Smog is a fairly general term which refers to forms of air pollution in which atmospheric visibility is hindered by a mist of solid and/or liquid particulates. The name stems from the modest combination of the words “smoke” and “fog”- not the most creative name in the world, but it does the job! Encompassed in the broad definition of smog, we can categorise two main types “Classical Smog” also known as ”London Smog” and “Photochemical Smog” or “Los Angeles Smog”
Classical Smog (Top Photo):
This type of smog is so named as it is associated with a traditional fuel source: coal. It is categorised by a high concentration of unburned carbon soot as well as high levels of sulphur dioxide (SO2). SO2 is a reducing agent and the precursor of the formation of a weak acid – which makes the overall chemical properties of this smog slightly acidic. Where the climate is humid, the carbon particles can serve as bases for the condensation of water droplets forming an irritating fog. Classical fog is sometimes referred to a London smog as during the 19th Century industrialised areas, like London, suffered greatly from this phenomenon. As late as 1952 in London a severe smog lasting several weeks led to the death of more than 4000 persons, mainly accredited to acute aggravation of underlying respiratory problems. As technology and legislation have progressed, classical smog has been eradicated for the most part in London and elsewhere. However, there are still present day examples where coal burning industry is still booming.
Photochemical Smog (Bottom Photo):
Unlike classical smog, photochemical smog is attributed to emissions from petroleum combustion, primarily from motor vehicles. These emissions cause a sequence of chemical and photochemical reactions which occur under specific conditions. Photochemical smog is essentially a 21st Century problem, resulting from the presence of unburned hydrocarbons (HC’s) and nitrogen oxides (NOx), both of which are the result of internal combustion engines. For this type of smog to occur, there needs to be certain climatic conditions- some of the reactions are thermal and require a warm atmosphere while others require sunlight- so this type of smog is most prevalent in warm climates, like LA. Here, the warm dry weather can also cause dust from the ground to become airborne. The combinations of the gaseous and particulate pollutants forms a uniquely characteristics haze. In contrast to the classical variety, photochemical smog is increasing worldwide.
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