.
Thus, adoption of zig-zag kiln technology is compulsory to control pollution from brick kilns.
Final answer: Option 3
RRB JE CBT-1 2025-26 – 19 February 2026 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q40. Which of the following is NOT a feature of the Comprehensive Environmental Pollution Index?

  1. (a) It was first introduced in 1983.
  2. (b) All the polluting sources identified in the area shall be notified and brought into the public domain.
  3. (c) It shall be an early warning tool.
  4. (d) It can applied to locations other than industrial clusters.
Answer & solution

Answer: (a) It was first introduced in 1983.

Correct option: 1
Comprehensive Environmental Pollution Index (CEPI) was introduced by the Central Pollution Control Board of India much later than 1983, around 2009.
The other features are correct:
- CEPI notifies all polluting sources in an area publicly.
- It acts as an early warning tool to identify pollution levels.
- It can be applied to locations beyond just industrial clusters.
Therefore, 'It was first introduced in 1983' is NOT a feature of CEPI.
Answer closed.
SSC MTS 2025 – 16 February 2026 Shift 2 · Greenhouse effect and greenhouse gases

Q41. How does the increased level of carbon dioxide in the atmosphere contribute to global warming?

  1. (a) By increasing cloud cover
  2. (b) By trapping heat rays reflected by the earth
  3. (c) By causing more snowfall
  4. (d) By cooling the earth's surface
Answer & solution

Answer: (b) By trapping heat rays reflected by the earth

Carbon dioxide (CO2) is a greenhouse gas that absorbs and re-emits infrared (heat) radiation reflected from the earth's surface. This trapping of heat prevents it from escaping into space, causing the temperature of the earth's atmosphere to rise — a phenomenon known as the greenhouse effect, which leads to global warming.
Why other options are incorrect:
- By increasing cloud cover: CO2 does not directly increase cloud cover; this is a separate atmospheric process.
- By causing more snowfall: Increased CO2 leads to warming, which generally reduces snowfall and causes glacial melting.
- By cooling the earth's surface: The opposite is true; CO2 increases global temperatures by trapping outgoing heat.
Delhi Police Head Constable (Ministerial) 2025-26 – 12 January 2026 Shift 3 · Air pollutants and their sources

Q42. What main pollutant is used for AQI calculation in Delhi as of January 2025?

  1. (a) PM10
  2. (b) SO2
  3. (c) PM2.5
  4. (d) Ozone
Answer & solution

Answer: (c) PM2.5

PM2.5 is correct. In Delhi, PM2.5 (fine particulate matter with diameter less than 2.5 micrometres) is the dominant pollutant driving the Air Quality Index (AQI) calculations, especially during winter months. PM2.5 originates from vehicle emissions, stubble burning in neighboring states, construction dust, and industrial activity. It is the most health-hazardous pollutant in Delhi's context, capable of penetrating deep into the lungs and bloodstream.
• PM10: Also monitored in Delhi, but PM2.5 is the primary driver of Delhi's dangerously high AQI readings, particularly in winter.
• SO2: Sulfur dioxide levels in Delhi are relatively lower and not the dominant AQI pollutant.
• Ozone: Ground-level ozone is a pollutant monitored in AQI, but it peaks in summer, not winter; PM2.5 dominates Delhi's AQI concerns in January.
Delhi Police Head Constable (Ministerial) 2025-26 – 8 January 2026 Shift 1 · Air pollutants and their sources

Q43. Which pollutant is NOT a part of India's AQI calculation?

  1. (a) PM10
  2. (b) Carbon Dioxide (CO2)
  3. (c) Ozone (O3)
  4. (d) Sulphur Dioxide (SO2)
Answer & solution

Answer: (b) Carbon Dioxide (CO2)

Correct Answer: Option 2 – Carbon Dioxide (CO2)
India's AQI (Air Quality Index), as defined by the Central Pollution Control Board (CPCB), is calculated based on eight pollutants:
1. PM2.5 2. PM10 3. NO2 (Nitrogen Dioxide) 4. SO2 (Sulphur Dioxide)
5. CO (Carbon Monoxide) 6. O3 (Ozone) 7. NH3 (Ammonia) 8. Pb (Lead)
Carbon Dioxide (CO2) is NOT included in India's AQI calculation. CO2 is a greenhouse gas responsible for climate change but does not directly affect air quality in the way AQI measures it. AQI focuses on pollutants that have immediate health impacts.
Why other options are wrong:
Option 1 (PM10): PM10 IS part of India's AQI.
Option 3 (Ozone/O3): O3 IS part of India's AQI.
Option 4 (SO2): SO2 IS part of India's AQI.
Delhi Police Head Constable (Ministerial) 2025-26 – 7 January 2026 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q44. What does AQI stand for in the context of environmental monitoring in India?

  1. (a) Air Quality Index
  2. (b) Annual Quota Indicator
  3. (c) Average Quotient of Industry
  4. (d) Air Quota Initiative
Answer & solution

Answer: (a) Air Quality Index

Correct Answer: Option 1 – Air Quality Index
AQI stands for Air Quality Index. It is a scale used to measure and communicate the level of air pollution in a given area. In India, the Central Pollution Control Board (CPCB) uses AQI to report daily air quality based on pollutants like PM2.5, PM10, NO2, SO2, CO, O3, NH3, and Pb.
AQI ranges:
• 0–50: Good
• 51–100: Satisfactory
• 101–200: Moderate
• 201–300: Poor
• 301–400: Very Poor
• 401–500: Severe
Why other options are wrong:
Options 2, 3, 4: These are fabricated expansions. AQI is universally known as Air Quality Index in environmental science.
Delhi Police Head Constable (Ministerial) 2025-26 – 7 January 2026 Shift 2 · Air pollutants and their sources

Q45. In the 2024 AQI (Air Quality Index) reports, several Indian cities were listed among the most polluted globally. Which of the following accurately reflects the AQI's assessment criteria and India's environmental challenge?

  1. (a) AQI evaluates only ozone levels.
  2. (b) AQI reports on green cover expansion.
  3. (c) AQI assesses multiple pollutants like PM2.5, PM10, NO2, CO, and SO2, with Indian metros frequently exceeding safe levels.
  4. (d) AQI includes carbon tax performance.
Answer & solution

Answer: (c) AQI assesses multiple pollutants like PM2.5, PM10, NO2, CO, and SO2, with Indian metros frequently exceeding safe levels.

Correct Answer: Option 3 – AQI assesses multiple pollutants including PM2.5, PM10, NO2, CO, and SO2
India's AQI (Air Quality Index), as defined by the Central Pollution Control Board (CPCB), evaluates eight pollutants:
PM2.5, PM10, NO2 (Nitrogen Dioxide), SO2 (Sulphur Dioxide), CO (Carbon Monoxide), O3 (Ozone), NH3 (Ammonia), and Pb (Lead).
Indian metro cities like Delhi, Mumbai, Kolkata, and Lucknow frequently exceed safe AQI levels, especially during winter months due to crop stubble burning, vehicular emissions, and industrial pollution.
Why other options are wrong:
Option 1 (Only ozone): AQI does not evaluate only ozone; it covers eight pollutants.
Option 2 (Green cover expansion): AQI does not measure green cover; it measures air pollution levels.
Option 4 (Carbon tax performance): AQI is not a carbon tax metric; it is an air quality measurement tool.
Delhi Police Constable (Exe) 2025-26 – 6 January 2026 Shift 1 · Greenhouse effect and greenhouse gases

Q46. Which gas in Earth's atmosphere primarily traps infrared radiation, contributing significantly to the greenhouse effect and global warming, thereby affecting the planet's overall climate?

