According to the Environmental Protection Agency (EPA), which one of the following is the largest source of sulphur dioxide emissions?

Answer: (d) Power plants using fossil fuels

Per the US EPA, burning fossil fuels at power plants is the largest source of SO₂.

The idea

Sulphur is an impurity in coal and in heavy oil. When the fuel burns, the sulphur burns too and leaves the chimney as sulphur dioxide. So the biggest source is simply whoever burns the most sulphur-bearing fuel, and that is coal-fired power plants.

Coal contains sulphur → Burnt in a power plant → Sulphur + oxygen → sulphur dioxide → Leaves through the chimney → Forms acid rain and fine sulphate particles

Why the others are wrong

(a) Locomotives burn diesel with little sulphur today.

(b) Ships burn high-sulphur fuel, but in total they emit less SO₂ than power plants.

(c) Smelting metal ores releases SO₂, but it is second to power plants.

The trap

Ships are notorious for sulphur fuel, so (b) tempts. Power plants burn far more fuel.

Also know

The fix is flue gas desulphurisation: the exhaust is passed through limestone, which absorbs the SO₂ and gives gypsum. India set such norms for power plants in 2015 and has extended the deadlines several times. Ships were told to cut fuel sulphur to 0.5% from 2020 by the International Maritime Organization. The largest natural source is volcanoes.