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Air pollution and its control

தமிழில் விளக்கம்

Pollution · Part 1 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Primary and secondary pollutants; particulate matter and the main gases; sources of India's air pollution and its winter peak; classical and photochemical smog and temperature inversion; acid rain and the Taj Trapezium Zone; the 2009 air quality standards, the National Air Quality Index, NCAP, GRAP and the Commission for Air Quality Management; vehicle norms and cleaner fuels.

Each idea is explained first in Tamil (தமிழ்), ending with the English and Hindi exam terms, followed by the same point in exam English. UPSC Prelims is in English or Hindi, so understand it in Tamil and revise it in English.

மாசு என்றால் காற்று, நீர், மண்ணில் தீங்கு செய்யும் அளவில் ஏதாவது கலப்பது. காற்று மாசு முதலில், ஏனெனில் மூச்சை நிறுத்த முடியாது. முதன்மை மாசுபடுத்தி நேரடியாகப் புகைபோக்கி, புகைக்குழாயிலிருந்து வரும். இரண்டாம்நிலை மாசுபடுத்தி காற்றிலேயே உருவாகும், வெயிலில் இரண்டு பொருட்கள் சேர்ந்து புதிய விஷம் சமைப்பது போல; உதாரணம் தரைமட்ட ஓசோன். (→ English: pollutant; primary pollutant; secondary pollutant; ground-level ozone · हिन्दी: प्रदूषक; प्राथमिक प्रदूषक; द्वितीयक प्रदूषक; भू-स्तरीय ओज़ोन)

In exam EnglishPollution means adding to air, water or soil a substance or form of energy, such as noise or heat, in amounts that harm living things or property; the substance itself is the pollutant. This chapter has three parts: air pollution here, water, soil and noise in the second, and waste with its rules in the third. Air comes first because it does the most damage to health: nobody can choose not to breathe, and fine particles reach deep into the body. A primary pollutant leaves its source already harmful, as sulphur dioxide does from a coal-fired chimney or carbon monoxide from an exhaust pipe. A secondary pollutant never leaves a chimney or a pipe; it is created in the air itself as other pollutants react, often with sunlight's help; ground-level ozone is the standard example.

நுண்துகள் பொருள் என்றால் காற்றில் மிதக்கும் சிறு துகள்கள்: தூசி, கரி, சாம்பல். PM10 மூக்கு, தொண்டையில் நின்றுவிடும், சல்லடை பெரிய கற்களைத் தடுப்பது போல. PM2.5 முடியை விட முப்பது மடங்கு மெல்லியது; நுரையீரலின் ஆழத்துக்கும், இரத்தத்துக்கும் போகும், அதனால் இதய நோய்க்கும் காரணம். தூசி பெரிய துகள்களைத் தரும்; எரித்தல், அதாவது இயந்திரம், அடுப்பு, வயலில் தீ, மெல்லியவற்றைத் தரும். (→ English: particulate matter; PM10; PM2.5 · हिन्दी: कणिकीय पदार्थ; PM10; PM2.5)

In exam Englishparticulate matter is the pollutant that matters most for health in India. It is a mix of tiny solid or liquid bits suspended in the air: dust, soot, ash, salts and droplets. PM10 covers particles of 10 micrometres or less across, which lodge in the nose, throat and upper airways. PM2.5 takes in the still finer particles, 2.5 micrometres or smaller, so tiny that a single strand of hair is roughly thirty times as wide; these slip deep into the lungs and the finest pass into the blood, which links them with heart disease and stroke as well as asthma and lung cancer. Coarse particles come mostly from road and construction dust and from grinding. Fine particles come mainly from burning, in engines, power plants, brick kilns, stoves and fields, and some form in the air from gases such as sulphur dioxide and ammonia. So PM2.5 is both a primary and a secondary pollutant.