  1. (a) Carbon dioxide
  2. (b) Oxygen
  3. (c) Nitrogen
  4. (d) Argon
Answer & solution

Answer: (a) Carbon dioxide

The correct answer is (a) Carbon dioxide.
Carbon dioxide (CO₂) is the primary greenhouse gas responsible for the enhanced greenhouse effect and global warming due to human activities (burning fossil fuels, deforestation, cement production). CO₂ molecules absorb and re-emit infrared radiation, trapping heat in the atmosphere. Its atmospheric concentration has risen from ~280 ppm (pre-industrial) to over 420 ppm today.
(b) Oxygen (O₂): Makes up about 21% of the atmosphere; does not absorb infrared radiation significantly and is not a greenhouse gas.
(c) Nitrogen (N₂): Makes up about 78% of the atmosphere; transparent to both visible and infrared radiation; not a greenhouse gas.
(d) Argon: An inert noble gas making up about 0.93% of the atmosphere; does not absorb infrared radiation; not a greenhouse gas.
Delhi Police Constable (Exe) 2025-26 – 29 December 2025 Shift 2 · Air pollutants and their sources

Q47. Which international organisation sets the global guidelines for air quality standards?

  1. (a) World Health Organization (WHO)
  2. (b) United Nations Environment Programme (UNEP)
  3. (c) Intergovernmental Panel on Climate Change (IPCC)
  4. (d) Food and Agriculture Organization (FAO)
Answer & solution

Answer: (a) World Health Organization (WHO)

The World Health Organization (WHO) sets global air quality guidelines. WHO's Air Quality Guidelines provide targets for key pollutants such as PM2.5, PM10, ozone, NO2, SO2, and CO to help countries reduce health risks from air pollution worldwide.
(b) Incorrect: UNEP deals with broader environmental policy but does not set air quality standards.
(c) Incorrect: IPCC assesses climate change science; it does not set air quality guidelines.
(d) Incorrect: FAO focuses on food and agriculture, not air quality standards.
Delhi Police Constable (Exe) 2025-26 – 29 December 2025 Shift 3 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q48. What determines the overall AQI for a location?

  1. (a) The average concentration of all pollutants.
  2. (b) The worst sub-index value among the pollutants.
  3. (c) The total number of pollutants measured.
  4. (d) The concentration of only the most harmful pollutant.
Answer & solution

Answer: (b) The worst sub-index value among the pollutants.

The overall Air Quality Index (AQI) for a location is determined by the worst (highest) sub-index value among all the pollutants measured — including PM2.5, PM10, NO2, SO2, CO, Ozone (O3), Ammonia (NH3), and Lead (Pb). Each pollutant gets its own sub-index, and the AQI equals the highest of these values. This ensures the AQI always reflects the most severe pollution concern at that location.
(a) Incorrect: AQI is not the average of all pollutant concentrations; it takes the worst value, not the mean.
(c) Incorrect: The number of pollutants measured is not how AQI is determined; it is the worst sub-index.
(d) Incorrect: AQI is not based solely on one pollutant; all measured pollutants are assessed and the worst sub-index is used.
Delhi Police Constable (Exe) 2025-26 – 22 December 2025 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q49. What does AQI stand for?

  1. (a) Air Quality Index
  2. (b) Air Quality Indicator
  3. (c) Air Quality Information
  4. (d) Air Quality Insight
Answer & solution

Answer: (a) Air Quality Index

AQI stands for Air Quality Index — a standardized scale used by government agencies to communicate how polluted the air currently is or how polluted it is forecast to become. In India, the National AQI measures six pollutants: PM2.5, PM10, NO2, SO2, CO, O3, and is measured on a scale from 0 to 500. Option 1 is correct.
Option 2 is wrong: 'Air Quality Indicator' is not the established full form of AQI; the correct expansion is Air Quality Index.
Option 3 is wrong: 'Air Quality Information' is not the correct full form of AQI; it is Air Quality Index.
Option 4 is wrong: 'Air Quality Insight' is not the correct full form of AQI; the universally accepted expansion is Air Quality Index.
Delhi Police Constable (Exe) 2025-26 – 19 December 2025 Shift 2 · Air pollutants and their sources

Q50. Which body releases the Air Quality Index (AQI) in India?

  1. (a) Environmental Pollution Authority (EPA)
  2. (b) Central Pollution Control Board (CPCB)
  3. (c) World Health Organization (WHO)
  4. (d) NITI Aayog
Answer & solution

Answer: (b) Central Pollution Control Board (CPCB)

The Central Pollution Control Board (CPCB) releases the Air Quality Index (AQI) in India under the National Air Quality Monitoring Programme (NAMP). The AQI measures eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb. The CPCB publishes real-time AQI data through the Sameer app and website. The AQI was launched in India in 2015 (initially as NAQI - National Air Quality Index).
Option 1 is incorrect: There is no regulatory body called 'Environmental Pollution Authority (EPA)' in India; this designation is used in the USA. India's pollution regulation falls under CPCB (central) and SPCBs (state level).
Option 3 is incorrect: The WHO sets global air quality guidelines but does not release India's national AQI; that is done by CPCB.
Option 4 is incorrect: NITI Aayog is a policy think tank; it does not monitor or release air quality data.
Delhi Police Constable (Exe) 2025-26 – 18 December 2025 Shift 1 · Air pollutants and their sources

Q51. Which of the following is NOT included in India's Air Quality Index (AQI) calculation?

  1. (a) Ammonia (NH3)
  2. (b) Lead (Pb)
  3. (c) Ozone (O3)
  4. (d) Methane (CH4)
Answer & solution

Answer: (d) Methane (CH4)

India's National Air Quality Index (AQI) measures eight major pollutants: PM10, PM2.5, NO2 (Nitrogen Dioxide), SO2 (Sulfur Dioxide), CO (Carbon Monoxide), O3 (Ozone), NH3 (Ammonia), and Pb (Lead). Methane (CH4) is NOT included in India's AQI calculation, even though it is a potent greenhouse gas.
Option (a) is incorrect: Ammonia (NH3) IS one of the 8 pollutants included in India's AQI calculation.
Option (b) is incorrect: Lead (Pb) IS one of the 8 pollutants included in India's AQI calculation.
Option (c) is incorrect: Ozone (O3) IS one of the 8 pollutants included in India's AQI calculation.
Option (d) is CORRECT: Methane (CH4) is NOT part of India's AQI. While it is a greenhouse gas, it is not monitored as part of the 8 standard AQI pollutants.
SSC CPO Paper-1 2025 – 10 December 2025 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q52. Which international organization publishes air quality guidelines that many nations use as a reference?

  1. (a) United Nations Educational, Scientific and Cultural Organization (UNESCO)
  2. (b) World Health Organization (WHO)
  3. (c) International Monetary Fund (IMF)
  4. (d) World Trade Organization (WTO)
Answer & solution

Answer: (b) World Health Organization (WHO)

Correct Answer: World Health Organization (WHO)
The WHO publishes the Global Air Quality Guidelines, which are used by countries worldwide as benchmarks for setting air quality standards and policies. The latest WHO Air Quality Guidelines were updated in 2021.
Option 1 (UNESCO): Incorrect. UNESCO deals with education, science, and culture.
Option 3 (IMF): Incorrect. IMF deals with monetary and financial matters.
Option 4 (WTO): Incorrect. WTO regulates international trade.
SSC CPO Paper-1 2025 – 9 December 2025 Shift 1 · Air pollutants and their sources

Q53. Which of these is a secondary pollutant that is included in Air Quality Index (AQI) calculations?

