Acid Rain Risk Calculator

Normal rainfall is slightly acidic at around pH 5.6 due to dissolved CO₂ forming carbonic acid. Rain is considered 'acid rain' when its pH falls below 5.0. Severely acidic rain can reach pH 4.0 or lower in heavily industrialised regions.

Sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) released from burning fossil fuels and industrial processes react with water vapour and oxygen in the atmosphere to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These acids dissolve in precipitation, lowering its pH and causing acid deposition.

Wet deposition includes acid rain, sleet, fog, and snow that carry SO₂ and NOₓ acids to the ground. Dry deposition occurs when acidic particles and gases settle directly onto surfaces without precipitation. Both types damage ecosystems, though wet deposition typically accounts for a larger share of total acid load in high-rainfall regions.

Soils with high buffering capacity — such as those rich in calcium carbonate (limestone or chalk) — can neutralise incoming acid through chemical reactions, protecting plant roots and water quality. Sandy or granite-derived soils have little buffering ability, so they acidify faster and cause more severe ecological damage.

Forests, freshwater lakes, streams, and wetlands are most sensitive. Acid rain leaches essential nutrients like calcium and magnesium from soils, impairs root function, and mobilises toxic aluminium ions. Aquatic ecosystems are particularly vulnerable because many fish and amphibians cannot survive below pH 5.0.

Taller stacks disperse pollutants higher into the atmosphere, spreading them over a larger geographic area rather than concentrating deposition near the source. While this reduces local risk, it can transport acid precursors across long distances and national borders — a key reason acid rain is considered a transboundary pollution problem.

The Risk Index is a composite score combining estimated SO₂ and NOₓ deposition rates, terrain sensitivity, soil buffering capacity, and projected pH change. Scores below 2 indicate low risk, 2–5 moderate risk, 5–8 high risk, and above 8 indicate critical risk requiring immediate mitigation measures.

Effective measures include installing flue-gas desulfurisation (scrubbers) on industrial stacks to reduce SO₂, using catalytic converters and low-NOₓ burners to cut NOₓ emissions, switching to low-sulfur fuels or renewables, and applying agricultural lime to acidified soils to restore buffering capacity.