Understanding CFCs and the Stratosphere

The question asks us to evaluate two statements concerning Chlorofluorocarbons (CFCs) and their behavior in the atmosphere, specifically their interaction with the stratosphere.

Let's break down each statement:

Assertion (A) Analysis: CFCs Penetrate the Stratosphere

The assertion states that CFCs are able to penetrate the stratosphere.

• CFCs are anthropogenic chemicals, meaning they are created by human activity.
• They were widely used in refrigerants, aerosols, and foam blowing agents.
• When released, CFCs enter the lowest layer of the atmosphere, the troposphere.
• Unlike many other chemicals released at ground level, CFCs are not easily broken down or removed in the troposphere.
• Over long periods, these stable molecules can gradually mix and move upwards through the atmosphere.
• This slow process eventually allows a significant amount of CFCs to reach the stratosphere, which is the atmospheric layer above the troposphere.

Based on scientific understanding, Assertion (A) is true.

Reason (R) Analysis: CFCs are Highly Unreactive

The reason states that CFCs are highly unreactive.

• In the lower atmosphere (troposphere), CFC molecules are remarkably stable.
• This stability is due to the strong chemical bonds between the carbon, chlorine, and fluorine atoms within the CFC molecule.
• They do not readily react with other chemicals, are not easily dissolved in water, and are not broken down by visible or UV light in the troposphere.

Based on scientific understanding, Reason (R) is also true.

Connecting Assertion (A) and Reason (R)

Now let's consider if Reason (R) is the correct explanation for Assertion (A).

• Assertion (A) says CFCs reach the stratosphere.
• Reason (R) says CFCs are highly unreactive.
• The key to CFCs reaching the stratosphere is their persistence in the lower atmosphere. If they were reactive, they would break down or be removed before they had a chance to travel high enough to reach the stratosphere.
• Their high unreactivity in the troposphere means they have a very long atmospheric lifetime (decades or even centuries). This long lifetime allows them to eventually diffuse upwards into the stratosphere.
• Therefore, the high unreactivity of CFCs is precisely why they are able to survive the journey through the troposphere and penetrate the stratosphere.

Reason (R) provides a correct explanation for Assertion (A).

Conclusion

Both the Assertion (A) and the Reason (R) are correct statements. Furthermore, the unreactivity of CFCs (R) is the primary reason why they are able to persist in the atmosphere and reach the stratosphere (A).

Revision Table: CFCs and Stratosphere

Additional Information: CFCs and Ozone Depletion

While CFCs are unreactive in the troposphere, their behavior changes once they reach the stratosphere.

• In the stratosphere, CFCs are exposed to intense ultraviolet (UV) radiation from the sun.
• This high-energy UV radiation can break the strong carbon-chlorine bonds in CFC molecules, releasing free chlorine atoms ($\text{Cl}$).
• The released chlorine atoms are highly reactive and act as catalysts in the destruction of stratospheric ozone ($\text{O}_3$). The reaction sequence is simplified as follows:
  • $\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$
  • $\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2$
• In this cycle, the chlorine atom is regenerated, meaning a single chlorine atom can destroy thousands of ozone molecules before being removed from the stratosphere.
• The stratospheric ozone layer is crucial because it absorbs most of the sun's harmful UV-B radiation, protecting life on Earth. Depletion of this layer leads to increased UV-B radiation reaching the surface, which can cause skin cancer, cataracts, and harm to ecosystems.
• The Montreal Protocol is an international treaty designed to phase out the production and consumption of ozone-depleting substances like CFCs.