Crude Oil

Sulfur dioxide from sulfur impurities in fuels

• Crude oil contains a small percentage of sulfur compounds; when fossil fuels are burned, the sulfur is oxidised to sulfur dioxide:

S(s) + O₂(g) → SO₂(g)

• SO₂ escapes into the atmosphere through the chimneys of power stations and the exhausts of unfiltered vehicles
• Many modern power stations now use flue-gas desulfurisation with limestone or calcium hydroxide to scrub SO₂ out before the gas reaches the atmosphere

Nitrogen oxides from hot engines

• The air drawn into a car engine, a jet engine, or an industrial blast furnace is mostly nitrogen and oxygen
• At the high temperatures inside these engines, the normally unreactive N₂ and O₂ react to form nitrogen monoxide (NO) and nitrogen dioxide (NO₂), collectively called nitrogen oxides (NO_x)
• Modern petrol cars carry a catalytic converter in the exhaust system that converts NO_x and CO back to N₂ and CO₂ before they leave the tailpipe

Acid rain

• Both SO₂ and NO_x dissolve in rain droplets high in the atmosphere
• SO₂ reacts with water (and more oxygen) to give sulfuric acid:

2 SO₂(g) + O₂(g) + 2 H₂O(l) → 2 H₂SO₄(aq)

• NO₂ reacts with water and oxygen to give nitric acid:

4 NO₂(g) + 2 H₂O(l) + O₂(g) → 4 HNO₃(aq)

• The mixture of acids falls as acid rain, with a pH typically of 4–5 (rainwater unaffected by pollution sits around pH 5.6, due only to dissolved CO₂)

Effects of acid rain

• Acidifies lakes and rivers, killing fish and aquatic invertebrates
• Damages tree leaves and dissolves nutrients out of soil — large areas of forest decline
• Corrodes metal structures (bridges, railings) and dissolves carbonate building materials (limestone, marble) — historic statues lose facial detail over decades