  1. (a) SO₂ (Sulphur Dioxide)
  2. (b) NO₂ (Nitrogen Dioxide)
  3. (c) O₃ (Ozone)
  4. (d) PM₁₀(Fine Particulate Matter)
Answer & solution

Answer: (c) O₃ (Ozone)

O₃ (Ground-level Ozone) is a secondary pollutant — it is not directly emitted but is formed in the atmosphere.
It forms when Nitrogen Oxides (NOx) and Volatile Organic Compounds (VOCs) react in the presence of sunlight (photochemical reaction).
India's National AQI monitors 8 pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb.
Among these, O3 is the classic secondary pollutant.
SO2 and NO2 are primarily primary pollutants (directly emitted from vehicles and industries).
PM2.5 can be both primary and secondary.
Therefore, the correct answer is O₃ (Option 3).
SSC Stenographer 2025 – 8 December 2025 Shift 1 · Greenhouse effect and greenhouse gases

Q54. Which specific environmental condition found in waterlogged soils of rice paddies and wetlands directly facilitates the significant microbial production of methane, a potent greenhouse gas?

  1. (a) Frequent soil tilling
  2. (b) Low soil temperatures
  3. (c) Anaerobic (oxygen-depleted) conditions
  4. (d) High levels of dissolved oxygen
Answer & solution

Answer: (c) Anaerobic (oxygen-depleted) conditions

In rice paddies and wetlands, waterlogging leads to anaerobic (oxygen-depleted) conditions . In the absence of oxygen, methanogenic bacteria thrive and decompose organic matter, producing methane (CH₄) as a by-product. This methane then escapes into the atmosphere, making paddy fields one of the largest agricultural sources of greenhouse gas emissions.
SSC Stenographer 2025 – 6 December 2025 Shift 2 · Air pollutants and their sources

Q55. Which AQI range corresponds to “Unhealthy for Sensitive Groups”?

  1. (a) 51–100
  2. (b) 101–150
  3. (c) 151–200
  4. (d) 0–50
Answer & solution

Answer: (b) 101–150

The correct answer is 101–150 . The Air Quality Index (AQI) is a numerical scale used to measure and report air quality levels. It classifies air pollution into categories that indicate health impacts. The range 0–50 indicates “Good,” 51–100 is “Moderate,” 101–150 is “Unhealthy for Sensitive Groups” (such as children, elderly people, and those with respiratory or heart conditions), 151–200 is “Unhealthy,” 201–300 is “Very Unhealthy,” and 301–500 is “Hazardous.” In the range 101–150, healthy people may not be affected, but sensitive groups can experience breathing difficulties and aggravated health conditions.
SSC CGL Tier-1 2025 – 21 September 2025 Shift 3 · Air pollutants and their sources

Q56. Read the below statements marked as Assertion (A) and Reason (R). Mark the correct options:
Assertion (A): AQI value above 400 is classified as 'Severe'.
Reason (R): Such levels cause respiratory effects even on healthy individuals.

  1. (a) Both A and R are true, and R is the correct explanation of A.
  2. (b) Both A and R are true, but R is not the correct explanation of A.
  3. (c) A is true, but R is false.
  4. (d) A is false, but R is true.
Answer & solution

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Assertion (A) is true: as per CPCB, AQI 401-500 is classified as 'Severe'.
Reason (R) is true: such severe pollution causes respiratory effects even on healthy individuals and serious health impacts on those with existing conditions — this is the very reason for the 'Severe' classification.
Hence both are true and R correctly explains A.
SSC CGL Tier-1 2025 – 16 September 2025 Shift 2 · Air pollutants and their sources

Q57. Read the below statements marked as Assertion (A) and Reason (R):
Assertion (A): PM2.5 is more harmful than PM10 in AQI.
Reason (R): PM2.5 particles are smaller and can reach deep into the lungs and bloodstream.

  1. (a) Both A and R are true, and R is the correct explanation of A.
  2. (b) Both A and R are true, but R is not the correct explanation of A.
  3. (c) A is true, but R is false.
  4. (d) A is false, but R is true.
Answer & solution

Answer: (a) Both A and R are true, and R is the correct explanation of A.

• Air Quality Index (AQI) science:
• Assertion A: PM2.5 (particulate matter ≤2.5 micrometers ) IS more harmful than PM10 (≤10 micrometers ) — TRUE ✓
• Reason R: PM2.5 particles are smaller and can penetrate deep into lung alveoli and enter the bloodstream — TRUE ✓
• R correctly explains why PM2.5 is more harmful than PM10
• Answer: Both A and R are true, and R is the correct explanation of A.
SSC CGL Tier-1 2025 – 15 September 2025 Shift 2 · Air pollutants and their sources

Q58. Read the below statements marked as Assertion (A) and Reason (R). Mark the correct options:
Assertion (A): Delhi frequently records the highest AQI levels among Indian cities.
Reason (R): It lies in a low-lying basin surrounded by the Himalayas.

  1. (a) Both A and R are true, and R is the correct explanation of A.
  2. (b) Both A and R are true, but R is not the correct explanation of A.
  3. (c) A is true, but R is false.
  4. (d) A is false, but R is true.
Answer & solution

Answer: (c) A is true, but R is false.

Assertion is true – Delhi frequently records highest AQI due to vehicular emission, stubble burning and dust.
Reason is false – Delhi lies in the Indo-Gangetic plain, not in a basin surrounded by the Himalayas (the Himalayas are far to the north).
Hence A is true, R is false.
RRB NTPC UG CBT-1 2024-25 – 2 September 2025 Shift 3 · Greenhouse effect and greenhouse gases

Q59. The increase in Earth's average temperature due to trapped gases is called:

  1. (a) Climate Change
  2. (b) Ocean acidification
  3. (c) Greenhouse effect
  4. (d) Ozone depletion
Answer & solution

Answer: (c) Greenhouse effect

SSC Selection Post 2025 (Phase XIII) – 29 August 2025 Shift 2 · Ozone layer and its depletion

Q60. Which equation correctly represents the formation of ozone in the upper atmosphere?

  1. (a) CO₂ + UV → CO + O
  2. (b) O₂ + UV → O + O; O + O₂ → O₃
  3. (c) O₂ + Heat → O + O
  4. (d) O₂ → O + O; O + O → O₂
Answer & solution

Answer: (b) O₂ + UV → O + O; O + O₂ → O₃

Correct Answer: Option 2 – O₂ + UV → O + O; O + O₂ → O₃
Ozone (O₃) formation in the stratosphere follows the Chapman mechanism: Step 1: O₂ + UV radiation (λ < 242 nm) → O + O (photodissociation of oxygen molecule into two oxygen atoms). Step 2: O + O₂ + M → O₃ + M (atomic oxygen combines with molecular oxygen; M is a third body for energy transfer). Net: 3O₂ + UV → 2O₃. This process creates the ozone layer in the stratosphere (15–50 km altitude), which absorbs harmful UV-B and UV-C radiation.
Option 1 (CO₂ + UV → CO + O): This describes photodissociation of carbon dioxide, not ozone formation.
Option 3 (O₂ + Heat → O + O): Thermal dissociation of O₂ requires extremely high temperatures; stratospheric ozone formation is driven by UV photons, not heat.
Option 4 (O₂ → O + O; O + O → O₂): This describes dissociation and recombination producing O₂, not O₃.
RRB NTPC UG CBT-1 2024-25 – 28 August 2025 Shift 1 · Acid rain

Q61. Which of the following pollutants is/are primarily responsible for causing acid rain?

