How do sulfur dioxide (SO2) and nitrogen oxides (NOx) contribute to acid deposition, and what are two environmental effects?

• A. SO2 and NOx neutralize rain acidity.
• B. SO2 and NOx are not involved in acid deposition.
• C. SO2 and NOx form sulfuric and nitric acids in the atmosphere; deposition lowers pH of rain and waters; effects include acidification of lakes/streams and soils, and damage to forests and aquatic life.
• D. Acid deposition only affects urban air, not environmental ecosystems.

Answer: (C)

Sulfur dioxide and nitrogen oxides released from burning fossil fuels rise into the atmosphere and react with water, oxygen, and other oxidants to form sulfuric acid and nitric acid. These acids are carried back to Earth in precipitation (wet deposition) or attach to particles that settle out (dry deposition). When the acids acidify rain and surface waters, the pH drops, and how severely this happens depends on the buffering capacity of soils and lakes.

Two environmental effects are most important: first, the acidification of lakes and streams, which can harm or kill sensitive aquatic organisms and reduce biodiversity; second, soil acidification, which leaches away essential nutrients and can mobilize toxic metals, harming plant growth and altering forest ecosystems. This combination of changes disrupts aquatic life and forest health, illustrating why sulfur dioxide and NOx contribute to broad ecological impacts beyond just the air.