Concepts of Photochemical Smog


Photochemical smog is a distinct form of air pollution that results due to chemical reactions between the sunlight and atmospheric pollutants, which are mostly in form of oxides. The formation of photochemical smog depends on the presence of three factors namely the presence of ultraviolet light, pollutants (gas oxides), and temperature inversion, to facilitate the buildup of pollutants in the lower inversion layer.

Common oxides associated with the formation of photochemical smog include hydrogen peroxide, nitrogen oxides (NOx), ozone (O3), aldehydes, volatile organic compounds (VOCs), and peroxyacetylnitrate (PAN). The primary source of most of these oxides burning fossil fuels, for example, coal, and exhaust emissions. Of all the oxides, Ozone is one of the most common components of photochemical smog, followed by nitrogen oxides (Canadian Council of Ministries of Environment 1-3).

Formation of Photochemical Smog

Because of the oxidizing nature of most oxides, once they come into contact with ultraviolet, most oxides will disintegrate, leading to emission of very hazardous gases into the environment. The process of formation of photochemical smog starts with the breaking down of Nitrogen dioxide (NO2) or Nitric oxide (NO) in the atmosphere. In the presence of oxygen, NO2 will change into nitrogen trioxide (NO3), which on further oxidation can form dinitrogen pentaoxide (N2O5). Once these products have been formed, depending on the prevailing environmental conditions, each of these can combine with other atmospheric substances, via a photochemical process, to form environmental pollutants, for example, nitric acid (HNO3), Alkyl or Peroxyalkyl, or alkyl, or peroxyacetyl nitrate, and nitrous or peroxynitrite, or nitric acid.

A diagram showing Chemical conversions of nitrogen oxides in the troposphere

Chemical conversions of nitrogen oxides in the troposphere

For photochemical smog to form, Under the action of ultraviolet light, NO2 breaks down to release a single atom of oxygen and nitric oxide (NO)( refer to equation 1). After this, the formed oxygen will react with more oxygen to form Ozone (refer to equation 2). This formed ozone can also break down to form atoms of oxygen, which will further react with other substances in the atmosphere to release elements of photochemical smog.

At the same time, the single oxygen atoms formed in step 1 can combine with water to form hydroxyl radicals (refer to equation 3), which will further combine with hydrocarbons radicals, leading to the formation of aldehydes (Equation 4). Depending on the prevailing environmental conditions the aldehydes formed can further react with oxygen, leading to the formation of aldehyde peroxides or peroxy acids. These end products of these reactions are the primary elements that make photochemical smog cause numerous health hazards. Once these compounds have been formed, they will combine with nitrogen oxides in the air to form photochemical smog (Equations 5, 6, and 7) (Foust 1).

Equation 1 NO2 + hv → NO + O

Equation 2 O + O2 → O3

Equation 3 4 O. +H2O → 2 OH.

Equation 4 RH +OH. → HO +R

R. + O2 → RO2 (very fast)

Equation 5 RO. 2 + NO → NO2 +RO.

RO. + O2 → RCHO. + H2O (very fast)

Equation 6 RCHO. + OH. → RCO. + H2O

RCO +O2 → RC (O) O2 . (very fast)

Equation 7 RC(O) O. 2 +NO2 →RC (O)NO. 2

Generalized scheme for the formation of photochemical smog

Effects of Photochemical Smog

Because of the intoxicating nature of photochemical smog’s chemical composition, photochemical smog can cause severe eye soreness, breathing, and visibility problems, because of the accumulation of gases in the lower inversion layer. In addition, the accumulation of oxides, for example, ozone, sulfur, nitrogen, carbon oxides in the atmosphere can cause numerous health hazards, more so to individuals who are suffering from heart and lung ailments, for example, bronchitis and emphysema. On the other hand, photochemical smog can cause numerous environmental associated hazards, for example, decaying of buildings, as a result of the numerous oxides associated with the formation of photochemical smog.

Preventing Photochemical Smog

Since the primary elements of photochemical smog are nitrogen oxides and hydrocarbons that result due to the burning of fossil fuels, there is a need for individuals to reduce the use of fossil fuels as a source of energy, more so in industries and during winter seasons. For automobiles, the primary way of reducing emissions of hydrocarbons is by emphasizing the need for motor vehicles to have catalytic converters, which can be made up of platinum or rhodium, or a combination of both. These catalysts help to convert carbon monoxide emitted by automobiles into less dangerous compounds namely carbon dioxide and water (Cudmore 1).

Works Cited

Canadian Council of Ministries of Environment. What is smog? 2008. Web.

Cudmore, John. Photochemical smog. 2001. Web.

Foust, Richard. Photochemical smog. Northern Arizona University. 2010. Web.

Shodor Education Foundation. Application: The science Smog photochemistry. 1998. Web.