ஒவ்வொரு வாயுவுக்கும் ஒரு மூலம்: சல்பர் டை ஆக்சைடு நிலக்கரி எரிப்பிலிருந்து, நைட்ரஜன் ஆக்சைடுகள் வாகனங்களிலிருந்து. கார்பன் மோனாக்சைடு இரத்தத்தில் ஆக்சிஜனின் இருக்கையைப் பிடித்துக்கொள்ளும், பேருந்தில் நம் இருக்கையில் வேறொருவர் அமர்வது போல; அதனால் மூடிய அறையில் கரி அடுப்பு ஆபத்து. மேலே வானில் ஓசோன் பாதுகாப்பு, கீழே மூச்சில் அதுவே தீங்கு. ஈயம் பெட்ரோலில் இருந்து வந்தது, இப்போது இல்லை. (→ English: sulphur dioxide; nitrogen oxides; carbon monoxide; ground-level ozone; lead · हिन्दी: सल्फ़र डाइऑक्साइड; नाइट्रोजन ऑक्साइड; कार्बन मोनोऑक्साइड; भू-स्तरीय ओज़ोन; सीसा)

In exam EnglishThe main gases each have a typical source and effect. Sulphur dioxide comes chiefly from burning coal and heavy oil in power plants, smelters and refineries; it irritates the lungs and is a parent of acid rain. Nitrogen oxides form whenever fuel burns hot, so vehicles and power plants are the main sources; they also feed acid rain and ozone. Carbon monoxide, from incomplete burning, binds to haemoglobin far more readily than oxygen does and starves the body of oxygen, which is why a charcoal stove in a closed room can kill. Ground-level ozone, formed from nitrogen oxides and hydrocarbons in sunlight, damages lungs and crops; it is the same gas as in the protective layer high in the stratosphere, but harmful when breathed. Lead once came mainly from leaded petrol, which India had phased out by 2000. Ammonia, largely from fertiliser and animal waste, helps form fine particles.

நம் காற்று அழுக்குக்குப் பல மூலங்கள்: சாலையில் வாகனங்கள், தொழிற்சாலைகள், செங்கல் சூளைகள், தூசி. அக்டோபர்-நவம்பரில் பஞ்சாப், ஹரியானா விவசாயிகள் கோதுமைக்காக நெல் வைக்கோலை எரிக்கிறார்கள். வீட்டில் விறகு அடுப்புப் புகை அதிகம் தாய்மாருக்கும் குழந்தைகளுக்கும் படுகிறது. குளிர்காலத்தில் காற்று அசையாது, மூடிய பாத்திரத்தில் ஆவி போல புகை அங்கேயே தங்கும்; அதனால் நவம்பரில் டெல்லிக் காற்று மோசம். (→ English: crop-residue burning; indoor air pollution; winter pollution · हिन्दी: पराली दहन; घरेलू वायु प्रदूषण; शीतकालीन प्रदूषण)

In exam EnglishIndia's dirty air has many sources, and their shares differ by city and season. Vehicles dominate along busy roads; coal-fired power plants, steel, cement and brick kilns load the air around industrial belts; road and construction dust adds large amounts of coarse particles. In October and November, farmers in Punjab and Haryana burn rice straw to clear fields quickly for wheat, and the smoke drifts over the plains. Households add a hidden share: smoke from wood, dung and crop waste burnt in stoves fills kitchens, so indoor air pollution falls hardest on the women and children who spend most time near the fire. In the north Indian winter, cool calm air close to the ground and light winds mean pollution is not carried away, which is why Delhi's air is usually worst in November.

