Formation of Photochemical Smog, 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. In this tutorial, we will discuss followings about photochemical smog.


  • What is a smog?
  • What is photochemical smog?
  • Physical and chemical characteristics (properties) of photochemical smog
  • Contents (chemicals)in the photochemical smog
  • Formation of photochemical smog
    • Automobile Exhausts
    • Stagnated Air Masses
    • Required reasons to photochemical smog
    • Steps of formation of photochemical smog with reactions
  • Effects of photochemical smog to humans and natural environment
  • Controlling the photochemical smog
  • Photochemical Smog and Acid Rain
  • Real photochemical smog cases around the world

What is a smog?

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

  1. Classical smog
  2. Photochemical smog

In this tutorial, we are specifically going to learn about photochemical smog which is considered as a significant environmental pollution factor.

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.

Physical and chemical characteristics (properties) of photochemical smog

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

Contents (chemicals) in the photochemical smog

  • Formaldehyde
  • Acrolein
  • Organic peroxides
  • Organic hydroperoxides
  • Peroxyacyl nitrates
  • Ozone (O3 )
  • Nitrogen dioxide (NO2)

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 gaseous pollutant.

If fuel contains sulfur, sulfur dioxide gas also can be formed as another gaseous pollutant. 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.

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

Required reasons to photochemical smog

There should be high concentration of photochemical oxidant.

Although it should have following environmental conditions.

  • warm area
  • should be dry
  • good sunny climate

Steps of formation of photochemical smog with reactions

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

  2. Ultra violet and visible radiations will brake up NO2 gas into Nitric oxide (NO) and free reactive oxygen atoms (O).
    NO2 break into NO and oxygen atoms

  3. These free radial oxygen atoms are very reactive. Reactive oxygen atoms combine with Oxygen gas molecules (O2) and produce ozone (O3) as the result.
    atomic O and O2 produce ozone O3

  4. Ozone reacts with earlier formed NO molecules and produce NO2 gas.
    NO and O3 reaction

  5. Again NO2 absorbs UV radiations breaks upto NO and O2 molecules 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) as below.

hydrocarbons and ozone

Acrolein and Peroxyacetylnitrate are very noxious substances.

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 to humans and natural environment

  • 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.

Real photochemical smog cases around the world

Photochemical cases are found everywhere in the world and we will list out some cases here for your reference.

Photochemical smog found in Kandy and Colombo, Sri Lanka

Photochemical smog found in Kandy

  • Province: Central
  • Elevation: 500m

Kandy, a world heritage city is known as a very scenic location around the world. However, there are concerns regarding the air pollution in this beautiful city.

Kandy city is covered with a range of mountains and lot of vehicles are entered to the city for official and tourism purposes. Unfortunately, roads and streets are not developed to the current requirement and huge traffics are occurred during the morning, school and evening times. Emissions caused due to the automobiles are stagnated in the City because emissions are poorly transferred to the outer areas due to the covering of mountains. Due to the accumulation of these emissions, Photochemical smog is occurred when other conditions are met. There were several experiments done by universities to observe air pollution in Kandy, they have confirmed that air quality is not upto the standard values in long traffics.

Related tutorials to photochemical smog and environmental pollutions


Difference between photochemical smog and classical smog?

Photochemical smog and classical smog are different consequences according to the different aspects such as environment conditions, auttomobile running and industrial emittings.

  • Classical smog is formed in a cold and humid climate. As chemicals, smoke, fog and sulfur dioxide are formed when cold and humid climate is avialble.