Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Primary pollutants enter the air directly. Some are released by natural processes, like ash from volcanoes. Most are released by human activities. They pour into the air from vehicles and smokestacks. Several of these pollutants are described below. Answer: primary pollutant
A:
Ash that enters the air naturally as a result of a volcano eruption is classified as what kind of pollutant?