SO₂ is mainly emitted by burning coal and high-sulphur petroleum. In the atmosphere, it oxidises to sulphuric acid, contributing to acid rain and forming sulphate aerosols that reduce visibility and affect respiratory health.
Volcanic eruptions are the most important natural source: a large eruption can inject millions of tonnes of SO₂ into the stratosphere, cooling climate by 0.5–1 °C for 1–2 years (e.g., Pinatubo, 1991). Desulphurisation policies have drastically reduced SO₂ in Europe since the 1980s.