Photochemical Smog - What is it and How is it Formed?

Photochemical smog refers to a type of smog which is formed when the ultraviolet light from the sun reacts with nitrogen oxides present in the atmosphere. This smog can cause a lot of adverse effects on humans and other living organisms. Students can learn more about photochemical smog and its formation here.

What is Photochemical Smog?

Photochemical smog refers to a chemical reaction of sunlight, nitrogen oxides, and organic compounds in the atmosphere. This smog is visible as a brown haze. Furthermore, this smog depends on the primary pollutants and also on the formation of secondary pollutants.

Smog can occur at any time, both during the day and night. On the other hand, photochemical smog occurs only in the presence of sunlight.

The primary pollutants include nitrogen oxides, particularly nitric oxide and nitrogen dioxide, and organic compounds of volatile nature. The major secondary pollutants consist of peroxyacetyl nitrates, tropospheric ozone, and aldehydes.

Process of Photochemical Smog Formation

The formation of photochemical smog consists of three main ingredients. Moreover, these ingredients are nitrogen oxides, hydrocarbons, and sunlight.

Nitrogen oxides and hydrocarbons are the by-products of fossil fuels-burning energy plants. Furthermore, they can also come from natural processes. Above all, the main source, of course, is automobiles.

Dissociation of nitrous oxide and nitrogen dioxide takes place in sunlight. Then they combine with trace hydrocarbons. Consequently, this produces a large number of pollutants. This process takes place in three stages as follows:

Sunlight causes the photo-dissociation of nitrogen and oxygen. This yields the ozone and the atoms of oxygen.
Then the reaction of oxygen atoms with water takes place. This results in the formation of hydroxyl radicals.
These hydroxyl radicals oxidize hydrocarbons which results in the formation of hydrocarbons radicals.
Then the oxidization of hydrocarbons takes place which results in the formation of a class of chemicals. This class of chemicals is aldehydes.
Aldehyde oxidizes to formaldehyde peroxides as well as aldehyde peroxy acids. Most noteworthy, these are the pollutants that lead to the creation of many health problems.

The Pollutants in Photochemical Smog

Many cities contain a smog index. The main chemical pollutant they monitor is ozone. It is formed as a by-product of the dissociation of nitrogen compounds. This formation occurs early in the process of smog formation. Ozone is certainly corrosive in nature.

Besides ozone, there are also a number of other pollutants in photochemical smog which are below:

Peroxyacetyl nitrate (PAN)- This pollutant is notable for causing eye and respiratory irritation. Furthermore, it is responsible for eye-watering when there is heavy pollution.

Nitrous acid (HNO2)- This is a mildly toxic compound, Moreover, this compound causes respiratory discomfort.

Nitric acid (HNO3)- This pollutant is a strong acid. Furthermore, it is an essential component of acid rain. In higher concentrations, this can burn skin and eyes.