The Role of Sulphur Dioxide in Air Pollution

Sulphur dioxide (SO₂) is one of the most studied and consequential air pollutants on the planet. It is a colourless gas with a sharp, pungent odour – and its presence in the atmosphere sets off a chain of chemical reactions that affect ecosystems, human health, and even historical monuments. Understanding where SO₂ comes from, what it does once it enters the atmosphere, and how we can control it is central to any serious study of air pollution chemistry.

Sources of sulphur dioxide 🔗

SO₂ enters the atmosphere from both natural and human-driven sources, though the two are far from equal in their contribution.

Natural sources 🔗

Volcanic activity is the primary natural source of SO₂. According to the U.S. Geological Survey, SO₂ is one of the most common gases released during volcanic eruptions, after water vapour and carbon dioxide. It can cause acid rain and severe air pollution in downwind regions – at Kīlauea volcano in Hawaii, for example, elevated SO₂ concentrations have created persistent volcanic smog (known locally as “vog”) that poses long-term health risks to nearby populations. When eruptions are powerful enough to inject SO₂ into the stratosphere above 10 km, the gas converts to sulphate aerosols that reflect sunlight and contribute to short-term global cooling – a phenomenon documented after major events like the 1991 eruption of Mount Pinatubo. Globally, volcanoes contribute roughly 7-8 teragrams of sulphur per year in the form of SO₂ – a meaningful but comparatively small figure.

Other minor natural sources include the decomposition of organic matter and emissions from geothermal fields, both of which release small quantities of sulphur compounds into the lower atmosphere.

Human (anthropogenic) sources 🔗

The dominant source of atmospheric SO₂ today is human activity. The U.S. Environmental Protection Agency (EPA) identifies the combustion of sulphur-containing fossil fuels – particularly coal and oil – as the leading driver, with power generation, industrial boilers, and petroleum refineries as the main emitters. When coal burns, the sulphur it contains is oxidised almost entirely to SO₂, which is then released as part of the flue gas. Fossil fuel combustion and industry together release approximately 70 teragrams of sulphur per year, dwarfing volcanic contributions by nearly an order of magnitude.

Beyond power plants, the California Air Resources Board notes that SO₂ is also emitted from natural gas and petroleum extraction, oil refining, and metal processing industries. Smelting operations – which process sulphide ores – are particularly significant emitters in many parts of the world. In short, anywhere sulphur-containing materials are heated or burned, SO₂ is likely to follow.

Atmospheric reactions and acid rain 🔗

Once SO₂ is in the atmosphere, it does not simply drift harmlessly away. It undergoes a series of chemical transformations that ultimately produce some of the most damaging forms of air pollution known.

Formation of sulphuric acid 🔗

The core reaction pathway begins when SO₂ is oxidised in the atmosphere. In the presence of oxygen, SO₂ converts to sulphur trioxide (SO₃). SO₃ then reacts readily with atmospheric water vapour to form sulphuric acid (H₂SO₄) – a strong acid that can be carried long distances by wind before depositing on the ground. As the EPA explains, SO₂ and nitrogen oxides (NOₓ) react with water, oxygen, and other atmospheric chemicals to form sulphuric and nitric acids, which then mix with precipitation and fall as acid rain. This deposition can occur in wet form (rain, snow, fog) or as dry acidic particles settling on surfaces.

Normal rainwater has a pH of around 5.6, reflecting the natural dissolution of carbon dioxide to form weak carbonic acid. Acid rain typically registers a pH between 4.2 and 4.4 – significantly more acidic. In heavily industrialised areas, readings below pH 2.4 have occasionally been recorded.

Impacts on ecosystems 🔗

The ecological damage from acid rain is well-documented and wide-ranging. Acid rain causes the acidification of lakes and streams, which kills fish and disrupts aquatic food chains. It strips essential nutrients from soils and leaches toxic metals such as aluminium into waterways. Forests are affected too – acid rain reduces the durability of tree bark, leaving vegetation more vulnerable to pests, drought, and disease. Red spruce forests at high elevations in the northeastern United States have been particularly hard hit.

National Geographic notes that the buffering capacity of soil – its ability to neutralise incoming acids – varies considerably by region. In areas with naturally low buffering capacity, such as parts of the U.S. Northeast and Scandinavia, ecological recovery from acid rain is slow even after emission reductions. Additionally, SO₂ deposition promotes chemical reactions that facilitate mercury accumulation in water and soil, which in turn elevates mercury levels in food sources and poses further health risks to human populations.

Damage to infrastructure and human health 🔗

Acid rain’s corrosive effects extend well beyond natural environments. Sulphates and nitrates formed from SO₂ accelerate the deterioration of paint, metals, marble, and limestone, damaging bridges, buildings, and cultural monuments. The damage to India’s Taj Mahal is one widely cited example of acid rain’s corrosive impact on heritage structures.

For human health, the International Volcanic Health Hazard Network (IVHHN) describes SO₂ as irritating to the eyes, throat, and respiratory tract. Short-term overexposure causes inflammation, coughing, and difficulty breathing, while prolonged exposure at lower concentrations can be dangerous for people with pre-existing heart or lung conditions. People with asthma are particularly sensitive, with adverse effects documented at concentrations as low as 0.2-0.5 parts per million (ppm). Fine sulphate particles formed from SO₂ in the atmosphere can penetrate deep into the lungs, contributing to increased rates of respiratory illness and premature cardiovascular mortality.

