Pea soup fog in the context of Great Smog of London

Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words smoke and fog to refer to smoky fog due to its opacity, and odour. The word was then intended to refer to what was sometimes known as pea soup fog, a familiar and serious problem in London from the 19th century to the mid-20th century, where it was commonly known as a London particular or London fog. This kind of visible air pollution is composed of nitrogen oxides, sulfur oxide, ozone, smoke and other particulates. Man-made smog is derived from coal combustion emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.

Smog is often categorized as being either summer smog or winter smog. Summer smog is primarily associated with the photochemical formation of ozone. During the summer season when the temperatures are warmer and there is more sunlight present, photochemical smog is the dominant type of smog formation. During the winter months when the temperatures are colder, and atmospheric inversions are common, there is an increase in coal and other fossil fuel usage to heat homes and buildings. These combustion emissions, together with the lack of pollutant dispersion under inversions, characterize winter smog formation. Smog formation in general relies on both primary and secondary pollutants. Primary pollutants are emitted directly from a source, such as emissions of sulfur dioxide from coal combustion. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere.

The South Bank Lion is an 1837 sculpture in Central London. Since 1966 it has stood next to County Hall, on the South Bank of the River Thames. It is a significant depiction of a lion, along with the four that surround Nelson's Column in Trafalgar Square just across the river.

The statue is about 13 feet (4.0 m) long and 12 feet (3.7 m) high, and weighs about 13 tonnes (14 tons). It was cast in 1837, the year of Queen Victoria's accession, of Coade stone, one of the earliest types of artificial stone. The material is very resistant to weathering, and the fine details of the lion's modelling still remain clear after decades of exposure to the corrosive effects of London's severe air pollution, the infamous pea soup fog, prior to the passage of the Clean Air Act 1956. The statue was made in separate parts and cramped together on an iron frame. It was formerly known as the Red Lion, as it was painted that colour between 1951 and 1966.