புகைமூட்டம் இரண்டு வகை. ஒன்று குளிர், ஈரத்தில் நிலக்கரிப் புகை பனியுடன் கலந்து வருவது; 1952 லண்டனில் ஆயிரக்கணக்கானோர் இறந்தனர். இரண்டாவது ஒளி வேதியியல் புகைமூட்டம்: வெயில், வெப்பம், போக்குவரத்து இருக்கும்போது வெயில் வாகனப் புகையில் வேலை செய்து ஓசோன் உருவாகிறது; லாஸ் ஏஞ்சல்ஸ் உதாரணம். வெப்பநிலைத் தலைகீழ் மாற்றம் என்றால் மேலே சூடான காற்று மூடி போல அமர்வது; கீழே புகை வெளியே போகாது. (→ English: classical smog; photochemical smog; temperature inversion · हिन्दी: पारंपरिक धूम-कोहरा; प्रकाश-रासायनिक धूम-कोहरा; तापमान व्युत्क्रमण)

In exam EnglishSmog comes in two kinds. Classical or sulphurous smog forms in cold, damp weather when smoke and sulphur dioxide from coal mix with fog; it is chemically reducing, and the London smog of December 1952 caused thousands of deaths in a few days. photochemical smog needs the opposite conditions: warm, sunny, dry weather and heavy traffic. Sunlight acts on nitrogen oxides and hydrocarbons from exhausts to produce ozone and other irritants such as peroxyacetyl nitrate (PAN); this oxidising brown haze was first studied in Los Angeles in the 1940s. A temperature inversion makes either kind worse. Normally air cools with height, so warm polluted air near the ground rises and spreads. When a layer of warmer air sits above cooler surface air, as on long winter nights, it acts like a lid and keeps pollution trapped close to where people breathe.

Classical and photochemical smog compared
Feature	Classical (sulphurous) smog	Photochemical smog
Weather	Cold, damp, foggy	Warm, sunny, dry
Main ingredients	Coal smoke and sulphur dioxide	Nitrogen oxides and hydrocarbons from traffic
Chemical nature	Reducing	Oxidising
Key products	Sulphuric acid droplets, soot	Ozone, PAN, aldehydes
Classic case	London, December 1952	Los Angeles, 1940s

சாதாரண மழையும் சற்றுப் புளிப்பாக இருக்கும்; மாசு கலந்தால் இன்னும் புளிப்பாகி அமில மழை ஆகிறது. நிலக்கரி, வாகனப் புகை வாயுக்கள் காற்றில் அமிலங்களாக மாறி நூற்றுக்கணக்கான கிலோமீட்டர் பயணிக்கின்றன. குளங்களில் மீன்கள் சாகும், இலைகள் கெடும். பளிங்கின் மேல் எலுமிச்சைச் சாறு ஊற்றினால் குமிழி வருவது போல, அமிலம் தாஜ்மஹால் கல்லைத் தின்கிறது; அதனால் 1996-இல் நீதிமன்றம் அதைச் சுற்றி தொழிற்சாலைகளை மாற்றச் சொன்னது. (→ English: acid rain; pH 5.6; Taj Trapezium Zone · हिन्दी: अम्लीय वर्षा; pH 5.6; ताज ट्रेपेज़ियम ज़ोन)

In exam Englishacid rain is rain, snow, fog or dry deposits made acidic by pollution. Clean rain is already slightly acidic, at a pH of about 5.6, because carbon dioxide dissolves in it; rain is called acid rain when its pH falls below that. Sulphur dioxide and nitrogen oxides, rising from power plants, smelters and traffic, change in the air into sulphuric and nitric acids, and winds can carry them hundreds of kilometres before they fall. It kills fish in lakes, harms leaves and eats into metal and stone. Marble is especially exposed, because the acid reacts with its calcium carbonate. Concern for the yellowing marble of the Taj Mahal led the Supreme Court in 1996 to act in the Taj Trapezium Zone, an area of about 10,400 sq km around the monument where polluting coal-based industries were ordered to shift to natural gas or relocate.

