The contribution of SO₂ to acid rain

   Despite being recognized as far back as the 1870s, the environmental problems associated with ‘acid rain’ came to the fore in the 1960s with the decline of fish stocks in European and North American lakes. Two of the major contributors towards acid rain are SO₂ and NO_x.  Although SO₂ emissions arise from natural sources such as volcanic eruptions, artificial sources contribute ≈ 90% of the sulfur in the atmosphere.  Fossil fuels such as coal contain ≈ 2–3% sulfur and combustion produces SO₂, and the gas is released when metal sulfide ores are roasted in the production of metals such as Co, Ni, Cu (equation 21.6) and Zn. Once released, SO₂ dissolves in the atmospheric water vapour, forming H₂SO₃ and H₂SO₄. Acid formation may take several days and involves multistage reactions, the outcome of which is:

   By the time acid rain falls to the Earth’s surface, the pollutants may have travelled long distances from their industrial sources so, for example, prevailing winds in Europe may carry SO₂ from the UK, France and Germany to Scandinavia. The effects of acid rain can be devastating. The pH of lakes

and streams is lowered, although the composition of the bedrock is significant, and in some cases provides a natural buffering effect. A second effect is that acid rain penetrating the bedrock can react with aluminosilicate minerals, or can leach heavy metal ions from the bedrock; as the acid rain makes its ways through the bedrock and into waterways, it carries with it the metal pollutants. Acidified and pollutedwaters not only kill fish, but also affect the food chain. Acid rain falling on soils may be neutralized if the soil is alkaline, but otherwise the lowering of the pH and the leaching of plant nutrients has devastating effects on vegetation. The effects of acid rain on some building materials are all around us: crumbling gargoyles on ancient churches are a sad reminder of pollution by acid rain. International legalization to reduce acidic gas emissions has been in operation since the 1980s, and recent environmental studies indicate some improvement in the state of Western European and North American streams and lakes. There is, however, a long way to go.

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