Removal techniques for sulphur dioxide 🔗

Controlling SO₂ emissions requires intervention at the point where it is generated – primarily at the smokestack of power plants and industrial facilities. Several methods have been developed and deployed at scale.

Limestone scrubbing (wet flue gas desulphurisation) 🔗

The most widely used SO₂ control technology is wet flue gas desulphurisation (FGD), commonly called limestone scrubbing. According to Wikipedia’s technical overview of FGD, approximately 79% of the world’s FGD capacity uses lime or limestone wet scrubbing. The process works by spraying flue gas with a slurry of ground limestone (calcium carbonate, CaCO₃) and water inside a scrubber tower. The SO₂ in the gas reacts with the limestone to form calcium sulphite (CaSO₃), which can be further oxidised to calcium sulphate – commonly known as gypsum (CaSO₄·2H₂O). This gypsum byproduct is commercially valuable and widely used in wallboard manufacturing, making the process economically attractive in addition to being environmentally effective.

The key chemical reactions in limestone scrubbing are straightforward. SO₂ first dissolves in water to form sulphurous acid, which then reacts with the calcium carbonate in limestone. Under optimal operating conditions, SO₂ removal efficiencies can reach 95-99%, making wet scrubbing the most effective available method for large power plants and industrial boilers.

Dry sorbent injection and spray dryer systems 🔗

Dry sorbent injection (DSI) is a simpler, lower-cost alternative suited to smaller facilities or retrofitting existing equipment. In this method, powdered sorbent – typically hydrated lime or sodium bicarbonate – is injected directly into the flue gas duct. The sorbent reacts with SO₂ and the resulting solid particles are captured downstream by a baghouse filter or electrostatic precipitator. FGD scrubbers as a category can achieve sulphur removal efficiencies between 50% and 98%, with wet systems consistently outperforming dry ones at the higher end of that range.

Spray dryer absorbers occupy a middle ground: a slurry of lime is atomised into a spray, which simultaneously dries and reacts with the SO₂ in the gas stream. The resulting dry powder is collected via a particulate control device. This approach works well for mid-sized installations and handles moderate SO₂ concentrations effectively.

Seawater scrubbing and other approaches 🔗

In coastal industrial settings, seawater scrubbing offers a practical alternative. SO₂ is absorbed into seawater – which is naturally alkaline due to its carbonate content – and oxidised to harmless sulphate ions before the treated water is returned to the sea. This approach has been applied successfully at large coastal power stations, such as the Aberthaw Power Station in Wales.

Other stack gas treatment approaches include the electron beam method, where high-energy electrons promote the oxidation of SO₂ in the presence of ammonia to produce ammonium sulphate – a material usable as fertiliser. While this technique has reached industrial scale in some applications, it remains less common than wet scrubbing due to higher operational complexity.

Regulatory context and emission trends 🔗

Technology alone does not drive emission reductions – regulation plays a central role. Under the U.S. Clean Air Act of 1990 and subsequent EPA programmes, sulphur dioxide emissions fell by 92% between 1990 and 2023, demonstrating that aggressive but achievable policy targets can deliver substantial environmental improvements. Similar programmes under the 1999 Gothenburg Protocol – coordinated through the United Nations Economic Commission for Europe (UNECE) – have established binding national emission ceilings for SO₂ across multiple countries.

Despite this progress, SO₂ remains a serious concern in rapidly industrialising regions. Countries that rely heavily on coal for electricity generation, particularly in Asia, continue to face elevated emission levels, underscoring the ongoing importance of both technological deployment and international policy coordination.

What do you think? Given that wet limestone scrubbing can remove up to 99% of SO₂ from flue gases, what are the main barriers preventing its universal adoption in coal-dependent economies? And as acid rain has been shown to damage ecosystems hundreds of kilometres from the pollution source, should international agreements carry stronger enforcement mechanisms to hold emitting nations accountable?

References

1. https://www.usgs.gov/programs/VHP/volcanic-gases-can-be-harmful-health-vegetation-and-infrastructure
2. https://en.wikipedia.org/wiki/Acid_rain
3. https://www.epa.gov/acidrain/what-acid-rain
4. https://ww2.arb.ca.gov/resources/sulfur-dioxide-and-health
5. https://pressbooks.uwf.edu/envrioscience/chapter/10-3-acid-rain/
6. https://www.nationalgeographic.com/environment/article/acid-rain
7. https://www.ivhhn.org/information/information-different-volcanic-gases/sulphur-dioxide
8. https://en.wikipedia.org/wiki/Flue-gas_desulfurization
9. https://torch-air.com/blog/flue-gas-desulfurization-fgd-scrubber
10. https://www.chartindustries.com/Articles/Reduce-The-Amounts-of-Sulfur-Dioxide-From-Flue-Gas
11. https://www.undrr.org/understanding-disaster-risk/terminology/hips/en0105