Air Pollution, AQI and Acid Rain in India

A real question on this chapter rarely settles for "what causes air pollution". It tests something narrower: how many pollutants actually feed into CPCB's own Air Quality Index and which six bands divide it, why sulphur dioxide and nitrogen oxides become acid rain through a specific atmospheric reaction rather than just "falling as pollution", or which scheme funds the machinery meant to stop Punjab and Haryana's farmers from burning their fields every October and November. This note works through four clusters examiners actually draw from: the real sources behind everyday ambient air pollution, the National AQI's verified methodology, the atmospheric chemistry that produces acid rain, and the specific policy response to crop and biomass residue burning, each anchored to a number or document rather than a general description.

Water, Soil and Chemical Pollution in India covers thermal power plant emission standards in detail, MoEFCC's 2015 particulate matter, sulphur dioxide, nitrogen oxide and mercury limits by plant vintage. This note covers ambient air pollution and the Air Quality Index instead, the pollution an aspirant actually breathes in a city street rather than what a plant's stack is permitted to release.

Sources of ambient air pollution

Ambient air pollution in an Indian city is rarely one source dominating every season; it is a mix that shifts with the calendar, and the exam tests whether a candidate can name the mix rather than pick one villain. Vehicular emissions are the most consistently tested source: India moved directly from Bharat Stage IV to Bharat Stage VI fuel and emission norms on 1 April 2020, skipping BS-V entirely, cutting permitted sulphur content in fuel from 50 ppm to 10 ppm, a five-fold reduction aimed specifically at particulate and sulphur dioxide output from vehicles.

Industrial sources and construction and road dust complete the mix, and their relative weight is genuinely seasonal rather than fixed. IIT Kanpur's 2016 source-apportionment study for Delhi, commissioned by the Delhi Pollution Control Committee, found that annual PM10 emissions were dominated by road dust (about 56%), followed by concrete batching and industrial point sources (about 10% each) and vehicles (about 9%). For PM2.5 in winter, the same study found a different ranking: secondary particles, formed when gases like sulphur dioxide and nitrogen oxides react in the atmosphere rather than being emitted as particles directly, contributed roughly 25-30%, vehicles roughly 20-25%, and biomass burning roughly 17-26%. The exam-relevant point is the reversal itself: road dust dominates the coarser PM10 fraction across the year, while combustion-linked secondary particles and vehicles dominate the finer, more health-damaging PM2.5 fraction in winter, and a question describing "the" source of Delhi's air pollution without specifying which pollutant and which season is incomplete by construction.

India's institutional response to ambient pollution is the National Clean Air Programme (NCAP), launched by the Ministry of Environment, Forest and Climate Change in January 2019, with CPCB as the nodal implementing agency. NCAP covers 131 non-attainment cities, cities that failed to meet the National Ambient Air Quality Standards (NAAQS) during 2011-2015 under the National Air Quality Monitoring Programme. Its original target was a 20-30% reduction in particulate matter concentration by 2024, against a 2017-18 baseline; this was revised in 2022 to a tighter goal of up to 40% reduction, or meeting the PM10 NAAQS of 60 micrograms per cubic metre, by 2025-26. A question testing whether NCAP's target changed, and in which direction, is testing whether a candidate has the current, revised figure rather than the original 2019 one.

The National Air Quality Index

The National Air Quality Index (AQI) was launched by Prime Minister Narendra Modi in New Delhi on 6 April 2015, developed by IIT Kanpur under a study sponsored by CPCB, and covered ten cities at launch: Delhi, Agra, Kanpur, Faridabad, Lucknow, Varanasi, Ahmedabad, Chennai, Bangalore and Hyderabad. Its stated design philosophy, "One Number, One Colour, One Description", is itself a testable phrase: the index collapses several pollutants into a single figure a layperson can act on, rather than requiring the public to interpret raw concentration data.

The AQI is built on eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3 and Pb, specifically the pollutants for which a short-term (up to 24-hour) NAAQS exists. This is a precise and commonly missed distinction: India's full NAAQS list, last revised in 2009, actually covers twelve parameters, adding benzene, benzo(a)pyrene, arsenic and nickel, but these four carry only annual-average standards, so they cannot generate a rolling 24-hour sub-index and are excluded from the AQI. A statement equating "the eight AQI pollutants" with "all NAAQS parameters" is a trap.

