How does industrial smog differ from photochemical smog?

Industrial smog, often grey, is caused by burning fossil fuels, while photochemical smog, typically brown, results from sunlight reacting with pollutants.

Industrial smog, also known as grey smog, is primarily caused by the combustion of fossil fuels, particularly in industries and factories. This type of smog is a mixture of sulphur dioxide, droplets of sulphuric acid, and a variety of suspended solid particles. It is often associated with colder, industrialised cities and is most common during the winter months when the air is stagnant and damp. The term 'industrial smog' was first used to describe the severe air pollution that affected London in the early 20th century, which was often so thick it was referred to as 'pea soup fog'.

On the other hand, photochemical smog, often referred to as 'brown air', is a type of air pollution derived from the reaction between sunlight and pollutants such as nitrogen oxides and volatile organic compounds (VOCs). These reactions form airborne particles and ground-level ozone, or smog. This type of smog is most common in warm, sunny climates and is often associated with vehicle emissions. The brown colour comes from the nitrogen dioxide in the smog, which absorbs sunlight and then re-radiates it as a brown light.

In summary, the key difference between industrial and photochemical smog lies in their causes and composition. Industrial smog is directly linked to the burning of fossil fuels and is characterised by the presence of sulphur compounds and solid particles. Photochemical smog, however, is the result of chemical reactions between sunlight and pollutants, primarily nitrogen oxides and VOCs, leading to the formation of ozone and other harmful substances.