Classical (London-type) Smog

Classical smog, sometimes called London-type smog after the notorious mid-20th-century London

episodes, is produced by the combination of smoke, fog and sulfur dioxide over an industrial city

in cold, humid winter weather. Coal and other sulfur-containing fuels burnt for heating and power

release large quantities of $\text{SO}_2$ and soot particles; in cold, still, foggy conditions

these pollutants become trapped close to the ground rather than dispersing upward, and the moisture

in the fog helps convert $\text{SO}_2$ into a fine sulfuric-acid mist (Section 13.3).

Chemically, classical smog is described as a reducing mixture, because its characteristic

pollutant, $\text{SO}_2$, is itself readily oxidised rather than being an oxidising agent -- it

consumes oxidants from its surroundings rather than generating them. This is the opposite chemical

character to photochemical smog (next section), and the distinction is a useful way of telling the

two types apart even before considering their different geography and season.

The conditions favouring classical smog are:

• Cold, humid weather, often with a temperature inversion that traps a layer of cool, polluted air close to the ground beneath a layer of warmer air above it, preventing normal vertical mixing and dispersal.
• Heavy reliance on coal or other sulfur-rich fuel for domestic heating and industrial power.
• Low wind speed, which allows pollutant concentrations to build up rather than being carried away. …