In environmental chemistry, which of the following gases is considered a major cause of acid rain among the options given?

Introduction / Context:
Acid rain is an important topic in environmental chemistry and pollution studies. It refers to precipitation with a pH lower than normal rain due to dissolved acidic oxides. The question asks which gas among the options contributes most to the formation of acid rain. Understanding this is crucial for recognising the impact of industrial emissions and vehicle exhaust on ecosystems, buildings, and human health.

Given Data / Assumptions:

• The options are carbon dioxide, carbon monoxide, nitrogen dioxide, and oxygen.
• We assume typical examination context where sulphur dioxide and nitrogen oxides are recognised as main acid rain contributors.
• Among the given choices, we must select the gas that best fits the known chemistry of acid rain.

Concept / Approach:
Acid rain primarily results from the reaction of sulphur dioxide and nitrogen oxides with water vapour and other components in the atmosphere, forming sulphuric acid and nitric acid. These acids then fall to the ground dissolved in rain, snow, or fog. Carbon dioxide forms weak carbonic acid in water but under normal atmospheric concentrations this does not by itself cause the strong acid rain associated with industrial pollution. The key is to see that of the gases listed, nitrogen dioxide is the one that forms nitric acid and is directly linked to acid rain in textbooks.

Step-by-Step Solution:
Step 1: Consider carbon dioxide. It forms carbonic acid when dissolved in water, but the resulting acidity is mild and part of the natural carbon cycle rather than the main cause of acid rain described in environmental problems. Step 2: Carbon monoxide is a toxic gas but does not significantly form strong acids in the atmosphere that cause acid rain. Step 3: Nitrogen dioxide is one of the nitrogen oxides produced from vehicle exhaust and industrial combustion. In the atmosphere, it reacts with water and other components to form nitric acid. Step 4: Oxygen is a non acidic gas and actually participates in oxidation reactions rather than being an acid rain precursor itself. Step 5: Therefore, among the options, nitrogen dioxide is the gas most clearly associated with the formation of acid rain.

Verification / Alternative check:
Environmental science texts usually state that acid rain is caused by emissions of sulphur dioxide and nitrogen oxides (NO and NO2). These gases react with water, oxygen, and other chemicals to form sulphuric acid and nitric acid. While sulphur dioxide is not listed as an option in this question, nitrogen dioxide is, and it fits the known process for nitric acid formation. Carbon dioxide is discussed in the context of climate change rather than acid rain, which confirms that nitrogen dioxide is the correct choice here.

Why Other Options Are Wrong:

• Carbon dioxide: Contributes to global warming but is not considered the primary cause of acid rain in environmental pollution discussions.
• Carbon monoxide: A poisonous gas that reduces oxygen carrying capacity of blood but does not significantly cause acid rain.
• Oxygen: A basic atmospheric gas necessary for life and combustion, but it does not itself create acid rain.

Common Pitfalls:
Some learners may confuse gases responsible for climate change with those that produce acid rain, leading them to choose carbon dioxide. Others may think any harmful or poisonous gas, like carbon monoxide, must be related to acid rain. To avoid these errors, remember that acid rain is primarily linked to sulphur dioxide and nitrogen oxides, and when sulphur dioxide is absent from the options, nitrogen dioxide is the gas that best represents the acid rain cause among the given choices.

Final Answer:
The gas listed that is a major cause of acid rain is nitrogen dioxide.