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Sulfur dioxide (SO₂) is a colourless gas. Major sources include burning sulfur-containing fuels, such as coal and oil, for heating, electricity generation and industrial processes.
Nitrogen dioxide (NO₂) comes mainly from fuel burning in vehicles, heating, power generation and industry. Gas stoves and other fuel-burning appliances can also release it indoors. It contributes to the formation of ozone and fine particles in the atmosphere.
Ground-level ozone (O₃) forms when sunlight drives reactions involving nitrogen oxides and volatile organic compounds. Levels often rise in sunny weather, and pollution can be transported away from its sources. Ground-level ozone is harmful; it is different from the protective ozone layer high in the atmosphere.