Animated Solution for Chemistry - Environmental Chemistry: Assertion (A) Ozone is destroyed by CFCs in the upper stratosphere. Reason (R) Ozone holes increase the amount of UV radiation reaching the earth.

Our Earth is protected by a fragile, invisible shield in the stratosphere known as the ozone layer. This layer is crucial for life because it absorbs the majority of the Sun's harmful ultraviolet (UV) radiation. However, human activities have introduced synthetic chemicals into the atmosphere that threaten this delicate balance. Let's break down the chemistry behind this environmental crisis by analyzing the given Assertion and Reason.

The Chemistry of Depletion

The Assertion states: Ozone is destroyed by CFCs in the upper stratosphere.

Chlorofluorocarbons (CFCs), such as Freon-12 ($C{F}_{2}C{l}_2$), are incredibly stable and unreactive in the lower atmosphere (troposphere). Because of their stability, they slowly drift upward over many years until they reach the upper stratosphere.

Once there, they are exposed to intense, high-energy UV radiation from the Sun. This radiation is powerful enough to break the carbon-chlorine bonds in the CFC molecules through a process called photolysis:

This reaction releases highly reactive chlorine free radicals ($\dot{Cl}$). These radicals act as catalysts in the destruction of ozone. A single chlorine radical reacts with an ozone molecule to form a chlorine monoxide radical and oxygen gas:

The $\dot{ClO}$ radical can then react with atomic oxygen to regenerate the $\dot{Cl}$ radical, creating a devastating chain reaction. Because of this, a single chlorine atom can destroy up to 100,000 ozone molecules before it is removed from the stratosphere. Therefore, the Assertion is absolutely correct.

The Consequences of Depletion

The Reason states: Ozone holes increase the amount of UV radiation reaching the earth.

When the ozone layer is severely depleted, it forms what we commonly refer to as an "ozone hole" (most notably over Antarctica). An ozone hole is not a literal hole in the sky, but rather a region where the concentration of ozone is exceptionally low.

Because the primary function of ozone is to absorb UV-B and UV-C radiation, a thinner ozone layer means less absorption. Consequently, a significantly higher amount of dangerous UV radiation penetrates the atmosphere and reaches the Earth's surface. This increased exposure can lead to severe health and environmental issues, including skin cancer, cataracts, and damage to marine phytoplankton. Thus, the Reason is also a factually correct statement.

Cause vs. Effect

Now, we must determine if the Reason correctly explains the Assertion.

- The Assertion describes the cause of ozone depletion (the chemical action of CFCs). - The Reason describes the effect or consequence of that depletion (increased UV radiation reaching the Earth).

Logically, an effect cannot explain its cause. The fact that more UV radiation reaches the Earth does not explain why CFCs destroy ozone. To correctly explain the Assertion, the Reason would need to discuss the photolysis of CFCs and the release of chlorine radicals.

Therefore, both the Assertion and the Reason are correct statements, but the Reason is not the correct explanation for the Assertion.