  1. (a) Sulphur dioxide and nitrogen oxides
  2. (b) Ozone
  3. (c) Carbon monoxide
  4. (d) Methane
Answer & solution

Answer: (a) Sulphur dioxide and nitrogen oxides

RRB NTPC UG CBT-1 2024-25 – 22 August 2025 Shift 2 · Greenhouse effect and greenhouse gases

Q62. Which of the following human activities contributes the most to the increase in atmospheric carbon dioxide levels?

  1. (a) Use of fertilisers in agriculture
  2. (b) Construction of dams
  3. (c) Burning of fossil fuels
  4. (d) Afforestation
Answer & solution

Answer: (c) Burning of fossil fuels

RRB NTPC UG CBT-1 2024-25 – 21 August 2025 Shift 2 · Acid rain

Q63. Acid rain is mainly caused due to the presence of which two gases in the atmosphere?

  1. (a) Carbon dioxide and Carbon monoxide
  2. (b) Nitrogen and Methane
  3. (c) Sulphur dioxide and Nitrogen oxides
  4. (d) Oxygen and Hydrogen
Answer & solution

Answer: (c) Sulphur dioxide and Nitrogen oxides

RRB NTPC UG CBT-1 2024-25 – 12 August 2025 Shift 2 · Water pollution and hardness of water

Q64. Which of the following is a water pollutant?

  1. (a) Oxygen
  2. (b) Nitrogen gas
  3. (c) Lead
  4. (d) Carbon dioxide
Answer & solution

Answer: (c) Lead

RRB NTPC UG CBT-1 2024-25 – 11 August 2025 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q65. Which of the following programs was launched in 2019 by Ministry of Environment, Forest and Climate Change (MoEF&CC) to improve air quality in 131 cities (Non-attainment Cities and Million Plus Cities) in 24 States/UTs?

  1. (a) National Association of Clean Air Agencies
  2. (b) National Clean Air Programme
  3. (c) Swach Vayu Survekshan
  4. (d) Air Quality Forecasting Systems
Answer & solution

Answer: (b) National Clean Air Programme

SSC Selection Post 2025 (Phase XIII) – 31 July 2025 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q66. Fill in the blank:
The AQI category for '____' ranges from 0 to 50.

  1. (a) Satisfactory
  2. (b) Moderate
  3. (c) Good
  4. (d) Poor
Answer & solution

Answer: (c) Good

Correct Answer: Option 3 – Good
India's Air Quality Index (AQI) is calculated by the Central Pollution Control Board (CPCB) on a scale of 0–500, divided into six categories: Good (0–50) — minimal impact; Satisfactory (51–100) — minor breathing discomfort to sensitive people; Moderate (101–200) — breathing discomfort to people with heart/lung disease; Poor (201–300) — breathing discomfort to most people; Very Poor (301–400) — respiratory illness risk; Severe (401–500) — serious health impacts, even healthy people affected. The category with AQI 0–50 is 'Good'.
Option 1 (Satisfactory): AQI 51–100 is Satisfactory — not 0-50.
Option 2 (Moderate): AQI 101–200 is Moderate.
Option 4 (Poor): AQI 201–300 is Poor.
SSC Selection Post 2025 (Phase XIII) – 31 July 2025 Shift 3 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q67. Which Indian government body is primarily responsible for monitoring and regulating the Air Quality Index (AQI) in India?

  1. (a) Ministry of Health and Family Welfare
  2. (b) Central Pollution Control Board (CPCB)
  3. (c) National Green Tribunal (NGT)
  4. (d) Indian Meteorological Department (IMD)
Answer & solution

Answer: (b) Central Pollution Control Board (CPCB)

Correct Answer: Option 2 – Central Pollution Control Board (CPCB)
The Central Pollution Control Board (CPCB), established under the Water (Prevention and Control of Pollution) Act, 1974, is the primary statutory body responsible for monitoring and regulating air quality in India, including the Air Quality Index (AQI). CPCB operates the National Air Quality Monitoring Programme (NAMP) and maintains the real-time air quality data portal (AQI India). The CPCB-developed AQI considers eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb. States have State Pollution Control Boards (SPCBs) working under CPCB's guidance.
Option 1 (Ministry of Health and Family Welfare): Deals with public health policy — not the primary AQI regulatory body.
Option 3 (National Green Tribunal): The NGT is a judicial body that adjudicates environmental disputes and can issue orders on pollution — but it is not the monitoring/regulatory authority for AQI.
Option 4 (Indian Meteorological Department): IMD monitors weather, forecasts rainfall, and issues cyclone warnings — not AQI.
SSC Selection Post 2025 (Phase XIII) – 30 July 2025 Shift 1 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q68. Fill in the blank:
The National Air Quality Index (AQI) is measured using _____ number of pollutants.

  1. (a) Three
  2. (b) Four
  3. (c) Five
  4. (d) Eight
Answer & solution

Answer: (d) Eight

Correct Answer: Option 4 – Eight
India's National Air Quality Index (AQI), launched by the Central Pollution Control Board (CPCB) in 2014 under the National Air Quality Monitoring Programme (NAMP), measures eight pollutants: (1) PM2.5, (2) PM10, (3) NO₂ (Nitrogen Dioxide), (4) SO₂ (Sulphur Dioxide), (5) CO (Carbon Monoxide), (6) O₃ (Ozone), (7) NH₃ (Ammonia), and (8) Pb (Lead). The AQI is calculated for each pollutant separately and the highest sub-index value is taken as the overall AQI. Scale: 0–50 Good, 51–100 Satisfactory, 101–200 Moderate, 201–300 Poor, 301–400 Very Poor, 401–500 Severe.
Option 1 (Three): Incorrect — India's AQI uses eight pollutants, not three.
Option 2 (Four): Incorrect — eight pollutants are used.
Option 3 (Five): Incorrect — the correct number is eight.
SSC Selection Post 2025 (Phase XIII) – 30 July 2025 Shift 2 · Acid rain

Q69. What is the main cause of acid rain?

  1. (a) Carbon monoxide and methane
  2. (b) Sulfur dioxide and nitrogen oxides
  3. (c) Chlorofluorocarbons (CFCs)
  4. (d) Lead and mercury compounds
Answer & solution

Answer: (b) Sulfur dioxide and nitrogen oxides

Correct Answer: Option 2 – Sulfur dioxide and nitrogen oxides
Acid rain is caused primarily by sulfur dioxide (SO₂) and nitrogen oxides (NOx) emissions. When SO₂ and NOx are released into the atmosphere (from burning fossil fuels in power plants, vehicles, and industries), they react with water vapour, oxygen, and other chemicals to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These acids fall as acid rain (pH < 5.6), acid snow, or dry acid deposition. SO₂ + H₂O → H₂SO₃; 2SO₂ + O₂ + 2H₂O → 2H₂SO₄.
Option 1 (CO and methane): Carbon monoxide contributes to photochemical smog; methane is a greenhouse gas — neither directly causes acid rain.
Option 3 (CFCs): Chlorofluorocarbons cause stratospheric ozone depletion — not acid rain.
Option 4 (Lead and mercury): These are heavy metal pollutants causing water/soil contamination; not the primary cause of acid rain.
SSC Selection Post 2025 (Phase XIII) – 29 July 2025 Shift 1 · Air pollutants and their sources

Q70. Fill in the blank:
AQI values above _____ indicate unhealthy air quality for sensitive groups.

