Acid Rain

Primary reference(s)

WMO/UNESCO, 2012. International Glossary of Hydrology. World Meteorological Organization (WMO) / United Nations Educational, Scientific and Cultural Organization (UNESCO). International Glossary of Hydrology - UNESCO Digital Library. Accessed 12 February 2025.

Annotations

Key relevant UN convention / multilateral treaty

The Convention on Long-range Transboundary Air Pollution (LRTAP Convention) (UNECE, 1979) serves as an umbrella convention for the international regime on the regulation of transboundary acidification in the member states of the United Nations Economic Commission for Europe (UNECE). The USA and Canada are members of the UNECE, as are all European countries.

Since 1979, the LRTAP Convention has addressed some of the major environmental problems of the UNECE region through scientific collaboration and policy negotiation. The Convention has been extended by eight protocols that identify specific measures to be taken by Parties to cut their emissions of air pollutants. The Convention, which now has 51 Parties, identifies the Executive Secretary of the UNECE as its secretariat.

The 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (Gothenburg Protocol) was amended in 2012. The Protocol establishes national emission ceilings for four pollutants—sulphur (SO₂), nitrogen oxides (NOₓ), volatile organic compounds (VOCs) and ammonia (NH₃)—covering the period from 2010 to 2020 (UNECE, no date).

Drivers

Acid rain occurs when carbon dioxide (CO₂), sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) are emitted into the atmosphere and transported by wind and air currents. The CO₂, SO₂ and NOₓ (see Chemical HIPs CH0300 and CH0901) carried in the air react with water, oxygen and other chemicals to form carbonic, sulphuric and nitric acids. These then mix with water and other materials before falling to the ground in the rain. While a small proportion of the SO₂ and NOₓ that cause acid rain is from natural sources such as volcanoes, most are from the burning of fossil fuels to generate electricity. Sixty percent of SO₂ and 15% of the NOₓ in the atmosphere are from electric power generation. Winds can transport SO₂ and NOₓ over long distances and across national borders, making acid rain an international problem—for everyone, not just those living near the emission sources (US EPA, 2025a).

Impacts

Acid rain impacts both terrestrial and aquatic ecosystems, affecting the acidity of soils and water, which causes the leaching of metals into soil and water columns, further impacting biodiversity. In some areas, acid rain has resulted in fish deaths and loss of forests (US EPA, 2025b). 

Plants are affected directly by acid rain due to reductions in growth, productivity and yield, by damaging photosynthetic mechanisms and reproductive organs, or indirectly by affecting underground components such as soil and root systems. Genes that play an important role in plant defence under abiotic stress are also modulated in response to acid rain. Acid rain induces soil acidification and disturbs the balance of carbon and nitrogen metabolism, litter properties, and microbial and enzymatic activities (Prakash et al., 2023). 

Acid rain increases the acidity of lake water. This causes changes in the assemblages of plants and animals that occur naturally in the lake. An acid-stressed lake is typically very clear, with filamentous algae along the bottom and a reduced fish population. Acid rain can leach copper, aluminium and other heavy metals out of the soil and into runoff and drinking water. This process, in turn, puts more harmful materials in the water and soil, thus reducing the populations of organisms in the waterbody or soil. In some lakes, fish are unable to survive due to the acidity of the water (US EPA, 2025b). 

Multi-hazard context

The figure below summarises common interactions between acid rain and other hazards. This information should be used with caution and not be solely relied upon in disaster risk management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

Addressing air pollution can reduce acid rain and its impacts. For example, the establishment of sulphur controls on power plants under the United States Acid Rain Program resulted in emissions reductions and subsequent mortality reduction (Barreca et al., 2021).

The 1986 U.S.-Canada Air Quality Agreement is a bilateral agreement aimed at addressing the transboundary air pollution that leads to acid rain (Government of Canada, 2021). The Ozone Annex was added to the Canada-United States Air Quality Agreement (December 2000) to address the transboundary air pollution leading to high air quality levels of ground-level ozone, a major component of smog. The long-term goal of the Ozone Annex is the attainment of the ozone air quality standards in both countries. Where there are transboundary flows of pollution that create ozone, the Annex commits both countries to reduce their emissions of nitrogen oxides and volatile organic compounds, the precursor pollutants to ground-level ozone

Monitoring

NADP (USA) and EMEP (Europe) monitor acid rain via pH/ion concentration networks (NADP, 2024).