Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are true? (1) Temperature inversion is caused by smog. (2) A product of photochemical smog is PAN. (3) \(\mathrm{O}_{3}\) is involved in photochemical smog. (4) \(\mathrm{O}_{3}\) layer is destroyed by fluorocarbons. (a) 1,2 and 3 (b) 1,2 and 4 (c) 2,3 and 4 (d) 1,3 and 4

Step-by-Step Solution

Answer

Option (c) 2, 3, and 4 are true.

Step 1: Understanding Statement 1

To determine if 'Temperature inversion is caused by smog' is true, we examine environmental science concepts. Temperature inversion occurs when a layer of warm air traps a layer of cooler air below it, often caused by geographic and climatic conditions rather than smog itself. Thus, this statement is false.

Step 2: Analyzing Statement 2

The statement 'A product of photochemical smog is PAN' refers to Peroxyacetyl nitrate (PAN), a known component formed during photochemical smog events involving hydrocarbons and nitrogen oxides with sunlight. Therefore, this statement is true.

Step 3: Evaluating Statement 3

The statement 'O3 is involved in photochemical smog' considers that ozone ( O_3 dot{O}_3) is indeed a major constituent of photochemical smog, formed by the reaction between sunlight and pollutants like nitrogen oxides and volatile organic compounds. This statement is true.

Step 4: Investigating Statement 4

For the statement ' O_3 layer is destroyed by fluorocarbons', it is established that fluorocarbons, specifically chlorofluorocarbons (CFCs), are responsible for ozone layer depletion in the stratosphere. Hence, this statement is true.

Step 5: Concluding Valid Statement Combinations

After evaluating the truth of each statement, statements 2, 3, and 4 are correct. Therefore, the correct option is (c) 2, 3, and 4.

Key Concepts

Temperature Inversion

Temperature inversion is a fascinating weather phenomenon where the normal temperature arrangement of the atmosphere is flipped. Instead of air getting cooler with height, a layer of warm air rests atop cooler air.
This can lead to several issues:

• Pollutants like smoke and exhaust gases can get trapped close to the ground, leading to increased pollution levels.
• The presence of trapped pollutants can create or worsen smog conditions.

Temperature inversions can occur due to geographic locations, such as valleys where cold air can settle, or due to certain weather conditions, like on a clear night with little wind. Hence, rather than being caused by smog, inversions can influence how pollution and smog behave.

Ozone Layer Depletion

The ozone layer in the stratosphere is crucial for life as it absorbs most of the sun's harmful ultraviolet radiation. However, human activities have led to its depletion, primarily through the emission of chlorofluorocarbons (CFCs), which are used in:

• Refrigerants
• Aerosols
• Foam-blowing agents

When CFCs reach the upper atmosphere, they release chlorine atoms through photodissociation, and these chlorine atoms catalyze the breakdown of ozone ( O_3 ), leading to thinning of the ozone layer.
This thinning increases the risk of skin cancer and cataracts in humans and can harm marine ecosystems by affecting plankton, a crucial part of the aquatic food chain.

Peroxyacetyl Nitrate (PAN)

Peroxyacetyl nitrate (PAN) is a significant secondary pollutant formed during photochemical smog episodes. When hydrocarbons react with nitrogen oxides (NOx) in the presence of sunlight, they produce PAN.
PAN is troublesome because:

• It is a respiratory irritant, affecting human health by exacerbating conditions like asthma.
• It can damage crops and vegetation, leading to reduced agricultural productivity.

PAN's ability to travel long distances in the atmosphere also means it can spread pollution across regions, affecting areas far from its original source.

Nitrogen Oxides

Nitrogen oxides (NOx) are a group of gases composed of nitrogen and oxygen, commonly emitted from vehicles, power plants, and industrial processes. These gases play a crucial role in the formation of photochemical smog:

• When nitrogen oxides react with volatile organic compounds (VOCs) in the presence of sunlight, they lead to the creation of ground-level ozone ( O_3 ), a major component of smog.
• NOx can contribute to acid rain, which can harm ecosystems and deteriorate buildings.

Reducing NOx emissions is crucial for improving air quality and decreasing the severity of smog in urban areas.

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons (CFCs) are man-made compounds traditionally used in applications such as air conditioning, refrigeration, and aerosol sprays. While stable and non-reactive in the lower atmosphere, CFCs release chlorine when they reach the stratosphere, disrupting the ozone layer.
This process happens because:

• The UV radiation breaks down CFC molecules, freeing chlorine atoms.
• Chlorine atoms then interact with ozone ( O_3 ), resulting in ozone molecules' destruction.

Efforts like the Montreal Protocol have been successful in phasing out the use of CFCs, leading to a slow but steady recovery of the ozone layer.