காற்று தூய்மையா என்று அளக்க 2009-இல் 12 மாசுபடுத்திகளுக்கு வரம்புகள் வைத்தார்கள், தேர்வுக்குத் தேர்ச்சி மதிப்பெண் வைப்பது போல. PM2.5-க்கு ஆண்டுச் சராசரி 40, ஒரு நாளுக்கு 60; PM10-க்கு 60, 100. உலக சுகாதார நிறுவனத்தின் அறிவுரை இன்னும் கடுமை: ஆண்டுக்கு 5 மட்டும், ஆனால் அது அறிவுரை, சட்டம் அல்ல. மீண்டும் மீண்டும் தோற்கும் நகரத்தை இலக்கை அடையாத நகரம் என்பார்கள். (→ English: National Ambient Air Quality Standards; non-attainment city; WHO air quality guidelines · हिन्दी: राष्ट्रीय परिवेशी वायु गुणवत्ता मानक; गैर-प्राप्ति नगर; WHO वायु गुणवत्ता दिशानिर्देश)

In exam EnglishIndia judges clean air by the National Ambient Air Quality Standards, notified by the Central Pollution Control Board in November 2009 under the Air Act. They set limits for 12 pollutants: sulphur dioxide, nitrogen dioxide, PM10, PM2.5, ozone, lead, carbon monoxide, ammonia, benzene, benzo(a)pyrene, arsenic and nickel. The limits for particles are the ones most often asked: for PM2.5, 40 micrograms per cubic metre as an annual average and 60 over 24 hours; for PM10, 60 annually and 100 over 24 hours. These are far looser than the World Health Organization's air quality guidelines of 2021, which advise only 5 micrograms per cubic metre of PM2.5 as an annual average. The guidelines are advice, not law. The CPCB and state boards monitor the air, and a city that keeps failing the standards is called a non-attainment city.

மைக்ரோகிராம் சாதாரண மக்களுக்குப் புரியாது, அதனால் 2014-இல் காற்றுத் தரக் குறியீடு: ஒரு எண், ஒரு நிறம். எட்டு மாசுபடுத்திகளில் எது மிக மோசமோ அதன் எண்ணே குறியீடு, வகுப்பில் குறைந்த மதிப்பெண் பாடமே முடிவைத் தீர்மானிப்பது போல. குறைந்தது மூன்றை அளக்க வேண்டும், அதில் ஒன்று PM. கார்பன் மோனாக்சைடு, ஓசோனுக்கு 8 மணி நேரச் சராசரி, மற்றவற்றுக்கு 24 மணி. 0 முதல் 500 வரை ஆறு நிறப் படிகள். (→ English: Air Quality Index; sub-index; averaging period · हिन्दी: वायु गुणवत्ता सूचकांक; उप-सूचकांक; औसत अवधि)

In exam EnglishStandards in micrograms mean little to the public, so in October 2014 the government launched the National Air Quality Index, with the slogan 'one number, one colour, one description'. It is built from up to eight pollutants: PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. Each pollutant's reading is turned into a sub-index, and the highest sub-index becomes the AQI for that place, so the worst pollutant decides the result rather than an average. An AQI is reported only when at least three pollutants are measured, one of which must be PM10 or PM2.5. Most pollutants are averaged over 24 hours, but carbon monoxide and ozone over 8 hours, since they rise and fall quickly. The scale runs from 0 to 500 in six bands, each with a colour and a health message, shown in the table below.

National Air Quality Index categories
AQI range	Category	Likely health effect
0–50	Good	Minimal impact
51–100	Satisfactory	Minor breathing discomfort to sensitive people
101–200	Moderately polluted	Discomfort to people with lung or heart disease, children and older adults
201–300	Poor	Breathing discomfort to most people on long exposure
301–400	Very poor	Respiratory illness on long exposure
401–500	Severe	Affects healthy people; serious for those with disease

