Answer
Sulfur dioxide reacts with oxygen to form sulfur trioxide:
$2SO_{2} (g) + O_{2} (g) → 2SO_{3} (g)$
Sulfur trioxide reacts with rain water to form sulfuric acid, which causes acid rain:
$SO_{3} (g) + H_{2}O (l) → H_{2}SO_{4} (l)$
Work Step by Step
Given above.