Acid Rain | Science for ACT PDF Download

Introduction

Acid rain, also known as acid deposition, encompasses various forms of precipitation containing acidic elements like sulfuric or nitric acid, which descend to the Earth's surface from the atmosphere in both wet and dry states. This encompasses rain, snow, fog, hail, or even acidic dust particles.

What Causes Acid Rain?

• Acid rain occurs when sulfur dioxide (SO₂) and nitrogen oxides (NOX) are discharged into the atmosphere and transported by wind and air currents. Upon contact with water, oxygen, and other chemicals, SO₂ and NOX undergo reactions to produce sulfuric and nitric acids. These acidic compounds then amalgamate with water and other substances before descending to the Earth's surface.
• Although a fraction of the SO₂ and NOX contributing to acid rain originates from natural sources like volcanoes, the majority stems from the combustion of fossil fuels. The primary emitters of SO₂ and NOX in the atmosphere include:
  • Power plants burning fossil fuels for electricity generation, accounting for two-thirds of atmospheric SO₂ and one-fourth of NOX.
  • Vehicles and heavy machinery.
  • Industrial activities such as manufacturing, oil refining, and other industrial processes.
• Wind patterns can carry SO₂ and NOX over extensive distances and across borders, making acid rain a concern not only for those in close proximity to emission sources but for everyone affected by atmospheric transport.

Forms of Acid Deposition

Wet Deposition

• Wet deposition, commonly known as acid rain, occurs when sulfuric and nitric acids formed in the atmosphere combine with rain, snow, fog, or hail and fall to the ground.

Dry Deposition

• In dry deposition, acidic particles and gases settle from the atmosphere without the presence of moisture. These particles and gases may deposit onto surfaces such as water bodies, vegetation, or buildings. They can react during atmospheric transport to form larger particles that may pose risks to human health. 
• When accumulated acids are washed off surfaces by subsequent rainfall, the acidic water flows over and through the ground, potentially harming plants and wildlife.

The extent of atmospheric acidity deposited through dry deposition depends on the amount of rainfall received in an area. For instance, desert regions typically experience a higher ratio of dry to wet deposition compared to areas with more significant annual rainfall.

Measuring Acid Rain

• Acidity and alkalinity are gauged using a pH scale, where 7.0 represents neutrality. A substance with a pH lower than 7 is acidic, while a pH higher than 7 indicates alkalinity. Normal rain typically has a pH around 5.6 due to the dissolution of carbon dioxide (CO₂) forming weak carbonic acid. Acid rain usually falls within a pH range of 4.2 to 4.4.
• Various stakeholders, including policymakers, research scientists, ecologists, and modelers, rely on the National Atmospheric Deposition Program’s (NADP) National Trends Network (NTN) to measure wet deposition. The NADP/NTN collects acid rain at over 250 monitoring sites across the US, Canada, Alaska, Hawaii, and the US Virgin Islands. Unlike wet deposition, measuring dry deposition is challenging and costly. Estimates for nitrogen and sulfur pollutants' dry deposition are provided by the Clean Air Status and Trends Network (CASTNET), which measures air concentrations at over 90 locations.
• When acid deposition enters lakes and streams, it can lead to increased acidity levels. The Long-Term Monitoring (LTM) Network monitors surface water chemistry at more than 280 sites to offer valuable insights into aquatic ecosystem health and responses to changes in acid-causing emissions and acid deposition.