The oppressive pollution was noticed to have abated after the unseasonal rainfall.

Abated means to become less intense or widespread; to lessen or reduce. Mitigated means made less severe, serious, or painful, which is the closest synonym. Enhanced means improved or increased (opposite meaning); discontinued means stopped completely; suppressed means forcibly put an end to. Abatement implies a gradual reduction rather than a complete stop, so mitigated best captures this nuance.