Question

Photochemical oxidants such as PAN and PBN are formed: (a) By the action of hydrogen sulphide on hydrocabons in the presence of sunlight (b) By the action of carbon dioxide on hydrocarbons in the presence of sunlight (c) By the action of nitrogen oxides on hydrocarbons in the presence of sunlight (d) None of these

Short answer

Option (c) is the correct answer.

Step-by-step solution

Identify photochemical oxidants

Photochemical oxidants are pollutants formed by the reaction of sunlight with other chemicals in the atmosphere. Common examples include peroxyacetyl nitrate (PAN) and peroxybenzoyl nitrate (PBN), which are secondary pollutants often found in photochemical smog.

Review the reaction components

Consider the possible reactants involved in the formation of photochemical oxidants. These are typically formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs), such as hydrocarbons, in the presence of sunlight.

Evaluate each option

- (a) Hydrogen sulphide is not involved in the formation of these oxidants. - (b) Carbon dioxide does not react with hydrocarbons in this photochemical process. - (c) Nitrogen oxides react with hydrocarbons in the presence of sunlight to form photochemical oxidants like PAN and PBN. - (d) Option (c) is correct, therefore this option is incorrect.

Select the correct option

The formation of PAN and PBN, which are types of photochemical oxidants, results from the reaction of nitrogen oxides with hydrocarbons in the presence of sunlight. The correct choice is (c).

Key concepts

PAN (peroxyacetyl nitrate)

PAN is a significant photochemical oxidant that is part of urban air pollution, especially in smog. It is a secondary pollutant, which means it is not directly emitted but forms through chemical reactions in the atmosphere.
PAN is produced when nitrogen oxides (NOx) and volatile organic compounds (VOCs) such as hydrocarbons react in the presence of sunlight. This process is part of the complex chemistry of smog formation.

• PAN contributes to eye irritation and respiratory problems.
• It is also responsible for damaging vegetation and agricultural crops.
• Its stability allows it to be transported far from its original source of emission, making it a regional pollution concern.

PBN (peroxybenzoyl nitrate)

PBN is another kind of photochemical oxidant similar to PAN but is less commonly discussed. It shares a similar formation process, arising from interactions involving nitrogen oxides and hydrocarbons in sunlight.
PBN's effects and concerns are very akin to those of PAN. It can cause eye irritation, affect lung function, and harm vegetative health.
Understanding the chemistry of PBN helps in comprehending the broader challenges of controlling smog and minimizing its health impacts.

• PBN contributes to the complex chemical mixtures found in polluted urban areas.
• Like PAN, it is considered a secondary pollutant with significant environmental impact.

Photochemical Smog

Photochemical smog is a type of air pollution created when sunlight reacts with pollutants such as volatile organic compounds and nitrogen oxides. These reactions result in a cocktail of secondary pollutants, including oxidants like ozone and PAN.

• It typically appears as a brownish haze in urban areas.
• Key components contributing to smog formation include anthropogenic emissions from vehicles and industrial processes.
• Smog poses significant health risks, such as respiratory diseases and reduced lung function, alongside environmental hazards like reduced visibility and plant damage.

Managing photochemical smog requires reducing emissions of precursor pollutants and enhancing public awareness about pollution control.

Nitrogen Oxides

Nitrogen oxides (NOx) are critically involved in forming photochemical oxidants. NOx refers to any of several compounds composed of nitrogen and oxygen, including nitric oxide (NO) and nitrogen dioxide (NO2).
They are primarily released into the atmosphere from burning fossil fuels, such as in vehicle emissions and power plants.

• NOx plays a vital role in the atmospheric reactions leading to ozone and photochemical oxidant formation.
• Controlling NOx emissions is key to reducing urban smog and improving air quality.

Effective strategies to reduce NOx include using cleaner energy sources, improving industrial processes, and enforcing stricter vehicular emissions standards.

Hydrocarbons

Hydrocarbons are organic compounds made up of hydrogen and carbon atoms. These volatile organic compounds (VOCs) are essential in the formation of photochemical oxidants when they react with nitrogen oxides in sunlight.
Sources of hydrocarbons include vehicle emissions, industrial discharges, and even natural sources like trees.

• Reducing hydrocarbon emissions is important to mitigate smog formation and improve air quality.
• Efforts include enhancing fuel efficiency, optimizing industrial processes, and adopting sustainable practices.

Understanding both anthropogenic and natural sources of hydrocarbons helps in developing comprehensive approaches for air pollution control.