Major Sources of Sulphur Dioxide: Natural, Industrial and Agricultural Emissions (2026)

Sulphur dioxide (SO2) occupies a paradoxical position in modern industrial chemistry: it is one of the most significant air pollutants in terms of human health and environmental damage, yet it is also an important industrial feedstock with legitimate and beneficial uses in food preservation, sulphuric acid manufacture, and specialty chemical production. Understanding both faces of SO2 — as a pollution source and as an industrial commodity — is essential for industry professionals, environmental managers, and anyone concerned with air quality and industrial chemistry in India.

This guide covers every major source of SO2 emissions (natural and anthropogenic), the health and environmental impacts of exposure, India’s specific emission profile and regulatory landscape, and the industrial applications that make SO2 a commercially important chemical. Ammoniagas supplies industrial-grade sulphur dioxide to customers requiring it for legitimate industrial applications across India.

1. What Is Sulphur Dioxide?

Sulphur dioxide is a colourless gas with the chemical formula SO2 — one sulphur atom bonded to two oxygen atoms. It has a molecular weight of 64.06 g/mol, a boiling point of -10°C (meaning it is a gas at ambient temperature but readily liquefied under modest pressure for storage and transport), and a sharp, pungent, suffocating odour detectable at concentrations as low as 0.5 ppm.

SO2 is produced whenever sulphur-containing materials are oxidised — in combustion of coal, oil, or natural gas containing sulphur impurities; in metal smelting where sulphide ores are roasted; and in volcanic activity where elemental sulphur and sulphide minerals are oxidised. Once in the atmosphere, SO2 can be further oxidised to sulphuric acid, making it a precursor to acid rain and secondary particulate matter.

2. Natural Sources of Sulphur Dioxide

Volcanic Activity

Volcanic eruptions and ongoing volcanic degassing are the largest natural sources of atmospheric SO2. Active volcanoes continuously emit SO2 from magma degassing — the gas dissolves in groundwater and is released as volcanic hot springs and fumaroles, or is emitted directly in eruption plumes. The global volcanic SO2 emission rate is estimated at 5–20 million tonnes per year during non-eruption periods. During major eruptions, this rate can spike dramatically — the 1991 eruption of Mount Pinatubo in the Philippines injected approximately 20 million tonnes of SO2 into the stratosphere in a single event, causing measurable global cooling for 1–2 years.

Biological Decay

Decomposition of organic matter in anaerobic environments (swamps, wetlands, ocean sediments) produces hydrogen sulphide (H2S), which can be oxidised to SO2 in the atmosphere. This biological source is relatively minor compared to volcanic and anthropogenic sources but contributes to background sulphur cycling in coastal and wetland environments.

Ocean Emissions

Marine phytoplankton produce dimethylsulphide (DMS), which is released from ocean surfaces and oxidised to SO2 and then to sulphate aerosols in the marine atmosphere. Marine SO2 from DMS oxidation is estimated at 15–25 million tonnes per year — a significant natural source that provides cloud condensation nuclei over remote ocean areas and influences global climate through cloud formation effects.

3. Coal Combustion: The Dominant Anthropogenic Source

Coal combustion is the single largest human-caused source of SO2 globally, accounting for approximately 45–50% of total anthropogenic SO2 emissions. All coal contains sulphur — typically 0.5–3.5% by weight, with some high-sulphur coals containing up to 5% or more. When coal burns, the sulphur oxidises to SO2:

S + O2 → SO2

A 1,000 MW coal power plant burning coal with 1% sulphur content emits approximately 50,000–80,000 tonnes of SO2 per year without flue gas desulphurisation. Multiplied across India’s approximately 200 GW of coal-fired power plant capacity, the SO2 emission from the power sector alone is a major air quality challenge — particularly in states like Uttar Pradesh, Jharkhand, Maharashtra, Chhattisgarh, and Odisha where coal plant clusters are concentrated.

