The depletion of the ozone layer in the stratosphere is mainly caused by the release of which substances?

Introduction / Context:
The ozone layer in the stratosphere protects life on Earth by absorbing harmful ultraviolet radiation from the Sun. In the late twentieth century, scientists observed significant thinning of this layer, often described as ozone holes. Understanding which human made substances are primarily responsible for this depletion is important in environmental chemistry and public awareness. This question tests your knowledge of the key chemical culprits.

Given Data / Assumptions:

• The focus is on ozone depletion in the upper atmosphere.
• Options include nitrous oxide, carbon dioxide, chlorofluorocarbons, methane, and sulfur dioxide.
• We consider their roles based on scientific studies of ozone layer chemistry.
• The question refers to the main cause of depletion, not general greenhouse effects.

Concept / Approach:
Chlorofluorocarbons, often abbreviated as CFCs, are stable compounds once used in refrigerants, aerosol propellants, and foam blowing agents. Because they are very stable in the lower atmosphere, they eventually reach the stratosphere, where ultraviolet radiation breaks them down to release chlorine atoms. These chlorine atoms participate in catalytic cycles that destroy ozone molecules. Carbon dioxide and methane are important greenhouse gases but are not the primary agents of ozone depletion. Nitrous oxide and sulfur dioxide have other environmental impacts but are not as directly associated with the observed ozone holes as CFCs.

Step-by-Step Solution:
Step 1: Recall that the ozone layer is located in the stratosphere and filters out much of the Sun ultraviolet radiation. Step 2: Chlorofluorocarbons are human made compounds containing chlorine, fluorine, and carbon that were widely used in refrigeration and aerosols. Step 3: These compounds are inert in the lower atmosphere, so they persist long enough to reach the stratosphere. Step 4: In the stratosphere, intense ultraviolet light breaks CFC molecules, freeing chlorine atoms. Step 5: A single chlorine atom can catalytically destroy many ozone molecules through a chain of reactions, leading to thinning of the ozone layer. Step 6: Therefore, chlorofluorocarbons are identified as the main cause of ozone layer depletion, which led to international agreements to phase them out.

Verification / Alternative check:
Scientific reports by organisations and the Nobel Prize winning work on ozone layer chemistry specifically highlight CFCs as the main drivers of ozone depletion. The Montreal Protocol, an international treaty, targeted CFCs and related halogenated substances for reduction and replacement. While nitrous oxide, methane, and carbon dioxide also interact with atmospheric chemistry, they are primarily discussed in the context of greenhouse warming rather than direct ozone destruction. This strong and specific link between CFCs and ozone loss confirms that chlorofluorocarbons are the correct answer.

Why Other Options Are Wrong:
Nitrous oxide: A greenhouse gas and contributor to some chemical reactions in the atmosphere, but not the primary substance responsible for observed ozone holes.
Carbon dioxide: The main greenhouse gas discussed in climate change, but it does not catalytically destroy ozone in the same way as chlorine from CFCs.
Methane: Another greenhouse gas that can affect atmospheric chemistry, yet it is not identified as the main cause of stratospheric ozone depletion.
Sulfur dioxide: Important for acid rain formation and atmospheric aerosols, but not recognised as the chief agent of ozone layer thinning.

Common Pitfalls:
Students sometimes confuse ozone depletion with global warming and therefore select carbon dioxide or methane because these gases are frequently mentioned in environmental discussions. Another error is to choose any gas that sounds industrial. To avoid confusion, remember that ozone depletion is strongly tied to chlorine and bromine containing compounds such as CFCs, whereas global warming is linked primarily with greenhouse gases like carbon dioxide and methane.

Final Answer:
The depletion of the ozone layer is mainly caused by chlorofluorocarbons (CFCs).