Step 1: Concept Overview:
Acid rain is precipitation with abnormally high acidity. It originates from precursor gases in the atmosphere that interact with water, oxygen, and other atmospheric components to yield acidic compounds.
Step 3: Detailed Explanation:
The two principal gases contributing to acid rain formation are:
\[\begin{array}{rl} \bullet & \text{Sulfur dioxide (SO\(_2\)): In the atmosphere, SO\(_2\) reacts with water to produce sulfurous acid (H\(_2\)SO\(_3\)). Subsequent oxidation can generate sulfuric acid (H\(_2\)SO\(_4\)), a potent acid.} \\ \bullet & \text{Nitrogen oxides (NOx), predominantly nitrogen dioxide (NO\(_2\)): NO\(_2\) reacts with water to form nitric acid (HNO\(_3\)) and nitrous acid (HNO\(_2\)), both of which increase rain's acidity.} \\ \end{array}\]While CO\(_2\) forms carbonic acid, making typical rain slightly acidic (pH ~5.6), it is not categorized as a cause of the "acid rain" issue, which refers to rain with a significantly lower pH. O\(_3\) (ozone) acts as an oxidant, aiding acid formation but is not a precursor. NH\(_3\) (ammonia) is a basic gas.
Step 4: Final Conclusion:
NO\(_2\) and SO\(_2\) are the primary acid-forming precursor gases responsible for acid rain.