Photochemical smog is a mixture of smoke, fog, and contaminants. It is formed when ozone-depleting substances such as nitrogen oxides, chlorofluorocarbons (CFCs), and volatile organic compounds (VOCs) react in the presence of sunlight. This results in the formation of brown haze that covers almost entire cities.

Nitrogen oxides (NOx), sulfur oxides (SOx), carbon oxides (COx), hydrocarbons (CH), and volatile organic compounds (VOCs) are primary pollutants. They get destroyed by sunlight through a series of chemical reactions leading to secondary pollutants. As these primary and secondary pollutants produce ozone (a troposphere pollutant), the NOx, SOx, COx, CH, and VOCs prove to be harmful to the environment. Many other substances like peroxyacetyl nitrate (PAN) are also involved in such pollution.

The smog formation process occurs due to the reaction of the above-mentioned compounds in sunlight, therefore, smog is mostly seen in summer when there is more sunlight available.

Photochemical Smog | Effects & Reduction

Sources of Photochemical Smog

There are different major sources of photochemical smog. The main sources include NOx, SOx, COx, CH, VOCs, etc. The sources of such contaminants are both natural and anthropogenic. The details are given below:

Natural sources

  • Natural bushfires, lightning, and microbial processes in soil are the basic source of nitrogen oxides.
  • Evaporation of natural compounds such as terpenes and eucalyptus produces volatile organic compounds.
  • Coal, petroleum, and natural gas are natural sources of hydrocarbons.

Anthropogenic Sources

  • Combustion of fossil fuels in power plants, industries, and vehicles are the main sources that concern human activities.
  • Industrial effluents also contain reactive oxides which on degradation form photochemical smog.
  • Evaporation of solvents and fuels.
  • Smoke from wood-burning stoves.
  • Automobile exhaust gases contain 66% nitrogen oxides and 44% VOCs.

Formation of Photochemical Smog

The series of the chemical reactions are listed below,

Nitrogen dioxide (NO2) reacts with sunlight and breaks down into nitric oxide (NO) and oxygen radical (O.).

formation of photochemical smog

The free radical of oxygen (O.) reacts with atmospheric oxygen (O2) to give ozone (O3).

formation of photochemical smog

Ozone (O3) reacts with nitric oxide (NO) to give nitrogen dioxide (NO2) and oxygen (O2).

formation of photochemical smog

Organic and inorganic compounds (ROx) react with nitrogen monoxide to produce more nitric oxides.

formation of photochemical smog

This series of reactions show that ozone reacts with nitrogen oxide in 3rd step. The ground-level ozone is formed when nitrous monoxide and nitrogen dioxide react with volatile organic compounds as a by-product.

Effects of photochemical Smog

Photochemical smog has various effects on the environment, human health, plants, animals, materials, etc.

Reduction of photosynthesis

Chemical compounds such as nitrogen oxide, ozone, and PAN have harmful effects on plants. These compounds stop the growth of the plants by reducing the process of photosynthesis. Ozone is also responsible for generating several diseases in plants. Similarly, PAN is more effective than ozone.

Other effects on living organisms

There are several effects of pollutants that are involved in photochemical smog on living organisms. Some important effects are breathing problems, heart and lung diseases, decreased immunity, cancer, eye irritations, etc.

Major effects of smog and seen in big cities where smog may exceed all limits and make everything almost invisible.

Photochemical smog

How to cope with Photochemical Smog?

To reduce the photochemical smog, it is necessary to stop the production of primary pollutants such as nitrogen oxides and VOCs, and other pollutants as well.

Addition of catalytic converters to vehicles’ exhaust:

The best method to reduce nitrogen oxide is catalytic reduction. The mechanism of catalytic reduction is to convert nitrogen oxides into nitrogen and oxygen. This can significantly help in the reduction of smog.

Use of alternative power sources:

Reduction in the use of air in the combustion of fuel in power plants and industries can help to cope with smog. It is because air combustion releases a large number of nitrogen oxides. Alternative sources of power can be solar, wind, and hydropower systems.

Electric vehicles:

A significant reduction in the emission of VOCs from vehicles can be made by using electric cars. By using electric vehicles with hybrid engines, the emission of toxic substances in the atmosphere can be minimized as well.

Concepts Berg

What is photochemical smog and what are its effects?

Photochemical smog is a brown haze formed by NOx, SOx, COx, CH, VOCs, PAN, and many other contaminants. It is formed as a result of the reaction between nitrogen oxides, VOCs, etc with sunlight. It has various effects on the environment such as lungs problem, heart diseases, skin diseases, eye irritation, etc.

What is the main cause of photochemical smog?

The main cause of photochemical smog is primary pollutants such as nitrogen oxides and volatile organic compounds.

Why is photochemical smog harmful?

It is harmful because it contains nitrogen oxides and volatile organic compounds that cause various problems in living organisms The resultant ozone and peroxyacetyl nitrate also damage the climate severely.

Can we make ozone artificially?

Obviously, we can make ozone artificially by electronic devices that generate ozone through chemical reactions.

What are photochemical smog gases?

These gases include NOx, SOx, COx, and VOCs are smog gases. The major sources of such gases are nitrogen oxides, ozone, and nitrogen dioxide because they produce oxygen-free radicals by photocatalytic reactions.

Why smog is named photochemical?

Smog gases are a mixture of different chemicals. These chemicals are toxic to the environment. They are formed by photocatalytic reactions of NOx with contaminants, therefore, named photochemical smog.

Is smog different from the fog?

Smog is somewhat different from fog because smog is a mixture of chemical and particulate matter with smoke and moisture whereas, fog is just water vapors that are suspended in the air.

Reference Books

  • On the Function of Hydrocarbon and Nitrogen Oxides in Photochemical-smog Formation By Basil Dimitriades (Bureau of Mines, US Department of Interior)

Reference links