For each of the eight pollutants, CPCB computes a sub-index using a piecewise linear function of measured concentration; the overall AQI for a location is the maximum of the individual sub-indices, not an average. This "worst pollutant" rule matters for interpretation: a city's reported AQI number is typically being driven by a single pollutant at a time (commonly PM2.5 or PM10 in winter, ozone in summer), which is why CPCB's own bulletins list a "prominent pollutant" alongside the number.

There are six AQI categories, and CPCB attaches a specific, verified health-impact statement to each:

• Good (0-50): minimal impact
• Satisfactory (51-100): minor breathing discomfort to sensitive people
• Moderate (101-200): breathing discomfort to people with lung disease, asthma and 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 disease

The exam trap here is usually sequencing or wording: swapping which category gets "breathing discomfort to most people" versus "respiratory illness", or assuming the scale runs to some other maximum instead of 500. Categories above 500 are not part of the defined scale; extreme readings are reported as an index value beyond 500 rather than a seventh category.

Acid rain: the atmospheric chemistry

Ordinary rain is already mildly acidic, with a pH of about 5.6, because atmospheric carbon dioxide dissolves into falling water droplets and forms weak carbonic acid. Acid rain is the sharper acidity that results when two additional gases enter the same process: sulphur dioxide (SO2) and nitrogen oxides (NOx), once emitted into the atmosphere and carried by wind and air currents, react with water, oxygen and other atmospheric chemicals to form sulphuric acid (H2SO4) and nitric acid (HNO3). These stronger acids then reach the ground as wet deposition (dissolved in rain, snow or fog) or, when the gases and particles settle directly and react with surface moisture later, as dry deposition. Rain carrying this additional acid load typically measures a pH of around 4.2 to 4.4, distinctly more acidic than unpolluted rainfall.

The exam-relevant distinction is which gas indicts which polluter: sulphur dioxide overwhelmingly points to coal combustion and industrial sources (this is the same SO2 the sibling note's thermal power emission standards are built to cap), while nitrogen oxides point more heavily toward combustion in vehicle engines and other high-temperature processes. A single acid rain event is usually the product of both gases together, but a question naming one specific gas is testing whether a candidate can trace it back to its more likely source rather than treating "acid rain" as one undifferentiated phenomenon.

Acid rain's effects are tested at two levels. In soil and freshwater systems, it lowers pH directly, harming pH-sensitive aquatic life such as amphibians and fish, while also leaching essential nutrients like calcium and magnesium out of soil and mobilising aluminium, which is otherwise chemically bound and relatively harmless but becomes toxic to plant roots once dissolved. On built structures, acid rain attacks calcium carbonate, the mineral in limestone and marble: the acid converts calcium carbonate into gypsum, a softer, more soluble compound that flakes and washes away, a mechanism widely cited in explaining the discolouration and pitting of marble monuments, including the Taj Mahal, which sits within the industrial and vehicular SO2 plume of the Mathura-Agra belt.

Crop and biomass residue burning

Punjab and Haryana's paddy-wheat cropping cycle creates the physical conditions for this problem every year. Paddy is harvested mechanically by combine harvesters through October, and wheat needs to be sown soon after; multiple agricultural studies put the actual gap available to farmers at only roughly 10 to 15 days. A combine harvester also leaves a taller standing stubble than manual harvesting does, stubble that blocks direct-drill sowing of the next crop unless it is cleared first. Given this narrow window, the labour and machinery cost of removing that stubble by other means, and a shortage of harvest-season labour, burning the field remains the fastest and cheapest way to clear it, which is why the fires cluster so tightly into the same six-to-eight week period each year.

The scale of the resulting impact on Delhi's air is real but genuinely variable year to year and is tracked in near-real time by SAFAR-India (System of Air Quality and Weather Forecasting and Research), run by the Ministry of Earth Sciences through the Indian Institute of Tropical Meteorology, Pune. On its worst tracked days, the effect has been large: SAFAR attributed 44% of Delhi's PM2.5 on 1 November 2019 to crop residue burning, illustrating how a source that contributes comparatively little across the full year can still dominate the city's air on specific days when wind direction and low mixing height combine to trap the smoke over Delhi-NCR. Because this share moves with weather and the extent of burning each season, it is not a fixed constant, and a question quoting one year's peak-day figure as if it applied every year is worth treating with caution.