  1. (a) 100
  2. (b) 50
  3. (c) 75
  4. (d) 25
Answer & solution

Answer: (a) 100

Correct Answer: Option 1 – 100
The Air Quality Index (AQI) scale used by India's Central Pollution Control Board (CPCB) and the US EPA both define AQI values above 100 as 'Unhealthy for Sensitive Groups' (US EPA category). In this range, people with respiratory diseases (asthma), cardiovascular conditions, elderly, and children may experience health effects. India's CPCB AQI scale: 0–50 (Good), 51–100 (Satisfactory/Moderate), 101–200 (Unhealthy for sensitive groups / Moderate–Poor), 201–300 (Very Poor), 301–400 (Severe), 401+ (Severe+/Hazardous). AQI above 100 marks the threshold where sensitive groups face risk.
Option 2 (50): AQI 0–50 is 'Good' — no health concerns for any group.
Option 3 (75): Not a defined threshold in standard AQI scales for sensitive group health impact.
Option 4 (25): Well within the 'Good' AQI range — no health concerns.
SSC Selection Post 2025 (Phase XIII) – 28 July 2025 Shift 3 · Air pollutants and their sources

Q71. Which technology is increasingly being used for real-time Air Quality Index (AQI) monitoring in urban areas?

  1. (a) Remote sensing and satellite data
  2. (b) Seismic sensors
  3. (c) Sonar technology
  4. (d) Ground penetrating radar
Answer & solution

Answer: (a) Remote sensing and satellite data

Correct Answer: Option 1 – Remote sensing and satellite data
Remote sensing and satellite data are increasingly used for real-time AQI monitoring in urban areas. Satellites like NASA's MODIS, Sentinel-5P (TROPOMI), and India's Cartosat/Resourcesat provide data on particulate matter (PM2.5, PM10), NO2, SO2, CO, and ozone concentrations over large urban regions. ISRO's satellite data is integrated with CPCB's (Central Pollution Control Board) ground monitoring network for comprehensive AQI assessment.
Option 2 (Seismic sensors): Seismic sensors detect ground vibrations and earthquakes; they have no application in air quality monitoring.
Option 3 (Sonar technology): Sonar uses sound waves to detect underwater objects; not used for AQI monitoring.
Option 4 (Ground penetrating radar): GPR uses radar pulses to image the subsurface; not applicable to air quality monitoring.
SSC Selection Post 2025 (Phase XIII) – 25 July 2025 Shift 2 · Air quality management (AQI, NAAQS, NCAP, GRAP)

Q72. What type of pollutants does the WHO Ambient Air Quality Database monitor?

  1. (a) CO₂ and ozone
  2. (b) PM₁₀ and PM₂.₅
  3. (c) Methane and CO
  4. (d) Lead and arsenic
Answer & solution

Answer: (b) PM₁₀ and PM₂.₅

Correct Answer: Option 2 – PM₁₀ and PM₂.₅
The WHO Ambient Air Quality Database tracks ground-level measurements of particulate matter: PM₁₀ (particles ≤10 micrometres in diameter) and PM₂.₅ (fine particles ≤2.5 micrometres ). These are the most harmful air pollutants tracked globally for public health purposes. The database covers thousands of cities worldwide and is used to assess compliance with WHO Air Quality Guidelines.
Option 1 (CO₂ and ozone): CO₂ is a greenhouse gas tracked separately by climate bodies; ground-level ozone is a pollutant but is not the primary focus of the WHO Ambient Air Quality Database.
Option 3 (Methane and CO): Methane is a greenhouse gas; CO (carbon monoxide) is a pollutant but not the focus of this specific WHO database.
Option 4 (Lead and arsenic): These are heavy metal pollutants tracked in specific contexts; not the primary pollutants in the WHO Ambient Air Quality Database.
RRB NTPC GL CBT-1 2024-25 – 20 June 2025 Shift 1 · Ozone layer and its depletion

Q73. What is the primary environmental impact of the release of freon from refrigeration plants on the atmosphere?

  1. (a) Increased nitrogen
  2. (b) Increased oxygen level
  3. (c) Stratospheric ozone depletion
  4. (d) Reduced greenhouse gas emissions
Answer & solution

Answer: (c) Stratospheric ozone depletion

RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Ozone layer and its depletion

Q74. Which of the following statements is/are correct related to eco-system?
(i) UV radiation is not damaging to organisms.
(ii) Ozone at the higher levels of the atmosphere is a product of UV radiation acting on oxygen molecule.
(iii) The amount of ozone in the atmosphere began to drop sharply in 1980.

  1. (a) (i), (ii) and (iii)
  2. (b) Only (i) and (ii)
  3. (c) Only (i)
  4. (d) Only (ii) and (iii)
Answer & solution

Answer: (d) Only (ii) and (iii)

Correct Answer: Option 4 — Only (ii) and (iii)
Statement (i) is wrong: UV (ultra-violet) radiation is highly damaging to living organisms. In human beings it causes skin cancer, cataract and damage to the immune system, and it also harms crops and planktons.
Statement (ii) is correct: high up in the atmosphere UV rays split molecular oxygen (O₂) into free oxygen atoms, and these atoms combine with O₂ molecules to form ozone (O₃). So ozone at higher levels is indeed a product of UV radiation acting on oxygen molecules.
Statement (iii) is correct: the amount of ozone in the atmosphere began to drop sharply in 1980, mainly because of CFCs (chlorofluorocarbons) used in refrigerators and fire extinguishers, producing the famous ozone hole over Antarctica.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — wrong because it accepts statement (i); UV radiation is definitely damaging to living organisms.
✗ Only (i) and (ii) — wrong; (i) is false, and this option also leaves out the true statement (iii).
✗ Only (i) — wrong; (i) is in fact the only false statement, while (ii) and (iii) are both true.
📌 SSC & Railway Exam Facts
• Ozone (O₃) occurs in the stratosphere and forms the protective ozone shield.
• The reactions are O₂ → O + O (by UV) and then O + O₂ → O₃.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) froze and then phased out CFC production world-wide.
• 16 September is observed as the International Day for the Preservation of the Ozone Layer.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Ozone layer and its depletion

Q75. Ozone shields the surface of the earth from ultraviolet (UV) radiation from the Sun and the amount of ozone in the atmosphere begins to drop sharply due to which of the following synthetic chemicals?

  1. (a) Insecticides
  2. (b) Chlorofluorocarbons
  3. (c) Pesticides
  4. (d) Carbon monoxide
Answer & solution

Answer: (b) Chlorofluorocarbons

Correct Answer: Option 2 — Chlorofluorocarbons
Ozone (O₃) forms a protective layer in the upper atmosphere (the stratosphere) and absorbs the harmful ultraviolet (UV) radiation of the Sun, which otherwise causes skin cancer, cataract and damage to crops.
Chlorofluorocarbons (CFCs) are man-made chemicals used as refrigerants and in fire extinguishers and aerosol sprays. In the stratosphere UV light splits a CFC molecule and frees a chlorine atom, which then destroys ozone molecules in a chain reaction — a single chlorine atom can break down thousands of O₃ molecules. NCERT records that the amount of ozone began to drop sharply in the 1980s, producing the ozone hole over Antarctica.
❌ Why the other options are incorrect
✗ Insecticides — chemicals that kill insects; they pollute soil and water and enter the food chain as biomagnification, but do not destroy ozone.
✗ Pesticides — a broader group of crop-protection chemicals; again a soil-and-water pollutant, not an ozone-depleting substance.
✗ Carbon monoxide — a poisonous gas from incomplete combustion that binds to haemoglobin; it is an air pollutant, not an ozone destroyer.
📌 SSC & Railway Exam Facts
• Ozone = O₃, formed when UV splits O₂ into free oxygen atoms which then combine with O₂.
• The Montreal Protocol (1987) froze and phased out CFC production.
• 16 September is celebrated as the World Ozone Day.
• The ozone hole was detected over Antarctica.
• Ozone is harmful at ground level (a pollutant) but essential in the stratosphere.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Ozone layer and its depletion

Q76. Which of the following statements is correct about the ozone?

