Which pollutants are precursors to acid rain, forming acids in the atmosphere?

• A. Carbon monoxide and methane
• B. Chlorofluorocarbons and ammonia
• C. Sulfur dioxide and nitrogen oxides
• D. Ozone and particulates

Answer: (C)

When acid rain is formed, it comes from gases in the air that turn into acids inside clouds and droplets. The best answer identifies the gases that actually become the acids: sulfur dioxide and nitrogen oxides. Sulfur dioxide, released by burning sulfur-containing fuels like coal and oil, is oxidized in the atmosphere to sulfur trioxide and then forms sulfuric acid in rain. Nitrogen oxides, produced by high-temperature combustion in vehicles and power plants, react to form nitric acid. These acids dissolve in raindrops as they fall, lowering the rain’s pH and harming ecosystems, buildings, and aquatic life. Other pollutants listed don’t directly form the main acids responsible for acid rain: carbon monoxide and methane are not acid rain precursors; ammonia can affect particulate acidity but isn’t the primary source of the acids; ozone and particulates are related air pollutants but not the primary precursors to acid rain.