2019-இல் தேசிய தூய காற்றுத் திட்டம் வந்தது: சுமார் 130 நகரங்கள் ஒவ்வொன்றும் தன் சொந்தத் திட்டம் எழுத வேண்டும், ஒவ்வொரு வீட்டுக்கும் தனி பட்ஜெட் போல. முதலில் 2024-க்குள் 20-30 சதவீதக் குறைப்பு இலக்கு, பிறகு 2025-26-க்குள் 40 சதவீதம் வரை. முடிவு காட்டினால்தான் பணம். இது திட்டம் மட்டுமே, சட்டம் அல்ல. (→ English: National Clean Air Programme; city action plan; non-attainment city · हिन्दी: राष्ट्रीय स्वच्छ वायु कार्यक्रम; नगर कार्य योजना; गैर-प्राप्ति नगर)

In exam EnglishThe National Clean Air Programme, launched by the Ministry of Environment, Forest and Climate Change in January 2019, was India's first national, time-bound plan for clean air. It covers the non-attainment cities and a few others, about 130 in all, each of which must prepare and carry out its own city action plan covering roads, dust, waste burning, industry and vehicles. Its first goal was a 20 to 30 per cent cut in particulate concentrations by 2024 against 2017 levels; the target was later revised to up to 40 per cent by 2025-26. Funds are tied to measured progress in each city. NCAP is a programme, not a law, so it relies on the existing powers of the Air Act and the pollution control boards for enforcement.

டெல்லிப் பிரச்சினை மாநில எல்லைகளைத் தாண்டுகிறது, அதனால் சிறப்பு ஏற்பாடு. படிநிலைத் திட்டம் என்றால் காய்ச்சல் ஏற ஏற மருந்தின் அளவை ஏற்றுவது போல: குறியீடு 'மோசம்' என்றால் முதல் நிலை, 'கடுமை கூடுதல்' என்றால் நான்காம் நிலை; கட்டுமானம் நிறுத்தம், பழைய டீசல் வாகனங்கள் தடை, பள்ளிகள் மூடல். 2021 முதல் இதை ஒரு சட்டப்பூர்வ ஆணையம் நடத்துகிறது. வைக்கோலுக்கு, நிற்கும் அடிக்கட்டைகளுக்கு நடுவே கோதுமை விதைக்கும் இயந்திரங்கள், வைக்கோலை மக்கச் செய்யும் தெளிப்பு. (→ English: Graded Response Action Plan; Commission for Air Quality Management; crop-residue management · हिन्दी: श्रेणीबद्ध प्रतिक्रिया कार्य योजना; वायु गुणवत्ता प्रबंधन आयोग; पराली प्रबंधन)

In exam EnglishDelhi and its region get special machinery because the problem crosses state borders. The Graded Response Action Plan, first notified in January 2017, lists measures that switch on automatically as air quality worsens. In its revised form it has four stages tied to the AQI: Stage I at 'poor' (201 to 300), Stage II at 'very poor' (301 to 400), Stage III at 'severe' (401 to 450) and Stage IV at 'severe plus' (above 450). Early stages control dust, open burning and diesel generators; later ones halt construction, restrict older diesel vehicles and trucks entering Delhi, and may close schools. Since 2021 the plan has been run by the Commission for Air Quality Management in National Capital Region and Adjoining Areas, a statutory body created by its own Act that replaced the earlier Environment Pollution (Prevention and Control) Authority. For crop residue, the answers include machines that sow wheat through standing stubble and microbial sprays that decompose straw in the field.

1998-இல் நீதிமன்றம் டெல்லிப் பேருந்துகள், ஆட்டோக்கள் சிஎன்ஜிக்கு மாற வேண்டும் என்றது. புதிய வாகனங்களுக்கு பாரத் ஸ்டேஜ் விதிகள்: நிலை ஏற ஏறப் புகை குறைய வேண்டும், வகுப்பு ஏற ஏறப் பாடம் கடினமாவது போல. நாம் BS-V-ஐத் தாண்டி நேரடியாக BS-VI-க்குத் தாவினோம், 1 ஏப்ரல் 2020 முதல். பெட்ரோலில் எத்தனால் கலப்பு இலக்கு 2025-26-க்கு 20 சதவீதம். வீட்டுப் புகையைக் குறைக்க உஜ்வலா எரிவாயு இணைப்புகள். (→ English: compressed natural gas; Bharat Stage VI; ethanol blending; Pradhan Mantri Ujjwala Yojana · हिन्दी: संपीडित प्राकृतिक गैस; भारत स्टेज VI; एथेनॉल मिश्रण; प्रधानमंत्री उज्ज्वला योजना)

