Acid Rain

Acid rain is an environmental problem caused by pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) in the atmosphere reacting with water vapor to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃), which then fall to the Earth’s surface in the form of rain, snow, fog, or dry particles. It was first identified in 1872 by Robert Angus Smith through his study of industrial precipitation around Manchester.

Formation of Acid Rain

Acid rain originates from both natural and anthropogenic sources. Natural sources include volcanic activity, biological processes, and lightning, while the primary human-caused sources are the combustion of fossil fuels, industrial activities, and motor vehicle exhaust emissions.

The main chemical processes are as follows:

• Sulfuric acid formation:
• • SO₂ + O₂ → SO₃
  • SO₃ + H₂O → H₂SO₄
• Nitric acid formation:
• • N₂ + O₂ → 2NO
  • 2NO + O₂ → 2NO₂
  • 4NO₂ + 2H₂O + O₂ → 4HNO₃

Acid rain can reach the Earth’s surface in both wet (rain, snow, hail) and dry (deposition of acidic particles) forms.

Effects of Acid Rain on Ecosystems

Effects on Soil

Acid rain lowers the pH of soil, leading to the leaching of essential nutrients such as calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺). At the same time, low pH increases the solubility of heavy metals (Al, Fe, Mn, Zn), causing them to reach toxic levels. This reduces soil fertility and negatively affects plant growth.

Effects on Aquatic Ecosystems

The decline in pH of surface and groundwater sources leads to a reduction in biological diversity in aquatic ecosystems. Acidification results in declining fish populations and habitat loss for many species.

Effects on Plants

Increased concentrations of heavy metals in soil cause toxicity in plants. Effects include nutrient imbalances, leaf burn, reduced photosynthetic capacity, and stunted growth. It has been reported that acid rain enters plants through leaf stomata, acidifying the cytoplasm and inhibiting photosynthesis.

Effects on Human Health

Although acid rain does not directly affect human health, it indirectly impacts health through contamination of drinking water sources and the food chain. Additionally, precursor gases responsible for acid rain formation (SO₂, NOₓ) can contribute to increased respiratory diseases.

Effects on Buildings and Historical Monuments

Acid rain causes chemical erosion of historical monuments made primarily of carbonate stones such as limestone and marble. This results in surface dissolution, weathering, and permanent damage to these structures.

The Status of Acid Rain in Türkiye

Studies conducted in Türkiye have shown that acid rain constitutes a significant environmental threat, particularly in industrialized regions.

In a study conducted in the Istanbul region, significant reductions in adhesion resistance, hardness, and gloss were observed on water-based and synthetic varnished wooden surfaces exposed to acid rain in outdoor environments. This reduces the durability of outdoor construction materials.

Chaotic analyses of pH levels of precipitation across Türkiye have revealed that rainfall patterns exhibit irregular and complex temporal structures. These findings demonstrate that the irregular nature of air pollution directly influences the occurrence of acid rain.

Control of Acid Rain

To prevent acid rain, the primary goal must be to reduce emissions of SO₂ and NOₓ. The main methods include:

• Reducing the use of fossil fuels and promoting clean energy sources,
• Installing flue gas treatment systems in industrial facilities,
• Implementing emission control technologies in motor vehicles.

In addition, liming applications are used in affected soils and water bodies to restore pH balance.

Acid rain is a major environmental issue that threatens both the natural environment and human life on a global scale. It causes damage across multiple domains, from soils and water resources to plants, human health, and cultural heritage. Controlling acid rain is of great environmental and economic importance.