Which one of the following gases is both a greenhouse gas and a substance that can deplete the ozone layer in the stratosphere?

Introduction / Context:
Global environmental issues often involve gases that have more than one harmful effect. Some gases act as greenhouse gases that trap heat in the atmosphere, while others or the same ones can contribute to ozone layer depletion. This question asks which chemical among the options is both a greenhouse gas and capable of depleting the ozone layer in the stratosphere. Recognising chlorofluorocarbons is crucial for understanding ozone depletion and climate change.

Given Data / Assumptions:
- Four chemical formulas are provided: CCl2F2, NH3, CH2N2, and BBr3.
- The gas we seek contributes significantly to the greenhouse effect and also breaks down ozone molecules in the stratosphere.
- Standard environmental science knowledge about greenhouse gases and ozone depleting substances is assumed.

Concept / Approach:
CCl2F2 is a type of chlorofluorocarbon, often designated as CFC 12. Chlorofluorocarbons are stable in the lower atmosphere but break down under ultraviolet radiation in the stratosphere, releasing chlorine atoms. These chlorine atoms catalyse the destruction of ozone, thinning the ozone layer. CFCs also act as strong greenhouse gases because they absorb infrared radiation and persist for long periods. In contrast, ammonia (NH3) is not a major ozone depleting gas, CH2N2 (diazomethane) is a reactive laboratory reagent not typically discussed as a greenhouse gas, and BBr3 (boron tribromide) is a specialised chemical used in industry, not a known greenhouse or ozone depleting gas at global scale.

Step-by-Step Solution:
Step 1: Identify CCl2F2 as a chlorofluorocarbon, a class of compounds once widely used in refrigeration and aerosol propellants. Step 2: Remember that CFCs are important greenhouse gases and also destroy ozone when they release chlorine atoms under ultraviolet light in the stratosphere. Step 3: Consider NH3, which is ammonia used in fertilisers and refrigeration but is not a primary ozone depleting substance. Step 4: Consider CH2N2, a highly reactive laboratory gas, not recognised as a significant greenhouse gas or ozone depleting substance in atmospheric science. Step 5: Consider BBr3, a specialist reagent, again not listed among major greenhouse gases or ozone depleting chemicals affecting global climate. Step 6: Conclude that CCl2F2 is the only gas among the options that is both a greenhouse gas and contributes to ozone layer depletion.

Verification / Alternative check:
Environmental reports and textbooks on atmospheric chemistry discuss chlorofluorocarbons in detail, especially CFC 11 and CFC 12. They explain that these compounds are potent greenhouse gases with long atmospheric lifetimes and that they are responsible for significant ozone depletion, which led to international agreements such as the Montreal Protocol to phase them out. Such references do not treat ammonia, diazomethane, or boron tribromide as major global ozone depleting greenhouse gases. This consistent documentation confirms CCl2F2 as the correct answer.

Why Other Options Are Wrong:
NH3: While ammonia can have environmental impacts, it is not classified as a principal ozone depleting greenhouse gas in global climate discussions.
CH2N2: Diazomethane is a laboratory reagent that is toxic and explosive, but it is not a significant atmospheric pollutant in the way CFCs are.
BBr3: Boron tribromide is used in chemical synthesis and does not have the widespread atmospheric presence needed to be a major greenhouse or ozone depleting gas.

Common Pitfalls:
Students may focus on the presence of nitrogen or bromine and guess that any compound containing these elements could affect the atmosphere. Another mistake is to overlook the famous role of CFCs in ozone depletion and instead choose a less familiar formula. A good strategy is to remember that many ozone depleting substances are chlorofluorocarbons and halons, and that formulas with both chlorine and fluorine like CCl2F2 are classic examples of such gases.

Final Answer:
The gas that acts as a greenhouse gas and can deplete the ozone layer is CCl2F2, a chlorofluorocarbon commonly known as CFC 12.