Acid Rain: Causes, Effects and Solutions

How acid rain forms

Burning fossil fuels, particularly coal and oil, releases sulfur dioxide (SO₂) and nitrogen oxides (NO_x) into the atmosphere. Power stations, factories and vehicle engines are the main sources.

These gases dissolve in water vapour in the atmosphere, forming sulfuric acid (H₂SO₄) and nitric acid (HNO₃). When this water falls as rain, it has a lower pH than normal rain (which is already slightly acidic at pH 5.6 due to dissolved CO₂). Acid rain typically has a pH of 4 or lower.

Effects on ecosystems

Forests: Acid rain damages leaves by destroying the waxy cuticle, making them more susceptible to disease and reducing photosynthesis. It also leaches essential minerals from the soil (such as magnesium needed for chlorophyll), causing nutrient deficiency and stunted growth.

Lakes and rivers: Acid rain lowers the pH of water bodies. Many aquatic organisms, especially fish, are sensitive to pH changes. Acidification can release toxic aluminium ions from the soil into waterways, which damage fish gills and interfere with their ability to absorb oxygen. Below pH 5, most fish species cannot survive.

Other effects

Buildings and statues: Acid rain corrodes limestone and marble buildings through chemical weathering. The acid reacts with calcium carbonate in the stone, dissolving it over time.

Human health: While acid rain itself is not directly harmful to humans, the pollutants that cause it (SO₂ and NO_x) can aggravate respiratory conditions such as asthma and bronchitis when inhaled.

Acid rain can travel long distances in the atmosphere, meaning pollution produced in one country can cause damage in another. This makes it an international environmental problem requiring cooperation between nations.

Reducing acid rain

Solutions include: using flue gas desulfurisation in power stations to remove SO₂ from emissions before they enter the atmosphere, fitting catalytic converters to vehicle exhausts to reduce NO_x emissions, switching to cleaner fuels (natural gas produces less SO₂ than coal), and increasing use of renewable energy sources.

Liming (adding calcium carbonate to acidified lakes and soils) can neutralise the acid and restore pH, but this is a treatment of the symptoms rather than a solution to the cause.