[ksab]

Outdoors: Particles less than 50nm often arise from sulphuric acid. Nucleation occurs with sulphur dioxide and water in vehicle exhaust and these particles undergo growth via condensation of organic compounds as the exhaust plume cools.

Larger particles (50-100nm) are typically soot.

Indoor Sources: cooking (frying especially), burning candles/incense, heating elements on stove tops and toasters, chemical reactions between lemon/pine scented cleaners with ozone.

Source: PhD in exposure to ultrafine particles.