Air pollution sources: identify a clearly manmade (anthropogenic) source among the listed options, distinguishing it from natural background contributors such as wildfires or biogenic emissions.

Introduction / Context:
Air pollutants arise from both anthropogenic and natural sources. Differentiating these helps design targeted control strategies in urban airsheds where human activity dominates emissions linked to health and smog formation.

Given Data / Assumptions:

• We must pick a clearly manmade source.
• Examples include transport, industry, and energy generation.
• Natural emissions include wildfires, biogenic gases, dust, and sea salt.

Concept / Approach:
Anthropogenic sources are directly attributable to human activities. In most cities, mobile sources (vehicles) are a principal source of NOx, CO, PM, and VOCs, which drive ozone and secondary PM formation.

Step-by-Step Solution:

Verification / Alternative check:
Urban emission inventories consistently attribute a major fraction of NOx, CO, and VOCs to road transport, verifying its anthropogenic character.

Why Other Options Are Wrong:

• Forest fire: natural, despite possible human ignition.
• Bacterial action in swamps and soils: natural biogenic source (e.g., methane).
• Volcanic eruption: natural geologic source.
• Sea spray aerosols: natural marine emission.

Common Pitfalls:
Choosing natural sources due to their magnitude during events; confusing human-triggered wildfires with inherently anthropogenic emissions like routine traffic exhaust.

Final Answer:
Automobile exhaust