Acid deposition occurs when sulfuric acid, nitric acid, or hydrochloric acid, emitted into or produced in the air as a gas or liquid, deposits to soils, lakes, farmland, forests, or buildings. Deposition of acid gases is dry acid deposition, and deposition of acid liquids is wet acid deposition. Wet acid deposition can be through rain (acid rain), fog (acid fog), or aerosol particles (acid haze). On the Earth's surface, acids have a variety of environmental impacts, including damage to microorganisms, fish, forests, agriculture, and structures. In the air, acids in high concentrations are harmful to humans. Acid deposition problems have existed since coal was first combusted, but were exacerbated during the Industrial Revolution in the eighteenth century. The problem became more severe with the growth of the alkali industry in 19th-century France and the United Kingdom. In this chapter, the history, science, and regulation of acid deposition problems are discussed.

HISTORICAL ASPECTS OF ACID DEPOSITION

Acid deposition is caused by the emission or atmospheric formation of gas- or aqueous-phase sulfuric acid (H2SO4), nitric acid (HNO3), or hydrochloric acid (HCl). Historically, coal was the first and largest source of anthropogenically produced atmospheric acids. Coal combustion produces sulfur dioxide gas [SO2(g)] and hydrochloric acid gas [HCl(g)]. SO2(g) produces gas-and aqueous-phase sulfuric acid. Humans have combusted coal for thousands of years. In the 1200s, sea coal was brought to London and used in lime kilns and forges (Section 4.1).