UPSC Prelims 2024 Q90: Largest Source of Sulphur Dioxide — Explained
According to the Environmental Protection Agency (EPA), which one of the following is the largest source of sulphur dioxide emissions?
- Locomotives using fossil fuels
- Ships using fossil fuels
- Extraction of metals from ores
- Power plants using fossil fuels
Answer: (d) Power plants using fossil fuels
Statement-wise explanation
- a: incorrect — The US EPA lists locomotives among the smaller sources of SO2, together with ships, other vehicles and heavy equipment that burn high-sulphur fuel. Locomotives burn far less fuel than the fleet of coal- and oil-fired generating units. Rail diesel has also moved to low-sulphur grades, which cuts their SO2 further.
- b: incorrect — The EPA puts ships in its list of smaller SO2 sources. Heavy marine fuel oil does carry a lot of sulphur, which is why the IMO cut the global limit for sulphur in ship fuel from 3.50% to 0.50% from 1 January 2020. Even so, total SO2 from shipping is smaller than from fossil-fuel power plants.
- c: incorrect — The EPA names 'extracting metal from ore' as an industrial process that is a smaller source of SO2. Roasting and smelting sulphide ores of copper, zinc and lead releases SO2. However, these sources are few and localised, and smelters often capture the gas to make sulphuric acid.
- d: correct — The EPA's Sulfur Dioxide Basics page says: 'The largest source of SO2 in the atmosphere is the burning of fossil fuels by power plants and other industrial facilities.' Coal contains sulphur, and burning it oxidises that sulphur to SO2. Power plants burn very large amounts of coal, so they lead all sources. In India, coal-based capacity supplied about 69.5% of electricity in April–June 2026.
How to eliminate
Start with scale. SO2 comes from burning sulphur-containing fuel, so the biggest emitter is the sector that burns the most coal and oil. That sector is electricity generation, and it far exceeds rail, shipping or metal smelting. Locomotives are the easiest to eliminate, because rail moves to electric traction and low-sulphur diesel. Ships and smelters are the real distractors, because both are known as high-sulphur activities. Ship fuel was historically very sulphur-rich, and sulphide ores release SO2 when roasted. Still, each is a niche source next to power generation. The question also names the EPA as its source. The EPA's own wording is the fact to remember: power plants and other industrial facilities burning fossil fuels are the largest source; metal extraction, volcanoes, locomotives, ships and heavy equipment are 'smaller sources'.
Know more
Sulphur dioxide (SO2) is a colourless gas with a sharp smell. It forms when sulphur in a fuel or ore is oxidised. Coal usually contains about 0.5–5% sulphur, so burning coal is the main route by which humans emit SO2.
Sources, as the US EPA ranks them
| Rank | Source | Why it emits SO2 |
|---|---|---|
| Largest | Fossil-fuel power plants and other industrial facilities | Burn huge volumes of sulphur-bearing coal and oil |
| Smaller | Metal extraction from ore | Roasting and smelting sulphide ores (Cu, Zn, Pb) |
| Smaller | Natural sources | Volcanoes |
| Smaller | Locomotives, ships, heavy equipment | Burn high-sulphur fuel |
Effects
- Health: short-term exposure irritates the airways and makes breathing difficult. Children with asthma are most at risk.
- Secondary particles: SO2 turns into sulphate aerosols that add to PM2.5 and haze.
- Acid rain: SO2 oxidises to sulphuric acid, which harms forests, lakes and stone monuments. The 'marble cancer' of the Taj Mahal is a classic example.
Control in India
- The National Ambient Air Quality Standards (2009) cap SO2 at 50 µg/m³ as an annual average and 80 µg/m³ as a 24-hour average.
- In December 2015, MoEF&CC notified SO2 emission limits for coal-based thermal power plants. Most plants must install Flue Gas Desulphurisation (FGD) units to meet them.
- FGD scrubs SO2 from flue gas, usually with limestone, and produces gypsum as a by-product.
- The deadlines have been extended several times. CEA monitors the progress of FGD installation and reports it to CPCB.
- For vehicles, BS-VI fuel limits sulphur to 10 ppm.
Remember: SO2 is mainly a fuel-sulphur problem, and nitrogen oxides (NOx) are mainly a combustion-temperature problem. Questions often mix up these two.
NCERT Concepts to Revise
- Air pollution: Every option is a possible source of an air pollutant, so the question asks you to rank sources of one gas within air pollution.