Gases that are less dense than air are collected by downward displacement of air. Examples include ammonia and hydrogen. 

Sulphur dioxide (SO\(_2\)) is not collected by downward displacement of air. It is collected by upward displacement of air because it is heavier than air.Also, chlorine is collected by upward displacement of air, not downward displacement. This is because although, it is heavier and denser than air, it is soluble in water. Instead, chlorine is collected by upward displacement of air.