Acid Deposition (College Board AP® Environmental Science): Study Guide

What is acid rain & deposition?

• Acid deposition is a significant environmental issue

• It occurs when sulfur dioxide and nitrogen oxides are released into the atmosphere through anthropogenic activities such as vehicle emissions and coal-burning power plants and natural sources such as volcanic eruptions

• Deposition can be dry or wet

  • Dry deposition occurs close to the point of source of the pollution and is usually in the form of particulate matter

  • Wet deposition, also called acid precipitation or rain, happens when the pollutants dissolve in precipitation and fall at a distance from the point of source

    

Causes of acid deposition

Primary pollutants

• Most causes of acid deposition is from fossil fuel combustion in power plants, industrial processes, and motor vehicles, releasing sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere

  • Power plants that burn coal, oil, or natural gas are major contributors to SO₂ emissions, while NOₓ emissions primarily come from vehicle exhausts

  • Sulfur dioxide is released when fossil fuels containing sulfur compounds are burned and contributes to the formation of acid rain directly

  • Oxides of nitrogen are produced through high-temperature combustion processes, such as those occurring in vehicle engines and industrial furnaces

Secondary pollutants

• SO₂ and NOₓ gases react with naturally occurring ammonia gas and transform into secondary pollutants of nitric salts and sulfates in the form of a gas, particulate, or aerosol

Dry deposition

• Sulfuric acid and nitric acid can combine with ash and other particles present in the air, forming dry particles

  • Dry deposition occurs when these particles settle on surfaces, including vegetation, buildings, and soil, causing damage over time

Wet deposition

• Wet deposition occurs when NOₓ and SO₂ undergo chemical reactions with water to form nitric acid (HNO₃) and sulfuric acid (H₂SO₄)

• Sulfuric acid and nitric acid can dissolve in rainwater or snowflakes, forming acidic precipitation

  • Wet deposition occurs when this acidic precipitation falls to the ground, affecting surface water and soils