  1. (a) Decrease in the amount of ozone in the atmosphere is due to chlorofluorocarbons (CFC).
  2. (b) It is formed by combining only two atoms of oxygen.
  3. (c) It is not poisonous.
  4. (d) It causes blood cancer.
Answer & solution

Answer: (a) Decrease in the amount of ozone in the atmosphere is due to chlorofluorocarbons (CFC).

Correct Answer: Option 1 — Decrease in the amount of ozone in the atmosphere is due to chlorofluorocarbons (CFC).
Ozone (O₃) is a molecule of three atoms of oxygen. High up in the stratosphere it forms a shield that absorbs harmful ultraviolet (UV) radiation coming from the Sun and prevents it from reaching the Earth's surface.
CFCs used in refrigerators, air-conditioners and aerosol sprays drift up to the stratosphere. UV light breaks them and releases chlorine atoms, and a single chlorine atom destroys thousands of ozone molecules in a chain reaction. This thinning of the layer is the ozone hole, first reported over Antarctica.
❌ Why the other options are incorrect
✗ It is formed by combining only two atoms of oxygen — O₂ has two atoms; ozone has three (O₃), made when UV splits O₂ and the free O joins another O₂.
✗ It is not poisonous — ozone is a deadly poison at ground level; it is useful only in the upper atmosphere.
✗ It causes blood cancer — ozone does not cause blood cancer; the UV rays it blocks cause skin cancer and cataract.
📌 SSC & Railway Exam Facts
• Ozone layer lies in the stratosphere, about 15–35 km above the Earth.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) froze and phased out CFC production.
• 16 September is observed as the World Ozone Day.
• Ozone thickness is measured in Dobson Units (DU).
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Ozone layer and its depletion

Q77. Which of the following statements is/are correct related to the eco-system?
(i) Ozone is a molecule formed by three atoms of oxygen.
(ii) Oxygen (O₂) is essential for all aerobic forms of life.
(iii) Ozone is non-poisonous.

  1. (a) Only (i)
  2. (b) Only (i) and (ii)
  3. (c) Only (i) and (iii)
  4. (d) (i), (ii) and (iii)
Answer & solution

Answer: (b) Only (i) and (ii)

Correct Answer: Option 2 — Only (i) and (ii)
Checking the three statements against NCERT:
• (i) is correct — ozone (O₃) is a molecule made of three atoms of oxygen, unlike the ordinary oxygen we breathe, which has two atoms (O₂).
• (ii) is correct — O₂ is essential for all aerobic forms of life, because aerobic respiration oxidises glucose in the mitochondria to release large amounts of ATP.
• (iii) is INCORRECT — ozone is a deadly poison. At ground level it is a harmful pollutant that irritates the eyes and lungs and damages crops. It is useful only high up in the stratosphere, where it filters out harmful UV radiation.
So statements (i) and (ii) alone are correct.
❌ Why the other options are incorrect
✗ Only (i) — it leaves out (ii), although oxygen really is essential for every aerobic organism.
✗ Only (i) and (iii) — statement (iii) is false, since ozone is poisonous at the surface.
✗ (i), (ii) and (iii) — it wrongly accepts (iii); ozone is not non-poisonous.
📌 SSC & Railway Exam Facts
• Ozone is formed in the stratosphere when UV rays split O₂ into free oxygen atoms, which then join O₂ molecules: O₂ → O + O; O + O₂ → O₃.
• Excess UV radiation causes skin cancer, cataract and damage to the immune system and crops.
• CFCs (chlorofluorocarbons) used in refrigerators and aerosol sprays are the main ozone-depleting substances.
• The ozone hole over Antarctica was reported in 1985; the Montreal Protocol (1987) phased out CFCs.
• 16 September is observed as the International Day for the Preservation of the Ozone Layer (World Ozone Day).
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Ozone layer and its depletion

Q78. Which of the following statements is/are correct related to the eco-system?
(i) Ozone is a molecule formed by three atoms of oxygen.
(ii) Oxygen (O₂) is essential for all aerobic forms of life.
(iii) Ozone is non-poisonous.

  1. (a) (i), (ii) and (iii)
  2. (b) Only (i) and (ii)
  3. (c) Only (i) and (iii)
  4. (d) Only (i)
Answer & solution

Answer: (b) Only (i) and (ii)

Correct Answer: Option 2 — Only (i) and (ii)
Ozone (O₃) is a molecule made of three atoms of oxygen, so statement (i) is correct. It is formed in the stratosphere when high-energy UV radiation splits molecular oxygen (O₂) into free oxygen atoms, which then combine with other O₂ molecules.
Statement (ii) is also correct — ordinary oxygen (O₂) is essential for all aerobic organisms, because they use it in respiration to release energy from glucose as ATP.
Statement (iii) is wrong. NCERT states plainly that ozone is a deadly poison. At ground level it irritates the eyes and lungs and damages crops. Only high up in the stratosphere, far away from living beings, does it act as a protective shield by absorbing harmful UV rays.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — includes statement (iii), but ozone is in fact highly poisonous.
✗ Only (i) and (iii) — again wrongly accepts (iii) and unnecessarily drops the correct statement (ii).
✗ Only (i) — too narrow; statement (ii) about the need for oxygen in aerobic life is perfectly correct.
📌 SSC & Railway Exam Facts
• O₂ and O₃ are allotropes of oxygen; ozone has a pungent smell.
• The ozone layer lies in the stratosphere, roughly 15–35 km above the Earth.
• The ozone hole over Antarctica was reported in 1985; CFCs are the main culprits.
• The Montreal Protocol (1987) regulated CFC production; 16 September is World Ozone Day.
• Ozone thickness is measured in Dobson Units (DU).
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Ozone layer and its depletion

Q79. Which of the following statements is/are correct related to eco-system?
(i) UV radiation is not damaging to organisms.
(ii) Ozone at the higher levels of the atmosphere is a product of UV radiation acting on oxygen molecule.
(iii) The amount of ozone in the atmosphere began to drop sharply in 1980.

  1. (a) (i), (ii) and (iii)
  2. (b) Only (i)
  3. (c) Only (i) and (ii)
  4. (d) Only (ii) and (iii)
Answer & solution

Answer: (d) Only (ii) and (iii)

Correct Answer: Option 4 — Only (ii) and (iii)
Ozone (O₃) is a molecule of three oxygen atoms that forms a protective shield in the stratosphere.
Checking each statement:
• (i) is wrong — UV radiation is highly damaging to organisms; it causes skin cancer, cataract and damage to DNA.
• (ii) is correct — high up in the atmosphere, UV radiation splits an oxygen molecule (O₂) into free oxygen (O) atoms, which then combine with O₂ to form ozone (O₃).
• (iii) is correct — the amount of ozone in the atmosphere began to drop sharply in the 1980s because of synthetic chemicals, mainly CFCs.
Hence only (ii) and (iii) are correct.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — statement (i) is false; UV rays are seriously harmful to living beings.
✗ Only (i) — (i) is the only statement that is actually wrong, so this option is the exact opposite of the truth.
✗ Only (i) and (ii) — it wrongly includes (i) and wrongly leaves out the true statement (iii).
📌 SSC & Railway Exam Facts
• Ozone is formed high in the stratosphere but is a harmful pollutant in the troposphere.
• CFCs (Chlorofluorocarbons), used in refrigerators, ACs and fire extinguishers, are the main ozone-depleting substances.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) was signed to freeze and phase out CFC production.
• 16 September is observed as World Ozone Day; ozone thickness is measured in Dobson Units (DU).
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 2 · Ozone layer and its depletion

Q80. Which of the following synthetic chemicals was responsible for the sharp drop in the amount of ozone in the atmosphere in the 1980s?

