Acid rain damages terrestrial and aquatic ecosystems, contributes indirectly to human health problems, and accelerates the deterioration of buildings and metals. In IB ESS, this is core SL and HL content in subtopic 8.3 Urban air pollution.
Acid deposition forms when sulfur dioxide and nitrogen oxides react with water and oxygen in the atmosphere, producing sulfuric and nitric acids. These are secondary pollutants because atmospheric reactions form them rather than direct emission.
| Area affected | Main impacts and mechanisms |
|---|---|
| Ecology | Acidification lowers the pH of soils, lakes, and streams. It leaches calcium and magnesium from soil, damages roots and leaves, and may release toxic aluminium ions. In water, low pH reduces reproduction or kills fish, amphibians, and invertebrates, disrupting food webs and reducing biodiversity. |
| Humans | Acid rain is not normally acidic enough to burn skin. The main health effects come from sulfur dioxide, nitrogen oxides, and fine sulfate and nitrate particulates, which worsen respiratory and cardiovascular disease. Acidification may also mobilize toxic metals into water supplies. |
| Buildings | Acids react with calcium carbonate in limestone and marble, dissolving surfaces and damaging statues and historic buildings. Deposition also accelerates metal corrosion and paint deterioration, increasing repair costs. |
A common misconception is that acid rain affects only locations near emission sources. Pollutants can undergo transboundary movement, so downwind ecosystems and communities may be harmed despite producing few emissions. This creates an environmental justice issue.
For example, lakes in the Adirondack Mountains of the United States were acidified by transported industrial emissions, causing fish declines. Controls under the US Clean Air Act Amendments reduced acid deposition and supported partial recovery.
Exam technique: For an explain question, write a causal chain: precursor emission, atmospheric transformation, acidification, then an ecological, health, or structural consequence. Distinguish acidic deposition's impacts from the health effects of precursor pollutants.