The government's standing response is the Crop Residue Management (CRM) Scheme, formally "Promotion of Agricultural Mechanisation for In-Situ Management of Crop Residue in the States of Punjab, Haryana, Uttar Pradesh and the NCT of Delhi", run by the Ministry of Agriculture and Farmers Welfare since 2018-19. It subsidises crop residue management machinery, including the Happy Seeder, Super Seeder, Zero-Till Drill and Rotavator, which let wheat be sown directly into retained paddy stubble (in-situ management) instead of removing it first, and it funds Custom Hiring Centres so smallholders can rent this machinery rather than buy it outright. Punjab alone received over Rs 1,387.6 crore between 2018 and 2022 under the scheme and had deployed roughly 1.17 lakh CRM machines by 2023. Guidelines were revised again to also strengthen ex-situ management, building supply chains that collect paddy straw for use as fuel in biomass power and compressed biogas plants rather than only supporting on-field incorporation. The exam trap: in-situ and ex-situ management are two strategies funded within the same scheme, not two competing or separately named programmes.

Exam angle

This chapter is tested on precision, not general awareness, because almost every plausible wrong answer swaps one verified figure for a similar, close-sounding one. A question may claim the AQI tracks "all NAAQS pollutants" when it actually covers only the eight with a short-term standard, leaving out benzene, benzo(a)pyrene, arsenic and nickel, which have annual-only standards. It may quote the AQI as an average of pollutant sub-indices rather than the maximum, when CPCB's own "worst pollutant" methodology is the opposite of averaging. It may attribute a Delhi source-apportionment finding to the wrong pollutant or season, since road dust dominates PM10 across the year while secondary particles and vehicles dominate winter PM2.5. On acid rain, the trap is usually mixing up which gas, sulphur dioxide or nitrogen oxides, points to which polluter, or forgetting that even unpolluted rain is already mildly acidic at pH 5.6, so "acid rain" is a question of degree, not the mere presence of acid. On stubble burning, the trap is treating a single year's peak-day contribution figure as a fixed annual constant, or confusing in-situ and ex-situ management as separate schemes rather than two strategies inside the same CRM Scheme.

Quick revision points

• NCAP: launched January 2019, CPCB as nodal agency, 131 non-attainment cities (NAAQS exceeded 2011-2015). Revised 2022 target: up to 40% PM reduction, or meeting the 60 micrograms/m3 PM10 NAAQS, by 2025-26 (original target was 20-30% by 2024 against a 2017-18 baseline).
• BS-VI: effective 1 April 2020 nationwide, skipped BS-V, cut fuel sulphur from 50 ppm to 10 ppm.
• National AQI: launched 6 April 2015 (PM Modi, New Delhi), developed by IIT Kanpur for CPCB, initially 10 cities. Tracks 8 pollutants (PM10, PM2.5, NO2, SO2, CO, O3, NH3, Pb) out of NAAQS's full 12 parameters (excludes benzene, benzo(a)pyrene, arsenic, nickel, which are annual-only standards). Overall AQI = maximum, not average, of each pollutant's sub-index.
• Six AQI categories: Good (0-50, minimal impact), Satisfactory (51-100, minor discomfort to sensitive people), Moderate (101-200, discomfort to people with lung/heart disease and asthma), Poor (201-300, discomfort to most people on prolonged exposure), Very Poor (301-400, respiratory illness on prolonged exposure), Severe (401-500, affects healthy people, serious impact on those with existing disease).
• Acid rain: normal rain pH about 5.6 (carbonic acid from CO2); acid rain roughly pH 4.2-4.4. SO2 forms sulphuric acid (industrial/coal source), NOx forms nitric acid (combustion/vehicular source). Corrodes marble/limestone by converting calcium carbonate to gypsum.
• Stubble burning: roughly 10-15 day gap between paddy harvest and wheat sowing in Punjab/Haryana drives burning. SAFAR-India (Ministry of Earth Sciences, IITM Pune) tracks Delhi's daily contribution; it hit 44% of Delhi's PM2.5 on 1 November 2019. CRM Scheme (2018-19, Ministry of Agriculture) funds Happy Seeder/Super Seeder/Rotavator and Custom Hiring Centres for in-situ management, plus ex-situ straw supply chains; Punjab alone got over Rs 1,387.6 crore (2018-22).

Try a few questions now to see which specific number this chapter is quietly testing on you.