Final answer:
Sulfur trioxide (SO3) reacts with water in the atmosphere to form sulfuric acid, a process that contributes to acid rain.
Step-by-step explanation:
The substance that combines with water in the atmosphere to make sulfuric acid is sulfur trioxide (SO3). The reaction that occurs is:
SO3(g) + H2O(l) → H2SO4(aq)
Sulfur trioxide in the atmosphere is produced by both natural processes, such as volcanic activity, and human activities, including the burning of fossil fuels and metal-refining processes. The formation of sulfuric acid from sulfur trioxide and water is a primary chemical process leading to the phenomenon known as acid rain.