Photochemical smog
The yellowish-brown haze formed when chemicals in the exhaust smoke of motor vehicles react in sunlight, which irritates the eyes and blocks vision, is called photochemical smog.
For extra knowledge
Photochemical smog forms when the oxides of nitrogen and unburnt hydrocarbons in smoke from burning fossil fuels are changed, in sunlight and at temperatures above 15 °C, into ozone, aldehydes, peroxyacetyl nitrate (PAN), peroxybenzoyl nitrate (PBN) and similar substances.
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Some harmful effects of photochemical smog:
- It affects the breathing system, causing coughs and asthma.
- It is poisonous to plants, slowing growth and food production.
- It makes the air less transparent, so visibility falls.
- It weakens rubber and fabrics and bleaches their colours.
Why smog needs sunlight and traffic
The name says it: photo-chemical smog is made by light. Car exhausts supply the raw materials — nitrogen oxides and unburnt hydrocarbons — and strong sunshine supplies the energy that drives the reactions. So smog is worst in large, sunny, crowded cities on still, warm days, when the polluted air cannot blow away. The ozone made in smog near the ground is harmful to breathe, even though ozone high in the atmosphere protects us. Cleaner engines, public transport and fewer cars in city centres all reduce it.
Exhaust plus sunlight
Photochemical smog forms when exhaust gases react in sunlight; it harms breathing, plants, visibility, rubber and fabrics.