Sulfur dioxide gets reacts with water vapor to produce aqueous hydrogen sulfite.

SO₂ + H₂O → HSO₃⁻ + H⁺

Explanation:

Assuming, you want to know the final balanced equation of this reaction, which is in words.

Sulfur dioxide reacts with water vapor to produce aqueous hydrogen sulfite and H⁺ ions (acidic).

Sulfur dioxide: SO₂ (g)

Water vapor: H₂O (g)

Hydrogen sulfite: HSO₃⁻ (aq)

Before putting them in the equation, check for each element's oxidation state to identify the nature of the reaction.

OS of S in SO₂ is + 4, and in HSO₃⁻ is also + 4.

OS of H and O remain + 1 and - 2, respectively.

SO₂ + H₂O → HSO₃⁻ + H⁺

This further reacts to form sulfur trioxide (SO₃) gas, which is main cause for acid rain, as this reacts with water vapor in atmosphere to form sulfuric acid.