How is `SO_2` an air pollutant?

Explanation

**Step-by-Step Solution:**

1. **Definition of Sulfur Dioxide (SO₂)**: Sulfur dioxide is a colorless gas with a pungent odor, primarily produced from the burning of fossil fuels containing sulfur, such as coal and oil. It is also emitted from vehicles and industrial processes.

2. **Source of Emission**: SO₂ is released into the atmosphere from various sources, including vehicle exhaust, power plants, and industrial activities. This makes it a common air pollutant.

3. **Reaction with Atmospheric Moisture**: Once in the atmosphere, sulfur dioxide can react with water vapor (H₂O). This reaction is significant because it leads to the formation of sulfuric acid (H₂SO₄).

...