Step 1: Concept Clarification:
The objective is to identify a primary air pollutant from the provided options.Primary pollutants are directly released into the atmosphere from their origin (e.g., industrial emissions, volcanic activity).Secondary pollutants are formed in the atmosphere through the reactions of primary pollutants with each other or with atmospheric components.
Step 3: Detailed Analysis:
\[\begin{array}{rl} \bullet & \text{(A) Volcanic Ash: This is classified as a primary pollutant. Ash expelled during a volcanic eruption is particulate matter directly introduced into the atmosphere from a natural source.} \\ \bullet & \text{(B) Tropospheric Ozone (O\(_3\)): This is a quintessential secondary pollutant. It forms in the troposphere via chemical reactions involving primary pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), facilitated by sunlight.} \\ \bullet & \text{(C) Stratospheric Ozone: This is not considered a pollutant. The ozone layer in the stratosphere plays a vital role in shielding life on Earth by absorbing harmful ultraviolet (UV) radiation.} \\ \bullet & \text{(D) PAN (Peroxyacetyl nitrate): This is another common secondary pollutant, generated by the reaction of NOx and VOCs, and constitutes a component of photochemical smog.} \\ \end{array}\]
Step 4: Conclusion:
Volcanic ash is a primary air pollutant.