Which term describes a pollutant released directly into the air?

• A. Primary air pollutant
• B. Secondary air pollutant
• C. Ground-level ozone
• D. PM2.5

Answer: (A)

A pollutant released directly into the air is called a primary air pollutant. The idea here is the source is emitted straight into the atmosphere, rather than forming there through reactions. Examples include soot, sulfur dioxide, carbon monoxide, and dust from burning or blasting. In contrast, secondary air pollutants are not released already formed; they arise when emitted substances react in the atmosphere with sunlight, moisture, or other chemicals—ground-level ozone is a classic example of this. PM2.5 refers to particles of a certain size and can be either emitted directly as primary particles or formed secondarily, but the question specifically points to substances released directly, which is what primary air pollutant denotes.