Which of the following gases acts as a greenhouse gas and is also known to contribute to depletion of the ozone layer in the atmosphere?

Introduction / Context:
Climate change and ozone layer depletion are two major environmental issues. Some gases trap heat in the atmosphere and are called greenhouse gases, while others also participate in reactions that thin the protective ozone layer in the stratosphere. This question asks you to identify which gas among the options is a greenhouse gas and has a role in ozone depletion, making it important in environmental chemistry and general knowledge exams.

Given Data / Assumptions:

• The options include nitrous oxide (N2O), arsine (AsH3), nitrogen gas (N2), pentane (C5H12), and carbon monoxide (CO).
• We are focusing on gases that significantly contribute to the greenhouse effect and can influence the ozone layer.
• We assume typical atmospheric conditions where these gases may be present due to natural or human activities.

Concept / Approach:
A greenhouse gas is a gas that can absorb and emit infrared radiation, thereby trapping heat and contributing to the warming of the Earth's surface and lower atmosphere. Important greenhouse gases include carbon dioxide, methane, nitrous oxide, and water vapour. Among these, nitrous oxide is notable not only for its greenhouse effect but also because it takes part in reactions in the stratosphere that can lead to ozone destruction. Many other gases in the list are either not significant greenhouse gases at normal concentrations or are not associated strongly with ozone depletion.

Step-by-Step Solution:
Step 1: Recall that nitrous oxide (N2O) is recognised by climate scientists as a potent greenhouse gas with a high global warming potential. Step 2: Understand that N2O can reach the stratosphere, where it is involved in reactions that form nitrogen oxides, which can catalytically destroy ozone molecules. Step 3: Consider nitrogen gas (N2), which is the major component of air but is very inert and does not significantly absorb infrared radiation, so it is not a greenhouse gas in the usual sense. Step 4: Recognise that arsine (AsH3) is a highly toxic gas used in some industrial processes but is not a major greenhouse gas or ozone depleting substance in global environmental discussions. Step 5: Note that pentane (C5H12) and carbon monoxide (CO) may have some indirect roles in atmospheric chemistry but are not primary greenhouse and ozone depleting gases at the level highlighted for N2O. Step 6: Conclude that nitrous oxide (N2O) is the correct answer.

Verification / Alternative check:
Reports by international climate panels list carbon dioxide, methane, nitrous oxide, and some halogenated compounds as principal greenhouse gases. Nitrous oxide has a global warming potential significantly higher than carbon dioxide over a hundred year horizon. Additionally, atmospheric chemistry studies show that N2O is now a leading source of nitrogen oxides in the stratosphere, which participate in catalytic cycles that break down ozone. The other gases in the list simply do not appear in such environmental priority lists as combined greenhouse and ozone depleting agents, which confirms the selection of N2O.

Why Other Options Are Wrong:
- Arsine (AsH3): It is a toxic gas used in semiconductor manufacturing but not a major greenhouse gas or widely recognised ozone depleting substance.
- Nitrogen gas (N2): Although it makes up about 78 percent of the atmosphere, it is largely transparent to infrared radiation and is chemically inert under most atmospheric conditions, so it is not considered a greenhouse gas.
- Pentane (C5H12): This hydrocarbon can contribute indirectly to ozone formation in the lower atmosphere but is not a key greenhouse gas or ozone layer depleting agent in the sense used for stratospheric ozone.
- Carbon monoxide (CO): It affects local air quality and can influence the concentration of other greenhouse gases indirectly, but it is not a principal greenhouse gas or direct ozone depleting substance.

Common Pitfalls:
Students may mistakenly focus on the toxicity or flammability of a gas and assume that any dangerous gas must also be a greenhouse gas. Toxicity, however, is not the same as greenhouse potential. Another common confusion is mixing up nitrogen (N2), which is inert, with nitrous oxide (N2O), which is chemically active in the upper atmosphere. Paying attention to the full chemical formula and remembering that N2O is an important greenhouse and ozone affecting gas helps to avoid these mistakes.

Final Answer:
The gas that is both a greenhouse gas and contributes to ozone layer depletion is Nitrous oxide (N2O).