4. Industrial Process Sources

Non-Ferrous Metal Smelting

Smelting of sulphide ores — copper (CuFeS2, chalcopyrite), zinc (ZnS, sphalerite), lead (PbS, galena), and nickel — involves roasting the ore at high temperature in air to oxidise and remove sulphur. This produces very high-concentration SO2 streams (15–40% SO2) that are ideally suited for recovery as sulphuric acid. In modern smelters, SO2 capture and sulphuric acid production is standard practice — both for revenue (sulphuric acid is valuable) and regulatory compliance. Historical smelters without SO2 capture caused some of the most severe localised SO2 pollution incidents ever recorded.

Oil Refining

Crude oil contains sulphur compounds that are removed during refining by hydrodesulphurisation (HDS). The sulphur removed is recovered as elemental sulphur (via the Claus process) — a valuable raw material for sulphuric acid production. Some SO2 is emitted from catalytic cracking and other refinery operations. India’s large refinery complex at Jamnagar (Reliance Industries) and other refineries are significant sulphur handling facilities, though modern refineries are designed to minimise SO2 emissions.

Cement Manufacturing

Cement kilns burning coal or petroleum coke as fuel emit SO2 proportional to the fuel sulphur content. Cement production is India’s third-largest industrial source of SO2 after power and steel. Some SO2 is captured within the kiln chemistry — alkaline calcium compounds in the raw meal react with SO2 — but significant emissions occur, particularly with high-sulphur fuels.

Sulphuric Acid Manufacturing

Paradoxically, sulphuric acid plants are major SO2 handlers but ideally zero-emission sources. In the Contact process, elemental sulphur is burned to SO2 which is then catalytically oxidised to SO3 and absorbed in water. Modern double-absorption sulphuric acid plants achieve SO2 emission conversion efficiencies above 99.7%. Emissions primarily arise from plant start-up, shutdown, and process upsets.

5. Transport and Petroleum Sources

Combustion of diesel, heavy fuel oil, and marine bunker fuel (which historically had high sulphur content of 3.5%) in vehicles and ships is a significant SO2 source — particularly for shipping. The International Maritime Organization’s (IMO) 2020 global sulphur cap reduced permitted marine fuel sulphur content from 3.5% to 0.5%, dramatically reducing shipping SO2 emissions globally. India’s Bharat Stage VI fuel quality standards (equivalent to Euro VI, implemented nationwide from April 2020) limit diesel sulphur to 10 ppm — a 97% reduction from pre-BS VI standards.

6. Agricultural and Biomass Sources

Agricultural biomass burning — stubble burning after rice and wheat harvest, particularly in Punjab and Haryana — releases SO2 alongside other pollutants from the sulphur content of crop residues. While agricultural SO2 from biomass burning is minor compared to coal combustion, it creates acute local and regional air quality episodes during harvest seasons, contributing to the severe winter air pollution events in North India.

7. Health Impacts of SO2 Exposure

SO2 is a respiratory irritant that affects the airways at concentrations well below those of many other industrial gases. Its health effects are concentration-dependent and disproportionately affect people with pre-existing respiratory conditions.

Acute Respiratory Effects

At 1–3 ppm: detectable taste and odour; bronchoconstriction in asthmatics and sensitive individuals, causing coughing, wheezing, and chest tightness. At 3–10 ppm: severe bronchoconstriction in most people; significant reduction in lung function measurable by spirometry. At 10–50 ppm: acute respiratory distress; potential chemical burns to airway mucosa; dangerous for all individuals without respiratory protection. The WHO 24-hour guideline of 20 micrograms/m3 (approximately 8 ppb) reflects the level below which significant respiratory health effects are not expected in the general population.

Long-Term and Population-Level Effects

Long-term residence near high SO2 emission sources (coal power plants, smelters) is associated with increased prevalence of chronic obstructive pulmonary disease (COPD), asthma, reduced lung function development in children, and increased cardiovascular disease rates. Indian epidemiological studies have documented elevated respiratory disease rates in communities adjacent to large coal-fired power plant clusters.

8. Environmental Impacts: Acid Rain and Ecosystems

SO2 in the atmosphere reacts with water, oxygen, and oxidants to form sulphuric acid and ammonium sulphate aerosols. These species deposit on the Earth’s surface as acid rain (wet deposition) and dry acid particles (dry deposition), with several significant environmental consequences.

