Acid Deposition

Acid deposition is the transfer of sulfuric and nitric acidity from the atmosphere to the surface through precipitation and dry processes, driven by the oxidation of sulfur and nitrogen oxides.

Definition

The deposition of acidic species, chiefly sulfuric and nitric acid, from the atmosphere to terrestrial and aquatic surfaces by wet and dry processes.

Scope

This topic covers the gas- and aqueous-phase oxidation of sulfur dioxide and nitrogen oxides to sulfuric and nitric acid, wet and dry deposition pathways, the resulting acidification of soils and surface waters, and the role of buffering and neutralization.

Core questions

• How are sulfur and nitrogen oxides oxidized to strong acids in the atmosphere?
• What distinguishes wet from dry deposition?
• Why are some watersheds more vulnerable to acidification than others?
• How do buffering and base cation supply moderate ecosystem effects?

Key theories

Sulfur and nitrogen oxidation to acidity

Emitted SO2 and NOx are oxidized in gas and aqueous phases to sulfuric and nitric acid; the acids deposit and lower the pH of precipitation, soils, and waters unless neutralized by available bases.

Mechanisms

Sulfur dioxide is oxidized by OH in the gas phase and by hydrogen peroxide and ozone in cloud droplets to sulfate; nitrogen oxides are oxidized to nitric acid mainly through OH and nighttime N2O5 hydrolysis. The acids are scavenged by precipitation (wet deposition) or deposited as gases and particles (dry deposition), with ecosystem impact set by the buffering capacity of receiving soils and waters.

Clinical relevance

Acid deposition damages forests, acidifies lakes and streams, mobilizes toxic aluminum, and corrodes materials; understanding its chemistry guided emission-control programs that reduced sulfur and nitrogen loading.

History

The term acid rain dates to Robert Angus Smith's 19th-century work, but the regional ecological problem was characterized in the 1960s-1980s in Scandinavia and North America, prompting major sulfur and nitrogen emission controls.

Key figures

• Robert Angus Smith
• Svante Oden

Seminal works

• vanLoon2017
• finlaysonPitts2000

Frequently asked questions

Why is normal rain slightly acidic even without pollution?

Dissolved atmospheric carbon dioxide forms carbonic acid, giving clean rain a pH near 5.6; acid deposition refers to the stronger acidity added by sulfur and nitrogen pollutants.