Natural sources of air pollution: Which option represents an unequivocally natural source of air pollutants (independent of human activity)?

Introduction / Context:
Air pollution arises from both anthropogenic and natural sources. While some sources may be either human-caused or natural (e.g., wildfires), others are inherently natural geophysical phenomena. This question tests recognition of an unambiguously natural source among the options.

Given Data / Assumptions:

• “Natural source” means it can occur without human initiation.
• Forest fires can be natural or human-caused; deforestation is anthropogenic.
• Volcanic eruptions emit gases and particulates from Earth's interior without human cause.

Concept / Approach:
Volcanic eruptions inject sulphur dioxide, carbon dioxide, hydrogen chloride, ash, and other species directly into the atmosphere. They are classical examples of natural pollution events and have historically impacted climate and air quality. Forest fires do occur naturally via lightning, but the term in regulatory contexts often includes widespread human-caused fires; to avoid ambiguity, the unequivocally natural choice is volcanic eruption. Deforestation is by definition human-driven land-use change.

Step-by-Step Solution:
Evaluate each source for independence from human action.Forest fire: ambiguous; can be human-caused or natural.Deforestation: anthropogenic.Volcanic eruption: natural geologic source → select.

Verification / Alternative check:
Air quality inventories list volcanic emissions as natural background sources influencing sulphate aerosol formation and visibility, confirming the selection.

Why Other Options Are Wrong:
Forest fire: Not unequivocally natural; many are anthropogenic.Deforestation: Human activity; not a natural emission source.None of these: Incorrect because volcanic eruption qualifies.

Common Pitfalls:

• Interpreting “forest fire” as always natural; policies distinguish cause-based categories.
• Overlooking volcanic gas composition and its strong atmospheric impact.

Final Answer:
Volcanic eruption