Photochemical fog (smog). OBZhD

BASICS OF SAFE LIFE
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Photochemical fog (smog). Basics of safe life

Fundamentals of Safe Life Activities (OBZhD)

Photochemical fog (smog) is a multicomponent mixture of gases and aerosol particles of primary and secondary origin. The composition of the main components of smog includes ozone, nitrogen and sulfur oxides, numerous organic peroxide compounds, collectively called photooxidants. Photochemical smog occurs as a result of photochemical reactions under certain conditions: if the atmosphere has a high concentration of nitrogen oxides, hydrocarbons and other pollutants, with intense solar radiation and no wind, and also in cases of very weak air exchange in the surface layer with a powerful increased inversion for at least days. Sustained calm weather, usually accompanied by inversions, is necessary to create a high concentration of reactants.

Such conditions are created more often in June-September and less often in winter. In prolonged clear weather, solar radiation causes the breakdown of nitrogen dioxide molecules with the formation of nitric oxide and atomic oxygen. When atomic oxygen combines with molecular oxygen, ozone is formed. It would seem that the latter, oxidizing nitric oxide, should again turn into molecular oxygen, and nitric oxide, in turn, into dioxide. But that doesn't happen. The nitric oxide reacts with the olefins in the exhaust gases, which break down at the double bond, forming molecular fragments and excess ozone. As a result of the ongoing dissociation, new masses of nitrogen dioxide are split and give additional amounts of ozone. A cyclic reaction occurs, as a result of which ozone gradually accumulates in the atmosphere. At night, this process stops.

Ozone, in turn, reacts with olefins. Various peroxides are concentrated in the atmosphere, which together form oxidants characteristic of photochemical fog. The latter are the source of the so-called free radicals, which are characterized by a special reactivity. They are extremely dangerous because they affect the respiratory and circulatory systems of the human body and often cause premature death of urban residents with poor health.

Authors: Aizman R.I., Krivoshchekov S.G.

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