In exam EnglishVehicles and fuels show how courts and policy pushed change. In July 1998 the Supreme Court ordered Delhi's buses, taxis and auto-rickshaws to move to compressed natural gas. Emission limits for new vehicles follow the Bharat Stage norms, modelled on European standards; the higher the stage, the less sulphur in fuel and the lower the allowed emissions. India decided to skip BS-V and move straight from BS-IV to BS-VI, and in 2018 the Supreme Court held that no BS-IV vehicle could be sold from 1 April 2020, the date BS-VI took effect across the country. Blending ethanol made from sugarcane and grain into petrol cuts imports and some emissions; the National Policy on Biofuels, as amended in 2022, set 20 per cent blending as the target for 2025-26. Indoors, the Pradhan Mantri Ujjwala Yojana, launched on 1 May 2016, gives poor households LPG connections in place of smoky solid fuels.

இந்தப் பகுதி ஒரு சங்கிலி: மூலத்திலிருந்து மாசு, காற்றில் புதிய மாசு, வானிலை அதைத் தங்க வைப்பது, பிறகு தீங்கு. தீர்வு அதே சங்கிலி பின்னோக்கி: தரநிலைகள், குறியீடு, நகரத் திட்டம், டெல்லி அவசர நிலைகள், தூய எரிபொருள். சமையலறையில் புகை வந்தால் முதலில் அடுப்பைப் பார்ப்போம், பிறகு ஜன்னலைத் திறப்போம்; அதுபோல மூலமும் சூழலும் இரண்டும் முக்கியம். (→ English: source; standard; response · हिन्दी: स्रोत; मानक; प्रतिक्रिया)

In exam EnglishThe pieces of this part fit into one chain. Sources, from exhausts and chimneys to stubble fires and kitchen stoves, release primary pollutants; sunlight and chemistry turn some of them into secondary ones such as ozone and part of PM2.5; weather, especially winter inversions, decides how long they stay near the ground; and their effects range from damaged lungs to acid rain on marble. The response follows the same chain in reverse. The 2009 standards define what clean air legally means, the AQI translates readings for the public, NCAP sets city-level targets, GRAP and the Commission for Air Quality Management act in emergencies around Delhi, and cleaner fuels and vehicle norms cut emissions at the source. The next part applies the same approach to water, soil and noise.

Exam traps

Nobody's chimney or exhaust pipe releases ground-level ozone; sunlight builds it in the air out of nitrogen oxides and hydrocarbons, which makes it a secondary pollutant. Chemically it is the very O3 of the protective stratospheric layer, yet near the ground it does harm.
PM2.5 means particles of 2.5 micrometres or less, not exactly 2.5; PM10 includes PM2.5 within it.
Classical smog is a cold, damp, reducing smog from coal; photochemical smog is a warm, sunny, oxidising smog from traffic. Questions often swap their conditions.
Rain is called acid rain only below a pH of about 5.6, not below 7: normal rain is already slightly acidic.
The NAAQS (2009) cover 12 pollutants; the National AQI (2014) uses up to eight. Do not mix the two numbers.
The AQI is the highest sub-index, not the average; at least three pollutants are needed, one of them PM10 or PM2.5; CO and ozone use 8-hour averages.
NCAP (2019) is a programme with targets, not a statute; the Commission for Air Quality Management (2021) is a statutory body created by its own Act and now runs GRAP.
India skipped BS-V and moved from BS-IV to BS-VI on 1 April 2020; the WHO guideline of 5 micrograms per cubic metre for annual PM2.5 is advice, not India's legal standard (40).

