Question

The pollutants which come directly in the air from sources are called primary pollutants. Primary pollutants are sometimes converted into secondary pollutants. Which of the following belongs to secondary air pollutants? (A) \(\mathrm{CO}\) (B) Hydrocarbon (C) Peroxyacetyl nitrate (D) \(\mathrm{NO}\)

Short answer

(C) Peroxyacetyl nitrate is a secondary air pollutant because it is formed during the photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (\(\mathrm{NO_x}\)) in the presence of sunlight, rather than being directly emitted into the atmosphere.

Step-by-step solution

Understanding Primary and Secondary Air Pollutants

Primary air pollutants are substances that are emitted directly into the atmosphere from their source. Examples of primary pollutants are carbon monoxide (\(\mathrm{CO}\)), nitrogen oxides (\(\mathrm{NO_x}\)), and hydrocarbons. Secondary pollutants, on the other hand, are not directly emitted but are formed when primary pollutants react with components of the atmosphere. An example of a secondary pollutant is ozone (O3). Now that we know the difference between primary and secondary air pollutants, we can move on to evaluate the given options.

Analyzing the options and identifying the secondary air pollutant

(A) \(\mathrm{CO}\) - Carbon monoxide is directly emitted from combustion processes, such as engines and industrial activities. Therefore, it is a primary air pollutant. (B) Hydrocarbon - Hydrocarbons are organic compounds made up of hydrogen and carbon atoms. They are emitted directly from natural sources (such as vegetation) and anthropogenic sources (such as fossil fuel combustion and industrial processes). Hence, they are primary air pollutants. (C) Peroxyacetyl nitrate - Peroxyacetyl nitrate (PAN) is a powerful oxidant and is not directly emitted into the atmosphere but formed during the photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (\(\mathrm{NO_x}\)) in the presence of sunlight. Since it is not emitted directly but formed through reactions in the atmosphere, it is a secondary air pollutant. (D) \(\mathrm{NO}\) - Nitric oxide is a nitrogen oxide that is directly emitted into the atmosphere during combustion processes, such as engines, power plants, and industrial activities. Therefore, it is a primary air pollutant. In conclusion, the correct answer is: (C) Peroxyacetyl nitrate

Key concepts

Primary Pollutants

Primary pollutants are pollutants that are directly emitted into the atmosphere from a specific source. These can be natural sources, like volcanoes and wildfires, or human-made sources, like vehicles and industrial facilities.
These pollutants include substances such as:

• Carbon monoxide (CO): A colorless, odorless gas resulting from the incomplete combustion of carbon-containing fuels.
• Nitrogen oxides (NOx): Gases produced from vehicle emissions, power generation, and other combustion processes.
• Hydrocarbons: Compounds consisting solely of hydrogen and carbon atoms, released from both natural vegetation and the burning of fossil fuels.

These pollutants can affect air quality, human health, and the environment. They also serve as building blocks for secondary pollutants by undergoing further chemical reactions in the atmosphere.

Peroxyacetyl Nitrate

Peroxyacetyl nitrate, often abbreviated as PAN, is a well-known secondary air pollutant. Produced not by direct emission but through complex chemical reactions in the atmosphere, PAN plays a significant role in air pollution and smog formation.
PAN is formed when volatile organic compounds (VOCs) react with nitrogen oxides (NOx) in the presence of sunlight.
Here's how it forms:

• VOC emissions, from sources like industrial activities and motor vehicles, mix with NOx in the atmosphere.
• Under sunlight, these compounds undergo photochemical reactions, leading to the creation of various oxidants, including peroxyacetyl nitrate.

PAN acts as a potent eye irritant and can affect respiratory function. It also has the ability to travel long distances, spreading pollution far from its original source.

Additionally, PAN is part of the larger photochemical smog complex, contributing to the overall air quality problems in urban areas.

Photochemical Reactions

Photochemical reactions are chemical reactions that occur in the atmosphere due to the presence of sunlight. These reactions play a key role in the formation of secondary pollutants like ozone and peroxyacetyl nitrate.
The process starts when sunlight provides energy to drive chemical reactions between atmospheric components. Here’s what typically occurs:

• Emissions of primary pollutants such as NOx and hydrocarbons are released into the atmosphere.
• Sunlight serves as a catalyst, breaking down these pollutants into reactive radicals.
• These radicals then undergo further reactions, leading to the creation of secondary pollutants like ozone and PAN.

It's essential to understand that without sunlight, these reactions would not occur, hence their prevalence during sunny days.
Photochemical reactions are often responsible for urban smog, a major environmental and health concern affecting large populations.