smog

A dirty fog that forms in urban and industrial environments, in which atmospheric pollution is high. The term arises from the fusion of the English words smoke (smoke) and fog (fog) and is known internationally as such. The smog affecting large cities is mainly caused by traffic and is known as photochemical smog. The combustion of hydrocarbons in motor vehicles emits carbon monoxide and nitrogen oxides (NO_x) (NO as a primary pollutant and NO₂ as a secondary one, forming from it on combining with oxygen) into the air, among other gases, apart from various volatile organic compounds that are also incorporated into the atmosphere. In the presence of solar radiation, a series of photochemical reactions occur that form large quantities of new harmful substances (secondary pollutants) such as ozone, which gradually clouds the environment, reducing visibility and forming a kind of dome (that is what it is called) of brownish appearance that covers the whole city. Its presence irritates the eyes and respiratory tract, causing many people to develop illnesses such as rhinitis, bronchitis, asthma, or pneumonia. Persistent anticyclonic situations, particularly in winter, with a thermal inversion present, in which insolation is high and there is hardly any wind, favor the formation and progressive increase of photochemical smog. There is another type of smog, industrial smog - also known as gray or sulfurous smog -, due to particulate and gaseous pollutant emissions originating from industrial activities. The burning of coal in factories, thermal power plants, and some cities is the main source of atmospheric emission of soot and sulfur dioxide (and from it, other substances such as sulfuric acid and various sulfates) which, on being incorporated into the water droplets of a fog, result in the aforementioned industrial smog, which is highly polluting.