Formation of Photochemical Smog in Cities, Toxic Compounds, Effects

Photochemical smog exists in big cities with large quantities of exhausts from automobile, factories and more. These exhaust gases produce substances such as ozone (O3), aldehyde and peroxyacyl nitrates(PAN) which are harmful for humans, animals and plants in various ways.

What is a smog?

Smog is a mixture id smoke, dust particles and small drops of fog. Smog is the major pollutant in large cities. Smog is categorized to two sections.

  1. Classical smog
  2. Photochemical smog

In this tutorial, we are going to learn more about photochemical smog.

What is photochemical smog?

Photochemical smog is formed by the combination of smoke, dust and fog with an air pollutant in the atmosphere as a result of a photochemical smog.


  • It is chemically oxidizing because it has oxidants in higher concentrations.
  • Brown hazy irritating fumes

Requirements to formation of photochemical smog

Warm, dry and sunny climate is required to form a photochemical smog. Also it needs an atmosphere which is loaded with large quantities of automobile exhausts and stagnated air masses.

Automobile Exhausts

Automobile exhausts contain carbon dioxide as the main gas component after the combustion. Due to high temperature inside the engine, oxides of nitrogen (mainly NO and NO2) may be produced. Nitric oxide is a major

If fuel contains sulfur sulfur dioxide also can be formed. Like that, due to incomplete combustion, carbon monoxide and unburnt hydrocarbons (in fuel) are also given as the products.

Nitrogen dioxide and hydrocarbons are the primary precursors of photochemical smog.

Stagnated Air Masses

If a city is rounded from mountains, exhaust gases are unable to leave the surrounded area. We called this air "stagnated air". Stagnated air helps to increase the photochemical smog.

Contents in the photochemical smog

  • formaldehyde
  • acrolein
  • organic peroxides
  • organic hydroperoxides
  • peroxyacyl nitrates
  • ozone (O3 )
  • nitrogen dioxide (NO2 )

Los Angeles Smog and Photochemical Smog

Photochemical smog was observed in cities which have warm, sunny and dry climates such as Los Angeles and Denver in USA. So this was earlier called Los Angelis Smog.

Brown Air Cities

Due to existing characteristic brown haze, the cities such as Los Angeles are sometimes called as brown air cities

Reasons to photochemical smog

There should be high concentration of photochemical oxidant.

Although it should have following conditions

  • warm area
  • should be dry
  • good sunny climate

Formation of photochemical smog

  1. Automobiles exhaust emits carbon monoxide (CO) , unburned hydrocarbons and oxides of nitrogen. Nitrogen monoxide (NO) reacts with oxygen (O2) and produce brown colour nitrogen dioxide (NO2) easily.

  2. Ultra violet and visible radiations will brake up NO2 into NO and free reactive oxygen atoms (O).
    NO2 break into NO and oxygen atoms
  3. Reactive oxygen atoms combine with O2 and produce ozone( O3 ).
    atomic O and O2 produce ozone O3

  4. Ozone reacts with NO and produce NO2
    NO and O3 reaction

  5. Again NO2 absorbs UV radiations breaks upto NO and O2. and continue the cycle.
    NO2 and O3 are strong oxidizing agents and they can react with unburnt hydrocarbons.
    Those reaction will start a chain reaction and produce formaldehyde, acrolein, organic peroxides, organic hydroperoxides, peroxyacyl nitrates(PAN).
    hydrocarbons and ozone

    Acrolein and Peroxyacetylnitrate are very noxious substances.
    acrolein_photochemical_smog Peroxyacetylnitrate (PAN) photochemical smog

    Peroxyacetylnitrate is formed from aldehydes. This reaction is started by hydroxyl radicals.
    OH. is formed by reaction between atomic oxygen and water.
    formation of hydroxyl radicals

    PAN is formed due to presence OH..
    formation of PAN photochemical smog

Effects of photochemical smog

  • Ozone can cause coughing, wheezing, bronchial constriction and irritation to the respiratory mucous system.
  • Aldehyde and PAN cause to irritation of eyes.
  • Reduces visibility
  • Cause to several chronic diseases of eyes, heart, and lungs.
  • Affects to the human health and comfort.
  • Affects to growth of plants. O3 damages to plants and reduces crop. PAN has also highest toxicity to plants. It attacks younger leaves and causes bronzing and glazing of their surfaces.
  • Materials are also affected by photochemical smog. As an example, Rubber is cracked.

Controlling the photochemical smog

There are so many researches were conducted and are conducting in the world to reduce this photochemical smog. In this section, we are going to discuss some actions we can take to control or reduce the effect of photochemical smog.

Catalytic Converters

Catalytic converters are installed to reduce emission of nitrogen oxide and hydrocarbons. These catalytic converters will convert nitrogen oxide to nitrogen. Hydrocarbons will be converted to carbon dioxide and water.

Plant Growing

Some plants, pinus, Juniparus, Quercus, Pyrus and Vitis can metabolism nitrogen dioxide (NO2)

Trap Free Radicals

Some compounds which are capable of trapping free radicals in the photochemical smog are applied. These added compounds generate free radicals which combine with free radicals precursors of photochemical smog.

Photochemical Smog and Acid Rain

There are common constituents in acid rains and photochemical smog. Nitrogen dioxide and sulfur dioxide gases are the reasons for acidic rains. Also nitrogen dioxide initiate the photochemical smog. Difference is, for acid rains, hydrocarbons are not required.