  1. (a) Iodobromocarbons
  2. (b) Iodofluorocarbons
  3. (c) Chloroiodocarbons
  4. (d) Chlorofluorocarbons
Answer & solution

Answer: (d) Chlorofluorocarbons

Correct Answer: Option 4 — Chlorofluorocarbons
Chlorofluorocarbons (CFCs) are man-made compounds of carbon, chlorine and fluorine, used as refrigerants in refrigerators and air-conditioners, and as propellants in aerosol sprays and fire extinguishers.
CFCs are very stable, so they slowly rise to the stratosphere. There UV radiation breaks them down and releases chlorine free radicals, and a single chlorine atom can destroy thousands of ozone molecules in a chain reaction. This caused the sharp fall in ozone observed from the 1980s, including the ozone hole over Antarctica.
❌ Why the other options are incorrect
✗ Iodobromocarbons — not the chemical class blamed for the 1980s ozone decline; halons (bromine compounds) contribute, but CFCs were the main cause.
✗ Iodofluorocarbons — not a commercially used refrigerant/propellant responsible for ozone depletion.
✗ Chloroiodocarbons — not the recognised ozone-depleting industrial substance of that period.
📌 SSC & Railway Exam Facts
• Ozone (O₃) is formed in the stratosphere when UV rays split O₂ into free oxygen atoms which then combine with O₂.
• The ozone layer is a protective shield that absorbs harmful UV radiation, which causes skin cancer and cataract.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) froze and then phased out CFC production; 16 September is celebrated as World Ozone Day.
• In the troposphere, ozone is a harmful pollutant; in the stratosphere, it is essential for life.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · Ozone layer and its depletion

Q81. In which year did the amount of ozone begin to drop sharply in the atmosphere?

  1. (a) 1950
  2. (b) 1970
  3. (c) 1980
  4. (d) 1960
Answer & solution

Answer: (c) 1980

Answer: Option 3 — 1980
The amount of ozone in the atmosphere began to drop sharply in the 1980s, a fall linked to synthetic chemicals like chlorofluorocarbons (CFCs).
Revision note — Ozone layer and its depletion
• Ozone (O₃) is formed in the upper atmosphere when UV splits O₂: O₂ → O + O, then O + O₂ → O₃.
• It lies mainly in the stratosphere and shields the Earth from harmful UV radiation (which can cause skin cancer).
• CFCs, used as refrigerants and in fire extinguishers, release chlorine that destroys ozone; halons release bromine.
• The ozone hole was first reported over Antarctica in 1985.
• In 1987, UNEP forged an agreement to freeze CFC production at 1986 levels; the Montreal Protocol (1987) phases out ozone-depleting substances.
• World Ozone Day is observed on 16 September.
Also asked from this topic: where the ozone hole was first found (Antarctica); the main ozone-depleting chemical (CFCs); the treaty phasing out ozone-depleting substances (Montreal Protocol).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · Acid rain

Q82. Why is monitoring pH crucial for understanding the impacts of acid rain on soil and water ecosystems?

  1. (a) Acid rain decreases the pH, harming aquatic life and soil nutrient availability.
  2. (b) Acid rain does not affect soil and water ecosystems.
  3. (c) Acid rain increases the pH of soil and water bodies.
  4. (d) Acid rain only makes the water taste sour.
Answer & solution

Answer: (a) Acid rain decreases the pH, harming aquatic life and soil nutrient availability.

Answer: Option 1 — Acid rain decreases the pH, harming aquatic life and soil nutrient availability.
Acid rain carries sulphuric and nitric acid, so it lowers the pH of lakes and soil; fish and other organisms survive only in a narrow pH range, and acidic soil loses nutrients.
Revision note — Acid rain
• Rain with pH below 5.6 is called acid rain.
• Caused mainly by sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels; they form H₂SO₄ and HNO₃.
• When the pH of river water falls, the survival of aquatic animals becomes difficult — living organisms tolerate only a narrow pH range (7.0 to 7.8) in the body.
• Acidic soil leaches nutrients such as calcium and magnesium; farmers add quicklime (CaO) or slaked lime to neutralise it.
• Acid rain corrodes marble — the Taj Mahal is affected by SO₂ from the Mathura refinery ("marble cancer").
Also asked from this topic: the gas mainly responsible for acid rain (sulphur dioxide); the two gases that cause acid rain (sulphur dioxide and nitrogen oxides).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · Water pollution and hardness of water

Q83. Which of the following can be minimised through the implementation of proper sewage treatment?

  1. (a) Air pollution
  2. (b) Water pollution
  3. (c) Noise pollution
  4. (d) Soil pollution
Answer & solution

Answer: (b) Water pollution

Answer: Option 2 — Water pollution
Untreated sewage carries organic waste, pathogens and nutrients into rivers and lakes.
Treating it before discharge directly reduces water pollution.
Revision note — Sewage treatment and water pollution
• Primary treatment: physical removal of solids — screening, grit removal, sedimentation (sludge settles).
• Secondary (biological) treatment: aerobic bacteria digest the organic matter; this lowers the BOD (biochemical oxygen demand).
• Tertiary treatment: removes nitrates, phosphates; disinfection with chlorine or ozone / UV.
• Sludge is digested anaerobically to give biogas (mainly methane).
• High BOD = more organic pollution; clean water has low BOD.
• Excess nitrogen and phosphorus from sewage and fertilisers cause eutrophication — algal blooms and fish death from low dissolved oxygen.
Also asked from this topic: a non-point source of water pollution (agricultural runoff); the nutrients behind eutrophication (nitrogen and phosphorus); the disinfectant used for water and sewage (chlorine).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · Ozone layer and its depletion

Q84. What is the reason for CFC free refrigerators being manufactured for use throughout the world?

  1. (a) CFC is cheap for use in refrigerators.
  2. (b) CFC makes refrigeration a tough process.
  3. (c) CFC is a synthetic chemical which depletes the ozone layer.
  4. (d) CFC is tough to handle for the refrigeration industry.
Answer & solution

Answer: (c) CFC is a synthetic chemical which depletes the ozone layer.

Answer: Option 3 — CFC is a synthetic chemical which depletes the ozone layer.
Chlorofluorocarbons (CFCs) are man-made chemicals once used as refrigerants.
In the stratosphere, UV light breaks them to release chlorine atoms, which destroy ozone (O_(3)).
So refrigerators are now made CFC-free.
Revision note — Ozone layer and its depletion (NCERT Class 10)
• Ozone (O_(3)) — three oxygen atoms; a deadly poison at ground level but a shield in the stratosphere.
• The ozone layer absorbs ultraviolet (UV) radiation, which causes skin cancer and harms living beings.
• Formation: O_(2) + UV → O + O; then O + O_(2) → O_(3).
• Ozone levels fell sharply in the 1980s; the ozone hole was first reported over Antarctica (1985).
• UNEP agreement (1987) froze CFC production at 1986 levels — the Montreal Protocol.
• CFCs were used in refrigerators, air conditioners, aerosol sprays and fire extinguishers.
• World Ozone Day: 16 September.
Also asked from this topic: the compound mainly responsible for ozone depletion (chlorofluorocarbons); the devices that earlier used CFCs extensively (refrigerators); the halogen released by CFCs (chlorine).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · Ozone layer and its depletion

Q85. Which synthetic chemicals are responsible for the depletion of ozone in the atmosphere?

