What is acid rain — how does it form and what damage does it cause

Acid rain is precipitation (rain, snow, fog) with a pH lower than 5.6. Normal rain is slightly acidic (pH ~5.6) due to dissolved $\text{CO}_2$ forming carbonic acid. Acid rain is significantly more acidic — pH can be as low as 4 or below.

The primary acids in acid rain are sulphuric acid ($\text{H}_2\text{SO}_4$) and nitric acid ($\text{HNO}_3$).

Source of sulphur dioxide ($\text{SO}_2$):

Burning of coal and petroleum (which contain sulphur impurities):

$\text{SO}_2$ reacts with water and oxygen in the atmosphere to form $\text{H}_2\text{SO}_4$:

Source of nitrogen oxides ($\text{NO}_x$):

Lightning and combustion at high temperature in vehicle engines and power plants:

$\text{NO}_2$ dissolves in rainwater:

These strong acids lower the pH of rainwater significantly.

Damage 1: Soil and vegetation

Acid rain leaches nutrients (calcium, magnesium, potassium) from the soil. These nutrient ions are replaced by toxic aluminium ions ($\text{Al}^{3+}$) released from aluminium salts in soil. Plants suffer nutrient deficiency, root damage, and reduced photosynthesis. Forests in affected areas show widespread leaf discolouration and tree death.

Damage 2: Aquatic ecosystems

Acid rain makes lakes and rivers more acidic. At pH below 5, most fish cannot survive (their enzymes denature and reproduction fails). At pH below 4, most aquatic life is eliminated. Many lakes in Scandinavia and North America became “dead” (no fish or aquatic plants) due to acid rain from industrial activity.

Damage 3: Monuments and structures

Acid rain reacts with limestone and marble (both $\text{CaCO}_3$) — the material of many historical buildings and statues:

The surface is converted to gypsum ($\text{CaSO}_4$), which is soft and crumbles. India’s Taj Mahal has been damaged this way — the marble is turning yellowish due to pollution and acid rain, a phenomenon sometimes called the “marble cancer.”