Which one of the following is a secondary aerosol?

Sulphate particles form in the atmosphere from sulphur dioxide gas, making them a secondary aerosol.

Sea salt is ejected directly by ocean spray, so it is primary.

Mineral particles are raised directly as dust, so they are primary.

Black carbon is emitted directly by combustion, so it is primary.

Only the particle produced by in-air conversion of a gas qualifies as secondary.

Secondary sulphate aerosols contribute to haze and acid deposition.