  1. (a) Chlorofluorocarbons (CFCs)
  2. (b) Nitrogen oxides
  3. (c) Methane
  4. (d) Carbon dioxide
Answer & solution

Answer: (a) Chlorofluorocarbons (CFCs)

Answer: Option 1 — Chlorofluorocarbons (CFCs)
CFCs are man-made compounds of chlorine, fluorine and carbon.
In the stratosphere, UV rays break them to release chlorine atoms, which destroy ozone (O_(3)).
Revision note — The ozone layer
• Ozone (O_(3)) is formed in the upper atmosphere when UV splits O_(2) and a free O atom joins O_(2).
• The ozone layer lies in the stratosphere and absorbs harmful UV radiation from the Sun.
• CFCs were used as refrigerants (refrigerators, ACs) and in fire extinguishers and aerosol sprays.
• One chlorine atom can destroy thousands of ozone molecules.
• The ozone hole was first found over Antarctica (reported 1985).
• The Montreal Protocol (1987) phases out ozone-depleting substances.
• World Ozone Day: 16 September.
Do not mix these up
• Ozone depletion vs global warming — ozone loss is caused mainly by CFCs; global warming mainly by CO_(2) and methane.
Also asked from this topic: where the ozone hole was first discovered (Antarctica); the agreement that phases out ozone-depleting substances (Montreal Protocol).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 2 · Acid rain

Q86. When the pH of rainwater falls below 5.6, it is commonly referred to as:

  1. (a) acid rain
  2. (b) mineral rain
  3. (c) hard water
  4. (d) alkaline rain
Answer & solution

Answer: (a) acid rain

Answer: Option 1 — acid rain
Normal rain is slightly acidic (pH about 5.6) because of dissolved carbon dioxide. When the pH falls below 5.6, it is called acid rain.
Revision note — Acid rain
• Caused mainly by sulphur dioxide (SO_(2)) and nitrogen oxides (NO_(x)) from burning fossil fuels.
• These form sulphuric acid and nitric acid in the atmosphere, which fall with rain.
• Effects: kills aquatic life when lake water pH drops, damages crops and forests, corrodes buildings.
• The Taj Mahal is yellowing from acid rain and pollution ("marble cancer") — marble is CaCO_(3).
• Lower pH = more acidic; pH 7 is neutral; living organisms survive only in a narrow pH range (7.0 to 7.8) in the body.
Also asked from this topic: the main gas behind acid rain (sulphur dioxide); the two gases mainly responsible (sulphur dioxide and nitrogen oxides).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 3 · Acid rain

Q87. The pH of acid rain is:

  1. (a) equal to 7
  2. (b) more than 5.6 but less than 7
  3. (c) less than 5.6
  4. (d) more than 7
Answer & solution

Answer: (c) less than 5.6

Answer: Option 3 — less than 5.6
Normal rain is already slightly acidic, about pH 5.6, because CO_(2) dissolves in it to form weak carbonic acid.
Rain is called acid rain only when its pH falls below 5.6.
Revision note — Acid rain
• Main causes: sulphur dioxide (SO_(2)) and nitrogen oxides (NO_(x)) from burning fossil fuels — power plants, vehicles, industries.
• They form sulphuric acid (H_(2)SO_(4)) and nitric acid (HNO_(3)) in the air.
• Effects: damages marble monuments — "marble cancer" of the Taj Mahal (CaCO_(3) reacts with acid); harms forests, soil and aquatic life.
• Acidic lakes kill fish; farmers correct acidic soil with quicklime or slaked lime.
• Pure water has pH 7; pH below 7 is acidic, above 7 is basic.
Do not mix these up
• Normal rain vs acid rain — both are below pH 7; the dividing line for acid rain is 5.6, not 7.
Also asked from this topic: the gas that contributes most to acid rain (sulphur dioxide); the two gases mainly responsible for acid rain (sulphur dioxide and nitrogen oxides).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 2 · Ozone layer and its depletion

Q88. Which of the following is/are responsible for the depletion of the ozone layer in the atmosphere?

  1. (a) Hydrofluorocarbons
  2. (b) Chlorofluorocarbons
  3. (c) Ethane
  4. (d) Methane
Answer & solution

Answer: (b) Chlorofluorocarbons

Answer: Option 2 — Chlorofluorocarbons
CFCs rise to the stratosphere, where UV rays break them and release chlorine atoms.
One chlorine atom can destroy thousands of ozone (O_(3)) molecules.
Revision note — Ozone layer
• Ozone O_(3) is formed when UV splits O_(2) into single O atoms, which join another O_(2): O + O_(2) → O_(3).
• The ozone layer lies in the stratosphere (about 15–35 km) and absorbs harmful UV radiation.
• CFCs were used in refrigerators, air conditioners, aerosol sprays and fire extinguishers.
• The ozone thinning was noticed in 1980s; the ozone hole was first reported over Antarctica (1985).
• Montreal Protocol (1987) phases out ozone-depleting substances; 16 September is World Ozone Day.
• Halons release bromine; CFCs, HCFCs and carbon tetrachloride release chlorine.
• Extra UV reaching Earth causes skin cancer and cataract.
Do not mix these up
• HFCs vs CFCs — HFCs contain no chlorine, so they do not deplete ozone.
HFCs are greenhouse gases, controlled by the Kigali Amendment (2016).
• Ozone depletion vs global warming — methane and CO_(2) cause warming, not ozone loss.
Also asked from this topic: the agreement that phases out ozone-depleting substances (Montreal Protocol); where the ozone hole was first discovered (Antarctica).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 2 · Ozone layer and its depletion

Q89. Why is the depletion of the ozone layer a concern for humans?

  1. (a) It leads to increased water pollution.
  2. (b) It increases soil fertility.
  3. (c) It promotes plant growth.
  4. (d) It protects us from harmful radiation.
Answer & solution

Answer: (d) It protects us from harmful radiation.

Answer: Option 4 — It protects us from harmful radiation.
Here "it" is the ozone layer: it absorbs the sun's harmful ultraviolet (UV) radiation.
If it thins, more UV reaches Earth — so its depletion is a concern for humans.
Revision note — Ozone layer and its depletion
• Ozone (O_(3)) is formed in the stratosphere when UV splits O_(2): O_(2) → O + O, then O + O_(2) → O_(3).
• The ozone layer lies mainly 15–35 km above Earth and absorbs UV-B and UV-C.
• Main cause of depletion: chlorofluorocarbons (CFCs) from refrigerators, ACs and aerosols; halons release bromine.
• The ozone hole was first detected over Antarctica (1985).
• More UV causes skin cancer, cataract and weakened immunity, and harms crops and plankton.
• Montreal Protocol (1987) phases out ozone-depleting substances; World Ozone Day is 16 September.
Do not mix these up
• Ozone depletion vs global warming — depletion lets in more UV; global warming is caused by greenhouse gases trapping heat.
Also asked from this topic: the function of the ozone layer (absorbing UV radiation from the sun); the main cause of ozone depletion (chlorofluorocarbons); the primary harmful effect of UV on humans (skin cancer).

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