Photochemical Smog: Types, Difference, Formation & Effects

Photochemical smog is also called summer smog. It is a form of smog that develops when UV light from the sun reacts with the nitrogen oxides in the air. 

• This kind of smog frequently occurs in densely populated cities in hot climates and typically appears as a brown haze.
• In the mornings and afternoons, photochemical haze is most clearly seen. 
• A complicated series of chemical reactions involving sunlight, nitrogen oxides, and volatile organic molecules that are present in the environment as a result of air pollution create a photochemical form. 
• Ozone at ground level and other airborne particles are frequently produced as a result of these interactions. 
• The concentration of primary pollutants in the atmosphere has a direct impact on how photochemical smog develops. 

Key Terms: Photochemical Smog, Pollution, Pollutants, Nitrogen Oxides, Pan, Ozone, Uv Rays, Atmosphere

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What is Photochemical Smog?

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Photochemical smog occurs when:

• The presence of photochemical smog, commonly referred to as "Los Angeles smog," is most noticeable in cities with dense populations of cars. 
• The nitrogen oxides and hydrocarbon vapours that cars release are the cause of this sort of smog. 
• When the sun's ultraviolet rays interact with the atmosphere's nitrogen oxides, photochemical smog is created. 
• It appears as a brown haze and is most noticeable in the morning and late afternoon, particularly in warm, densely populated places. 
• Los Angeles, Sydney, Mexico City, Beijing, and many other cities are among those that often suffer from this haze.

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Types of Smog

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Smog is classified into at least two different categories: sulphurous smog and photochemical smog.

Read Also: Sulphur Dioxide (SO₂): Structure, Uses, Preparation and Properties 

Sulfurous Smog

It is often known as "London smog” that occurs when sulfur-containing fossil fuels, primarily coal is burned, and is characterised by a high level of sulphur oxides in the air. 

• Wetness and a high level of suspended particulate matter in the air worsen this type of smog.
• Brown clouds in the atmosphere that are caused by smog-like air pollution trapped higher in the atmosphere can have an impact on the weather and human health.

Photochemical Smog

The presence of photochemical smog, commonly referred to as "Los Angeles smog," is most noticeable in cities with dense populations of cars.

• The nitrogen oxides and hydrocarbon vapours that are released by vehicles and other sources, which subsequently take part in photochemical reactions in the lower atmosphere, are the primary causes of this type of smog.
• When sunlight is present, nitrogen oxides combine with hydrocarbon vapours to form the highly deadly gas ozone, and nitrogen oxides also react with sunlight to produce some nitrogen dioxide. 
• The following haze causes the atmosphere to turn a light brownish tint, impaired vision, plant damage, eye irritation, and respiratory distress. 
• In urban regions prone to photochemical smog, surface-level ozone concentrations that surpass 70 parts per billion for eight hours or more are considered harmful.

Smog

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Formation of Photochemical Smog

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Three components work together to create photochemical haze. These components include sunshine, hydrocarbons, and nitrogen oxides. 

• Energy facilities that burn fossil fuels produce byproducts like nitrogen oxides and hydrocarbons. 
• Additionally, they could result from organic processes. Cars are the primary source in the first place. 
• Nitrous oxide and nitrogen dioxide separate in the presence of sunshine. They mix after that with traces of hydrocarbons. As a result, this generates a lot of contaminants. 

There are different steps to this process, as follows:

• Nitrogen and oxygen are photo-dissociated as a result of sunlight. As a result, oxygen atoms and ozone are produced.
• After that, oxygen atoms and water start to react. As a result, hydroxyl radicals are produced.
• These hydroxyl radicals oxidise hydrocarbons, generating hydrocarbon radicals as a result.
• After that, hydrocarbons undergo oxidation, which leads to the creation of a group of compounds. Aldehydes are a class of chemicals.
• Both formaldehyde peroxides and aldehyde peroxy acids are produced when aldehyde oxidises. The contaminants that cause the most significant health issues are those that are most notable.

Pollutants That Cause Photochemical Smog

The following list of substances that contribute to photochemical smog in addition to ozone:

• Peroxyacetyl nitrate (PAN) is a contaminant known for irritating the eyes and lungs. When there is significant pollution, it also causes eye-watering.
• In addition to being a poisonous substance, nitrous acid (HNO₂) is also uncomfortable for the respiratory system. Strong acid nitric acid (HNO₃) is the pollutant.
• Additionally, it is a crucial part of acid rain. This can cause skin and eye burns in larger quantities.

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Photochemical Smog Effects

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As a result, there are two different types of smog: photochemical smog and traditional smog. Additionally, compared to ordinary smog, photochemical smog has far more negative impacts. 

• This is due to the oxidising nature of the photochemical smog's constituents. 
• Nitrogen dioxide, trioxygen, and ozone are a few of the typical compounds found in photochemical smog. 
• As a result, it causes corrosion in a variety of materials, including painted surfaces, rubber, and stones as well as open-air metal.
• It causes headaches, dry throat, and chest pain when there is a high concentration of photochemical smog in the air. 
• If not treated promptly, it may result in several respiratory conditions.
• Photochemical smog contains formaldehyde, acrolein, and PAN. 
• The primary cause of photochemical haze is the burning of fossil fuels. 
• When fossil fuels like gasoline and diesel burn, they release hydrocarbons into the atmosphere, which go on to produce ozone, PAN, and other compounds.

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Causes of Smog

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Complex photochemical processes involving very volatile organic molecules produce smog. The haze in a city is caused by automotive exhaust. Factories that manufacture goods using fossil fuels and power plants that burn coal to provide energy also cause smog.

Even everyday goods like plastic food packaging, hairspray, chemical solvents, and paints are responsible for smog.

• NO₂ + sunlight \( \rightarrow\) NO + O
• O + O₂ \( \rightarrow\) O₃
• O₃ + NO \( \rightarrow\) NO₂ + O₂
• NO₂ + R \( \rightarrow\) Forms Products such as PAN

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Difference Between Photochemical and Classical Smog

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The major differences Photochemical Smog and Classical Smog are:

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Things to Remember

• The effect of solar ultraviolet radiation on an atmosphere contaminated with hydrocarbons and nitrogen oxides results in photochemical smog
• Smog contains anthropogenic air pollutants trapped close to the ground by a temperature inversion, primarily ozone, nitric acid, and organic chemicals.
• Pollutants in smog have the potential to harm both plants and people.
• Photochemical smog frequently has an unpleasant smell. Instead of ozone, nitric acid, and organic chemicals,
• The term "smog" (not to be confused with photochemical smog) is frequently used to describe a near-ground haze comprised of a mixture of smoke and fog.
• Nitrogen oxides, a type of air pollution primarily produced by internal combustion engines, appear to be the cause of photochemical smog.
• Incompletely burned hydrocarbons and CO are made to react with a metal catalyst, usually platinum, in catalytic converters, producing CO₂ and water to reduce smog.

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Previous Year Questions

1. The book ‘Silent Spring’ is related to which pollution? [NEET 2015]
2. Sound above what level are considered hazardous? [NEET 2014]
3. Which statement about photochemical smog is incorrect?[NEET 2012]
4. What is Green Chemistry? [NEET 2008]
5. Which of these is not a green house gas? [NEET 2019]
6. Dobsons Units is used to measure thickness of? [NEET 2021]
7. Which of these is a green house gas? [KEAM]
8. Excess nitrate in drinking water can cause? [KEAM]
9. Which is not a green house gas? [KEAM]
10. Which metal causes the water unsuitble for drinking? [JEE MAIN 2019]