Explanation:

Volcanic eruptions can release a variety of materials into the atmosphere and surrounding environment. Let's analyze each of the items you've listed:

1. Pyroclastic debris: This is a direct product of volcanic eruptions and consists of a mix of gas, ash, and larger volcanic rock fragments ejected during an explosive eruption.

2. Ash and dust: Volcanic ash is also a direct product of eruptions. It consists of fine particles that are ejected during explosive volcanic activity.

3. Nitrogen compounds: While volcanic eruptions can emit various gases, nitrogen compounds are primarily a result of natural processes in the atmosphere or some biological processes, not directly a product of volcanic activity.

4. Sulphur compounds: These are produced during eruptions, particularly sulfur dioxide (SO2), which is released from the magma.

From the analysis, pyroclastic debris, ash and dust, and sulphur compounds are products of volcanic eruptions. However, nitrogen compounds are not considered direct products of eruptions.

Thus, the answer is: Only three.