Question

The secondary precursors of photochemical smog are (a) \(\mathrm{SO}_{2}\) and \(\mathrm{NO}_{2}\) (b) \(\mathrm{SO}_{2}\) and hydrocarbons (c) \(\mathrm{NO}_{2}\) and hydrocarbons (d) \(\mathrm{O}_{3}\) and \(\mathrm{PAN}\)

Short answer

The correct option is (d) O3 and PAN.

Step-by-step solution

- Understanding Photochemical Smog

Photochemical smog is a type of air pollution formed from the reaction between sunlight and pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs). It results in the formation of ozone (O3) and other compounds.

- Identifying Secondary Precursors

Secondary precursors are compounds formed from the primary pollutants through chemical reactions in the atmosphere. For photochemical smog, these secondary precursors include ozone (O3) and peroxyacetyl nitrate (PAN).

- Matching the Options

Compare the options with the identified secondary precursors. The correct answer must include ozone (O3) and peroxyacetyl nitrate (PAN).

Key concepts

Secondary Precursors

Secondary precursors are chemicals that form in the atmosphere after the primary pollutants undergo chemical reactions. Primary pollutants, like nitrogen oxides (NOx) and volatile organic compounds (VOCs), are released directly into the air through various sources such as vehicle emissions and industrial activities. These primary pollutants react in the presence of sunlight to create secondary pollutants.
In the case of photochemical smog, the main secondary precursors are ozone (O3) and peroxyacetyl nitrate (PAN).

• Ozone (O3) is a harmful gas found at ground level, known for its role in smog formation.
• Peroxyacetyl nitrate (PAN) is a toxic compound that irritates the eyes and respiratory system and acts as a reservoir for NOx.

Understanding the transformation from primary to secondary pollutants is key to grasping the mechanisms behind air pollution and photochemical smog.

Ozone Formation

Ozone (O3) is a major component of photochemical smog and forms through a series of complex reactions. Initially, sunlight breaks down nitrogen dioxide (NO2) into nitrogen monoxide (NO) and a free oxygen atom (O).
The free oxygen atom then combines with molecular oxygen (O2) to produce ozone (O3).
Slowly and continuously, these reactions contribute to the accumulation of ozone near Earth's surface, especially during sunny days.
This ground-level ozone differs from the ozone found in the stratosphere, which plays a crucial role in blocking harmful ultraviolet radiation.

• Ground-level ozone acts as a pollutant with adverse health effects.
• It's a key ingredient in smog.

Controlling precursor emissions like NOx and VOCs is essential to reducing ground-level ozone.

Peroxyacetyl Nitrate

Peroxyacetyl nitrate (PAN) is another significant secondary pollutant in photochemical smog. It forms when nitrogen oxides (NOx) react with volatile organic compounds (VOCs) in the presence of sunlight.

PAN is particularly concerning due to its stability and ability to travel long distances. It can remain in the atmosphere for days, spreading pollution far from its source.

• PAN is a strong eye irritant and causes respiratory issues.
• It also acts as a reservoir for nitrogen dioxide (NO2), releasing it slowly and sustaining the smog formation process.

Monitoring and regulating emissions that contribute to PAN formation are crucial for improving air quality and mitigating photochemical smog.