Aquatic Ecosystem Acidification

Acid deposition reduces the pH of lakes, rivers, and streams, particularly in areas with low-buffering-capacity (granitic) bedrock. pH values below 5.5 impair reproduction in fish; below 5.0, fish populations collapse entirely. Freshwater acidification from SO2 deposition devastated fisheries in Scandinavia, Canada, and the northeastern USA in the 1970s–1980s before emission controls reduced acid deposition substantially.

Soil and Forest Damage

Acid deposition leaches base cations (calcium, magnesium) from soils, reducing soil fertility and nutrient availability to forest trees. It also mobilises toxic aluminium ions from soil minerals into soil water, poisoning fine root systems. Combined with ozone stress, acid deposition contributed to widespread forest dieback in Central Europe in the 1980s (Waldsterben) and continues to damage forests in regions with inadequate SO2 emission controls.

9. India’s SO2 Emission Profile

India emits approximately 7–9 million tonnes of SO2 per year from anthropogenic sources — making it one of the world’s top three SO2 emitters alongside China and the USA. Power sector emissions account for approximately 60–65% of this total; industrial process sources (steel, cement, fertilisers) for approximately 20–25%; and transport for the remainder.

The CPCB’s 2015 emission standards for thermal power plants require: SO2 emission limits of 200 mg/Nm3 for new plants and 600 mg/Nm3 for existing plants (with stricter limits near sensitive areas); compliance through flue gas desulphurisation (FGD) systems or fuel blending with low-sulphur coal. Implementation timelines have been repeatedly extended, but FGD installation is now proceeding at scale — with significant implications for the gypsum market (FGD gypsum as a cement industry raw material) and for SO2 emissions reduction.

10. Control Technologies and Regulations

Flue Gas Desulphurisation (FGD)

The dominant SO2 control technology for coal power plants and large industrial boilers is wet limestone FGD — flue gas passes through a scrubber where it contacts a limestone (CaCO3) slurry, reacting to form calcium sulphite and then calcium sulphate (gypsum, CaSO4.2H2O). Modern FGD systems achieve SO2 removal efficiencies of 95–99%. The gypsum by-product is used in wallboard manufacturing and cement production — a circular economy product from a pollution control process.

Fuel Switching

Switching from high-sulphur coal to low-sulphur coal, natural gas, or renewable energy eliminates SO2 at source rather than treating the exhaust. Natural gas contains negligible sulphur and produces essentially zero SO2 on combustion — India’s gas-fired power plants are SO2-free. The ongoing transition from coal to renewables will reduce India’s SO2 emissions substantially over the coming decades as coal plant capacity is retired.

11. Industrial Uses of Sulphur Dioxide

Despite its pollutant status, SO2 has important and legitimate industrial applications. Industrial-grade sulphur dioxide from Ammoniagas is supplied to customers requiring it for the following applications.

Food Preservation and Winemaking

SO2 is one of the oldest food preservatives in history — used in winemaking since ancient Roman times. It inhibits oxidation and microbial spoilage in wine, dried fruits (apricots, raisins), fruit juices, and some processed foods. Food-grade SO2 is regulated by FSSAI in India, with maximum permitted levels specified for each food category.

Sulphuric Acid Raw Material

SO2 is the primary feedstock for sulphuric acid production via the Contact process. Elemental sulphur (from oil refining desulphurisation or natural deposits) is burned to SO2, which is then oxidised to SO3 over a vanadium pentoxide catalyst and absorbed in water. Sulphuric acid is one of the world’s most important industrial chemicals, consumed in fertiliser production, mining, batteries, and chemical manufacturing.

Textile and Paper Bleaching

SO2 is used to bleach wool, silk, straw, and some cellulosic materials — its reducing action removes colour from oxidised chromophores. In paper manufacturing, sulphite pulping uses SO2 and bisulphite chemistry to dissolve lignin from wood chips, producing cellulose pulp for paper production.

Industrial Disinfection

SO2 gas is used to fumigate food storage areas, wine tanks, and water treatment facilities as a disinfectant and antifungal agent. Its broad antimicrobial activity at relatively low concentrations makes it effective for surface disinfection in enclosed spaces.

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