SO₂ — Sulfur Dioxide Air Quality
Sulfur dioxide (SO₂) is a sharp-smelling gas released by burning sulfur-containing fuels (coal, oil) and from volcanic eruptions. It is a major cause of acid rain and a direct respiratory irritant, particularly for people with asthma.
What is SO₂?
SO₂ forms when sulfur in coal, oil, or diesel reacts with oxygen during combustion. In the atmosphere it can oxidise to sulfuric acid (H₂SO₄), which falls as acid rain — damaging ecosystems, forests, and buildings. SO₂ also reacts with ammonia and water to form fine sulfate particles (a component of PM2.5).
Concentrations near coal power plants and smelters can be extremely high. Modern coal plants use flue-gas desulfurisation (scrubbers) to reduce SO₂ by 90–95%, but older plants remain a major source globally — particularly in China, India, and Eastern Europe.
Sources of SO₂ Pollution
- Coal-fired power plants — largest anthropogenic source worldwide
- Metal smelting — copper, zinc, and lead smelters emit large SO₂ plumes
- Oil refining — processing high-sulfur crude oil
- Ships — bunker fuel combustion (high sulfur content historically)
- Diesel vehicles (low-sulfur standards have greatly reduced this)
- Volcanoes — major natural source (e.g., Hawaii's "vog")
- Household coal burning — a significant source in developing countries
Health Effects of SO₂
Respiratory effects
- Bronchoconstriction (airways narrow within minutes)
- Asthma attacks
- Coughing, wheezing, shortness of breath
- Mucus hypersecretion
Long-term and secondary effects
- Chronic bronchitis with prolonged exposure
- Contributes to PM2.5 via sulfate particle formation
- Acid rain damages ecosystems and water bodies
- Eye, nose, and throat irritation
SO₂ AQI Breakpoints (US EPA)
Based on 1-hour average concentration in parts per billion (ppb).
SO₂ Regulatory Standards
| Standard | Limit | Period |
|---|---|---|
| WHO (2021) | 40 μg/m³ (≈15 ppb) | 24-hour |
| US EPA NAAQS | 75 ppb | 1-hour (99th percentile) |
| US EPA NAAQS | 0.5 ppm (500 ppb) | 3-hour secondary |
| EU Directive | 350 μg/m³ (≈134 ppb) | 1-hour |
| EU Directive | 125 μg/m³ (≈48 ppb) | 24-hour |
| India NAAQS | 80 μg/m³ (≈31 ppb) | Annual mean |
SO₂ and Coal Power Plants in India and China
India and China together account for over 50% of global SO₂ emissions from coal power. India's coal fleet grew 7× between 2000 and 2020, concentrated in states like Uttar Pradesh, Chhattisgarh, and Odisha. Many older plants lack scrubbers.
Satellite data (Sentinel-5P) shows large SO₂ hotspots over the Singrauli coal belt (UP/MP border) and the Kutch industrial zone in Gujarat. These are detectable from space and correlate with elevated PM2.5 downwind.
Protection Tips
- Asthma patients should carry rescue inhalers during high-SO₂ advisory days
- Avoid areas near industrial zones and power plants on low-wind days
- Use HEPA + activated carbon air purifiers indoors (captures sulfate particles)
- Check SO₂ sub-index in AQI apps — some sensors report individual pollutants
- During volcanic SO₂ events, stay indoors and keep windows closed