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 answer is (c) 2, 3 and 4.

Step 1: Understanding the Statements

The exercise provides four statements, each addressing different environmental phenomena, and asks which combination of them are true. To solve this, we need to understand each statement critically.

Step 2: Analyzing Statement 1

Statement 1 claims that temperature inversion is caused by smog. Temperature inversion actually refers to a meteorological phenomenon where a layer of warm air sits above cooler air, trapping pollutants. It is not directly caused by smog but rather can be a condition that enhances the effects of smog. Thus, this statement is false.

Step 3: Analyzing Statement 2

Statement 2 declares that a product of photochemical smog is PAN (Peroxyacetyl nitrate). Photochemical smog does indeed produce PAN among other chemicals as a result of reactions involving sunlight, hydrocarbons, and oxides of nitrogen. Therefore, this statement is true.

Step 4: Analyzing Statement 3

Statement 3 suggests that \(\mathrm{O}_{3}\) (ozone) is involved in photochemical smog. Ozone is indeed a major component of photochemical smog. It forms when sunlight reacts with pollutants such as nitrogen oxides. So, this statement is true.

Step 5: Analyzing Statement 4

Statement 4 states that the \(\mathrm{O}_{3}\) layer is destroyed by fluorocarbons. This is true; fluorocarbons, specifically CFCs (chlorofluorocarbons), release chlorine atoms in the stratosphere which deplete the ozone layer. Hence, this statement is true.

Step 6: Evaluating the Answer Choices

Now, we know that statements 2, 3, and 4 are true, while statement 1 is false. The correct combination of true statements is (2, 3, and 4), which corresponds to option (c).

Key Concepts

Photochemical Smog

Photochemical smog is a type of air pollution that results from the reaction of sunlight with pollutants. These pollutants, primarily nitrogen oxides ( Ox ) and volatile organic compounds (VOCs), are mostly emitted by vehicles and industrial sources.

• When exposed to sunlight, these chemicals undergo complex photoreactions, leading to the formation of a brownish haze commonly referred to as smog.
• The presence of tertiary pollutants, such as ozone ( O_3 ), adds to the harmful effects of photochemical smog.
• This type of smog is most prevalent in urban areas with considerable vehicular traffic and sunny weather conditions.

Photochemical smog can cause respiratory issues, eye irritation, and reduces visibility. Efforts to minimize its occurrence involve reducing emissions from cars and industrial activities by adopting cleaner technologies.

Ozone Depletion

Ozone depletion refers to the thinning of the ozone layer in the Earth's stratosphere. This layer is crucial for blocking harmful ultraviolet (UV) radiation from reaching the Earth’s surface.

• The main culprits behind ozone depletion are chlorofluorocarbons (CFCs) and other related chemicals.
• These compounds release chlorine and bromine upon degradation in the stratosphere, which then catalyze the breakdown of ozone molecules ( O_3 ) into oxygen ( O_2 ).

A depleted ozone layer leads to increased UV radiation reaching the Earth, which can cause skin cancer, cataracts, and negatively impact ecosystems and agriculture. Steps such as the Montreal Protocol have been significant in curbing the emissions of ozone-depleting substances.

Peroxyacetyl Nitrate

Peroxyacetyl nitrate (PAN) is a significant component in photochemical smog. It is particularly notable for its role as a secondary pollutant formed by the photochemical reactions of precursor substances.

• PAN is formed through reactions involving hydrocarbons, nitrogen oxides, and sunlight.
• Unlike other pollutants, PAN is more stable at lower temperatures, which enables it to travel long distances from its original source.
• While in the atmosphere, PAN acts as a reservoir for nitrogen oxides, gradually releasing them, which can further contribute to ozone formation and smog in areas far away from its origin.

Being a strong eye irritant and harming vegetation, PAN is a concern for both human health and the environment. Mitigating its formation can help in controlling the effects of smog in urban areas.