Recap

Primary pollutants are emitted directly (SO2, CO); secondary ones form in the air (ground-level ozone, part of PM2.5).
PM10 lodges in the upper airways; PM2.5 reaches deep into the lungs and blood; burning is the main source of fine particles.
SO2 from coal, NOx from hot combustion, CO binds haemoglobin, lead came from leaded petrol (phased out by 2000).
Winter peak in north India: stubble burning, calm air and temperature inversion trap pollution near the ground.
Classical smog: cold, coal, reducing (London 1952); photochemical smog: sunny, traffic, oxidising (Los Angeles).
Acid rain: pH below 5.6 from SO2 and NOx; Taj Trapezium Zone (Supreme Court, 1996).
NAAQS 2009: 12 pollutants; PM2.5 limits 40 (annual) and 60 (24-hour); WHO 2021 guideline 5 (annual).
AQI 2014: eight pollutants, six categories from 0 to 500, worst sub-index decides.
NCAP 2019: about 130 cities, target up to 40 per cent cut by 2025-26; GRAP 2017, four stages, run by CAQM (2021).
CNG for Delhi (1998), BS-VI from 1 April 2020, 20 per cent ethanol target for 2025-26, Ujjwala (2016).

Practice questions

Q1. Which one of the following is a secondary air pollutant?

Carbon monoxide
Sulphur dioxide
Ground-level ozone
Lead

Show answer

Answer: C. Ground-level ozone

Ground-level ozone is not emitted directly; it forms when sunlight acts on nitrogen oxides and hydrocarbons. Carbon monoxide comes straight out of exhausts, sulphur dioxide straight out of coal-burning chimneys, and lead was released directly from leaded petrol, so all three are primary pollutants.

Q2. Consider the following statements about photochemical smog:
1. It forms most readily in cold, damp and foggy weather.
2. Ozone and peroxyacetyl nitrate (PAN) are among its products.
Which of the statements given above is/are correct?

1 only
2 only
Both 1 and 2
Neither 1 nor 2

Show answer

Answer: B. 2 only

Statement 2 is correct. Statement 1 is wrong: cold, damp, foggy weather suits classical (sulphurous) smog, while photochemical smog needs warm sunshine. Options 1 and 3 include statement 1 and fail; option 4 fails because statement 2 is true.

Q3. Rain is generally described as 'acid rain' when its pH falls below about:

Show answer

Answer: C. 5.6

Clean rain is already slightly acidic, at about pH 5.6, because carbon dioxide dissolves in it; acid rain is rain below that level. A pH of 7 is neutral, so it would wrongly make all normal rain acid rain; 6.5 has no special meaning here; and 3.0 is an extreme value seen only in badly polluted places, not the threshold.

Q4. Consider the following statements about the National Air Quality Index:
1. The AQI of a place is the average of the sub-indices of all pollutants measured.
2. Carbon monoxide and ozone are assessed on 8-hour averages.
3. At least one of PM10 and PM2.5 must be among the pollutants measured.
Which of the statements given above are correct?

1 and 2 only
2 and 3 only
1 and 3 only
1, 2 and 3

Show answer

Answer: B. 2 and 3 only

Statements 2 and 3 are correct. Statement 1 is wrong: the AQI is the highest sub-index, so the worst pollutant decides it. Options 1, 3 and 4 all include statement 1 and are therefore wrong.

Q5. Under the National Ambient Air Quality Standards, 2009, the annual average limit for PM2.5 is:

5 micrograms per cubic metre
40 micrograms per cubic metre
60 micrograms per cubic metre
100 micrograms per cubic metre

Show answer

Answer: B. 40 micrograms per cubic metre

India's annual PM2.5 limit is 40. Five is the WHO's 2021 guideline, which is advice rather than India's standard; 60 is the 24-hour limit for PM2.5 (and the annual limit for PM10); 100 is the 24-hour limit for PM10.

