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

The correct option is (c) 2, 3, and 4.

Step 1: Understanding the Statements

To solve the exercise, we need to evaluate each statement individually and determine whether they are true. This requires understanding the scientific concepts related to each statement.

Step 2: Evaluation of Statement 1

The statement 'Temperature inversion is caused by smog' is inaccurate. Temperature inversion is a meteorological phenomenon where a layer of warm air traps pollutants close to the ground, including smog, rather than being caused by it. Thus, this statement is false.

Step 3: Evaluation of Statement 2

The statement 'A product of photochemical smog is PAN' is true. PAN, or peroxyacetyl nitrate, is indeed a component of photochemical smog and is formed from the reaction of pollutants like hydrocarbons and nitrogen oxides under sunlight.

Step 4: Evaluation of Statement 3

The statement '\(\mathrm{O}_{3}\) is involved in photochemical smog' is true. Ozone (\(\mathrm{O}_{3}\)) is a major component of photochemical smog, formed from the reactions of sunlight with pollutants like hydrocarbons and nitrogen oxides.

Step 5: Evaluation of Statement 4

The statement '\(\mathrm{O}_{3}\) layer is destroyed by fluorocarbons' is true. Fluorocarbons, particularly chlorofluorocarbons (CFCs), are known to break down ozone molecules in the ozone layer, causing depletion.

Step 6: Conclusion

With the evaluation of each statement, statements 2, 3, and 4 are true. Therefore, the correct option is (c) 2, 3, and 4.

Key Concepts

Temperature Inversion

Temperature inversion is an interesting atmospheric occurrence. Under normal conditions, air temperature usually decreases with height. However, during a temperature inversion, a warm air layer sits above cooler air near the ground. This reversal, or 'inversion', can result in fascinating and sometimes concerning environmental effects.

In typical scenarios, pollutants would disperse due to rising warm air. But when an inversion happens, the warm air layer acts as a blanket, trapping pollutants like smog near the ground. This can lead to decreased air quality and potential health hazards.

• Traps pollutants close to the ground
• Occurs when warm air overlays cooler ground air
• Can worsen air quality in urban areas

Identifying this phenomenon helps in understanding pollution patterns and preparing for its effects.

Ozone Layer Depletion

Ozone layer depletion is a critical environmental challenge we face today. The ozone layer, found in the Earth’s stratosphere, plays a vital role in protecting life by absorbing most of the sun's harmful ultraviolet radiation. When this layer becomes depleted, more UV radiation reaches the Earth's surface.

One primary cause of ozone depletion is the presence of man-made chemicals known as chlorofluorocarbons (CFCs). When these chemicals reach the stratosphere, they undergo reactions that release chlorine atoms. These chlorine atoms then interact with ozone ( O_3 ) molecules, breaking them apart.

• Protects Earth from UV radiation
• Depleted by CFCs releasing chlorine
• Increased UV can cause skin cancer and other health issues

Monitoring and regulating substances that harm the ozone layer continues to be essential for environmental health.

Peroxyacetyl Nitrate (PAN)

Peroxyacetyl nitrate, more commonly known as PAN, is a significant component of photochemical smog. It is formed when hydrocarbons and nitrogen oxides react under sunlight. PAN is particularly interesting because of its role in air pollution and its effects on the environment.

PAN is known for being a strong eye and respiratory irritant and can damage vegetation. This compound is stable in colder conditions, which enables it to travel over long distances before breaking down. This longevity makes PAN a crucial factor to consider in regional air quality management.

• Formed from hydrocarbons and nitrogen oxides
• Stable and can travel far
• Causes eye irritation and damages plants

Understanding the behavior and impact of PAN helps in devising strategies to reduce photochemical smog.

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons, or CFCs, are synthetic compounds once widely used in industry, particularly as refrigerants, propellants in aerosol sprays, and solvents. They gained infamy due to their destructive effects on the ozone layer.

When CFCs are released into the atmosphere, they gradually make their way up to the stratosphere. There, they encounter ultraviolet (UV) light, which breaks them down, releasing chlorine atoms. These chlorine atoms are capable of destroying thousands of ozone molecules, leading to significant thinning of the ozone layer.

• Used in refrigeration and aerosols
• Break down under UV light in the stratosphere
• Release chlorine that depletes ozone

Efforts such as the Montreal Protocol aim to control the production and release of CFCs to protect the ozone layer.