What is acid deposition, and through which atmospheric processes does it occur?

• A. Acids formed in the atmosphere from sulfur dioxide (SO2) and nitrogen oxides (NOx) that return as rain, snow, or dry particles.
• B. Solid particles deposited from dust storms only.
• C. Acid rain produced by carbon dioxide dissolving in rainwater.
• D. Acid deposition is the deposition of base substances that raise pH.

Answer: (A)

Acid deposition happens when acids form in the atmosphere from sulfur dioxide and nitrogen oxides released by burning fossil fuels, and are then carried to Earth’s surface by rain, snow, or as dry particles. In the air these gases are transformed by reactions with oxidants like hydroxyl radicals, ozone, and hydrogen peroxide into sulfuric acid and nitric acid, which form sulfate and nitrate aerosols or dissolve in water droplets. When precipitation occurs, these acids dissolve in the rain or snow and fall to the surface as acid rain or acid snow (wet deposition). Even without rain, acidic gases and particles can deposit directly onto surfaces as dry deposition. This solidifies the idea that acid deposition is about acids formed in the atmosphere from SO2 and NOx and removed through wet or dry deposition.

Dust storms produce solid particles but aren’t the acids in question. Carbon dioxide dissolving in rainwater forms carbonic acid, but that isn’t the primary mechanism of acid deposition described here. Depositing bases would raise pH, not cause acidity.