Q6. Match List I with List II:
List I
A. Graded Response Action Plan
B. National Clean Air Programme
C. National Air Quality Index
D. Commission for Air Quality Management
List II
1. Launched in 2014 to report air quality as one number and colour
2. Statutory body for Delhi and NCR created in 2021
3. Stage-wise emergency measures for Delhi and NCR, first notified in 2017
4. National time-bound programme for cities, launched in 2019
Select the correct answer using the code given below:

A-3, B-4, C-1, D-2
A-4, B-3, C-1, D-2
A-3, B-4, C-2, D-1
A-2, B-4, C-1, D-3

Show answer

Answer: A. A-3, B-4, C-1, D-2

GRAP is the staged emergency plan (2017), NCAP the city programme (2019), the AQI the one-number index (2014) and CAQM the statutory commission (2021). Option 2 swaps GRAP and NCAP; option 3 swaps the AQI and CAQM; option 4 swaps GRAP and CAQM.

Q7. Consider the following statements:
1. India moved directly from Bharat Stage IV to Bharat Stage VI emission norms, skipping Bharat Stage V.
2. The Supreme Court ordered Delhi's public transport to shift to compressed natural gas in 1998.
Which of the statements given above is/are correct?

1 only
2 only
Both 1 and 2
Neither 1 nor 2

Show answer

Answer: C. Both 1 and 2

Both statements are correct: BS-VI replaced BS-IV on 1 April 2020 with no BS-V stage, and the CNG order came in July 1998. Options 1 and 2 leave out a true statement, and option 4 rejects both.

Q8. Smog in the cities of the north Indian plains is usually worst in early winter mainly because:

photochemical reactions are strongest in winter sunshine
calm winds and temperature inversion trap pollutants near the ground while crop-residue smoke adds to them
acid rain is most frequent in winter
vehicles are not used in the monsoon season

Show answer

Answer: B. calm winds and temperature inversion trap pollutants near the ground while crop-residue smoke adds to them

Calm air and inversions keep pollution close to the ground, and stubble smoke from October and November adds to it. Photochemical smog is a warm, sunny-weather problem, not a winter one; acid rain does not create smog; and vehicles run all year, so the monsoon difference comes from weather, not from fewer vehicles.

All parts Next: Part 2 →

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Sources

Central Pollution Control Board: National Ambient Air Quality Standards, notified 18 November 2009; National Air Quality Index (launched 17 October 2014) — https://cpcb.nic.in
Ministry of Environment, Forest and Climate Change: National Clean Air Programme (January 2019); Graded Response Action Plan for Delhi and NCR (notified January 2017, revised by the Commission for Air Quality Management in 2022) — https://moef.gov.in
The Commission for Air Quality Management in National Capital Region and Adjoining Areas Act, 2021; the Air (Prevention and Control of Pollution) Act, 1981 — https://www.indiacode.nic.in
Supreme Court of India: M. C. Mehta v. Union of India (Taj Trapezium, judgment of 30 December 1996); M. C. Mehta v. Union of India (CNG for Delhi's public transport, order of 28 July 1998); M. C. Mehta v. Union of India (BS-IV vehicles not to be sold from 1 April 2020, order of 24 October 2018) — https://main.sci.gov.in
United States Environmental Protection Agency: particulate matter, ground-level ozone and acid rain basics — https://www.epa.gov
World Health Organization: WHO global air quality guidelines (22 September 2021) — https://www.who.int
Press Information Bureau: Pradhan Mantri Ujjwala Yojana (launched 1 May 2016); National Policy on Biofuels as amended in 2022 (20 per cent ethanol blending target for 2025-26); Bharat Stage VI emission norms — https://pib.gov.in