Understanding impact categories: acidification
Acidification occurs when substances that contribute to the formation of acidity are released into the environment and deposited in terrestrial or aquatic ecosystems. Although natural processes exist, human activities—including fossil-fuel combustion, industry and agriculture—are important sources of acidifying emissions.
Key substances include sulphur dioxide (SO₂), nitrogen oxides (NOₓ) and ammonia (NH₃). These emissions can contribute to the acidification of soils and waters, affecting organisms, vegetation and ecosystem functions.
How is it assessed in LCA?
In Life Cycle Assessment, acidification is an impact category within life cycle impact assessment (LCIA). Methods apply characterisation factors to inventory emissions to estimate their potential contribution to acidification. Some methods express results in kilograms of sulphur dioxide equivalents (kg SO₂ eq.); others, including the European Union's Environmental Footprint method, use their own indicators, such as accumulated exceedance.
Results depend on the method, version and spatial scale considered. The deposition of acidifying substances and ecosystem sensitivity vary geographically, so indicators should not be interpreted as direct measurements of change in a specific ecosystem.
Ocean acidification is different
Ocean acidification is primarily associated with the uptake of atmospheric carbon dioxide (CO₂) by seawater, which changes carbonate chemistry and lowers pH. It should not be confused with the terrestrial or freshwater acidification category commonly used in LCA. Its assessment requires specific indicators and models.
How can emissions be reduced?
Reducing SO₂, NOₓ and NH₃ emissions can lower terrestrial and freshwater acidification pressures. Measures include industrial emission controls, cleaner combustion technologies, low-emission mobility and more efficient fertiliser management. Reducing CO₂ emissions is essential to limit ocean acidification.
LCA helps identify the life-cycle stages and activities contributing to these pressures, enabling comparisons and helping avoid shifting impacts between categories or life-cycle stages.