Question

what does sulfur dioxide react with in the atmosphere to form acids that return to the surface as either dry or wet deposition?
water vapor
carbon dioxide
nitrogen gas
ozone

Explanation:

Sulfur dioxide ($SO_2$) in the atmosphere first reacts with oxygen ($O_2$) in the presence of a catalyst (such as dust particles) to form sulfur trioxide ($SO_3$). Then sulfur trioxide reacts with water vapor ($H_2O$) to form sulfuric acid ($H_2SO_4$), which can return to the surface as acid - deposition.

Answer:

water vapor