What is photochemical smog? Explain its system of formation with the help of chemical equations.

Points to Remember:

• Photochemical smog is a type of air pollution.
• It forms through a complex series of chemical reactions involving sunlight, nitrogen oxides, and volatile organic compounds (VOCs).
• It has significant negative impacts on human health and the environment.
• Control strategies focus on reducing emissions of precursor pollutants.

Introduction:

Photochemical smog, also known as Los Angeles-type smog, is a complex mixture of air pollutants formed by the interaction of sunlight with nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere. Unlike classical smog (sulfurous smog), which is primarily composed of sulfur dioxide and particulate matter, photochemical smog is characterized by the presence of ozone (O3), peroxyacetyl nitrate (PAN), aldehydes, and other secondary pollutants. The formation of photochemical smog is a significant environmental concern, impacting air quality and public health globally. The World Health Organization (WHO) consistently reports high levels of ozone in many urban areas worldwide, directly linked to photochemical smog.

Body:

1. Precursor Pollutants:

The formation of photochemical smog begins with the emission of precursor pollutants:

• Nitrogen Oxides (NOx): Primarily emitted from vehicle exhausts, power plants, and industrial processes. NOx includes nitric oxide (NO) and nitrogen dioxide (NO2).
• Volatile Organic Compounds (VOCs): These are organic chemicals that easily evaporate at room temperature. Sources include vehicle exhaust, industrial solvents, paints, and natural sources like trees. Examples include methane (CH4), butane (C4H10), and benzene (C6H6).

2. Photochemical Reactions:

Sunlight plays a crucial role in initiating and driving the chemical reactions that lead to smog formation. The process can be summarized in several key steps:

• NO2 Photodissociation: Sunlight (UV radiation) breaks down nitrogen dioxide (NO2) into nitric oxide (NO) and a highly reactive oxygen atom (O).
  NO2 + hv → NO + O
  (where hv represents a photon of light)

• Ozone Formation: The oxygen atom (O) reacts with molecular oxygen (O2) to form ozone (O3).
  O + O2 → O3

• VOC Oxidation: Ozone and other reactive species (like hydroxyl radicals, OH•) react with VOCs, leading to the formation of a variety of secondary pollutants, including aldehydes, ketones, and peroxyacetyl nitrate (PAN). A simplified example with methane:
  CH4 + OH• → CH3• + H2O
CH3• + O2 → CH3OO•
… (further reactions leading to the formation of formaldehyde and other products)

• PAN Formation: PAN is formed through reactions involving peroxy radicals and nitrogen oxides. The exact mechanism is complex, but a simplified representation is:
  CH3COO• + NO2 → CH3COOONO2 (PAN)

3. Impact of Photochemical Smog:

Photochemical smog has several detrimental effects:

• Human Health: Ozone is a respiratory irritant, causing coughing, wheezing, and reduced lung function. PAN also irritates the eyes and respiratory system. Exposure to high levels of photochemical smog can exacerbate respiratory illnesses like asthma and bronchitis.
• Environmental Damage: Ozone damages vegetation, reducing crop yields and harming forests. It also contributes to acid rain and can damage materials like rubber and fabrics.
• Visibility Reduction: The pollutants in photochemical smog scatter sunlight, reducing visibility and creating hazy conditions.

Conclusion:

Photochemical smog is a significant air pollution problem resulting from the complex interaction of sunlight and precursor pollutants like NOx and VOCs. The formation involves a series of photochemical reactions leading to the production of ozone, PAN, and other harmful secondary pollutants. These pollutants have severe impacts on human health, the environment, and visibility. To mitigate photochemical smog, strategies must focus on reducing emissions of NOx and VOCs through stricter vehicle emission standards, promoting cleaner energy sources, and implementing effective industrial emission controls. A holistic approach involving technological advancements, public awareness campaigns, and strong regulatory frameworks is crucial to ensure cleaner air and a healthier environment, upholding the constitutional right to a healthy environment. Investing in sustainable transportation systems and promoting green technologies are vital steps towards achieving this goal.