Acid Rain and Deposition

Acid deposition is the transfer of acidic compounds from the atmosphere to the surface, commonly known as acid rain.

Definition

The deposition of acidic substances from the atmosphere, in precipitation as wet deposition or as gases and particles in dry deposition, resulting mainly from sulfur and nitrogen oxide emissions.

Scope

This topic covers the formation, transport, and effects of acid deposition as an air-pollution problem. It addresses how emissions of sulfur dioxide and nitrogen oxides are oxidized to sulfuric and nitric acids, the distinction between wet and dry deposition, and the impacts on lakes, soils, forests, and built structures. The role of buffering capacity in determining ecosystem sensitivity and the regulatory response to reduce precursor emissions are also included.

Core questions

• How are acidifying precursors converted to acids in the atmosphere?
• What is the difference between wet and dry acid deposition?
• Why are some lakes and soils more sensitive to acidification than others?
• How does acid deposition affect ecosystems and materials?

Key theories

Precursor oxidation to strong acids

Sulfur dioxide and nitrogen oxides are oxidized in the atmosphere to sulfuric and nitric acids, which lower the pH of precipitation and deposited material well below that of natural rainfall.

Buffering capacity and ecosystem sensitivity

The susceptibility of a lake or soil to acidification depends on its capacity to neutralize acids; systems on poorly buffered, base-poor geology acidify readily, whereas carbonate-rich systems resist change.

Clinical relevance

Acid deposition damages aquatic life, leaches nutrients and mobilizes toxic metals in soils, harms forests, and corrodes structures; understanding precursor chemistry justified controls on sulfur and nitrogen emissions.

Evidence & guidelines

Long-term deposition monitoring informs emission-control programs aimed at reducing acidifying pollutants; this context is described here to explain how acid deposition has been addressed rather than as prescriptive guidance.

History

Robert Angus Smith coined the term acid rain in the nineteenth century, and Likens and Bormann documented its regional severity in North America in the 1970s, prompting emission-reduction programs for sulfur and nitrogen oxides.

Key figures

• Gene E. Likens
• F. Herbert Bormann
• Robert Angus Smith

Seminal works

• likens1974
• seinfeld2016
• manahan2017

Frequently asked questions

What causes acid rain?

Acid rain forms when sulfur dioxide and nitrogen oxides, largely from fossil-fuel combustion, are oxidized in the atmosphere to sulfuric and nitric acids that fall to the surface in precipitation or deposit as gases and particles.

Why are some lakes harmed by acid rain while others are not?

Lakes and soils differ in their ability to neutralize acids; those on base-poor, poorly buffered bedrock acidify quickly, while those rich in carbonate minerals can neutralize incoming acid and resist acidification.