Acid Rain
Primary reference(s)
WMO/UNESCO, 2012. . Accessed 15 November 2019
Additional scientific description
Acids form when certain atmospheric gases (primarily carbon dioxide, sulphur dioxide, and nitrogen oxides) come into contact with water in the atmosphere or on the ground and are chemically converted to acidic substances. Oxidants play a major role in several of these acid-forming processes. Carbon dioxide dissolved in rain is converted to a weak acid (carbonic acid). Other gases, primarily oxides of sulphur and nitrogen, are converted to strong acids (sulphuric and nitric acids). Rain is naturally slightly acidic owing to carbon dioxide, natural emissions of sulphur and nitrogen oxides, and to certain organic acids, however, emissions from human activities can make it much more acidic. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialised areas (NASA, 2014).
The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and from biological processes that occur on land, in wetlands, and in the oceans. The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe (Pawar, no date).
The main human sources are industrial and power-generating plants, and transportation vehicles. Since the industrial revolution, emissions of sulphur and nitrogen oxides to the atmosphere have increased. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulphur oxide emissions (NASA, 2019). Acidity and alkalinity are measured using a pH scale for which 7.0 is neutral. The lower the pH of a substance (below 7.0), the more acidic it is. The higher the pH of a substance (above 7.0), the more alkaline it is. Normal rain has a pH of about 5.6; while the pH of acid rain is typically between 4.2 and 4.4 (US EPA, 2019).
Metrics and numeric limits
Acidity and alkalinity are measured using a pH scale for which 7.0 is neutral. The lower the pH of a substance (below 7.0), the more acidic it is. The higher the pH of a substance (above 7.0), the more alkaline it is. Normal rain has a pH of about 5.6; while the pH of acid rain is typically between 4.2 and 4.4 (US EPA, 2019).
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 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 (Government of Canada, 2021). 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 the pollution that creates ozone, the Ozone Annex commits both countries to reduce their emissions of nitrogen oxides and volatile organic compounds, the precursor pollutants to ground-level ozone.
Examples of drivers, outcomes and risk management
Acid rain results when sulphur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents, and the SO2 and NOX carried in the air react with water, oxygen and other chemicals to form sulphuric and nitric acids. These then mix with water and other materials before falling to the ground in rain. While a small proportion of the SO2 and NOX that cause acid rain is from natural sources such as volcanoes, most is from the burning of fossil fuels. The major source of SO2 and NOX in the atmosphere is burning of fossil fuels to generate electricity. Two-thirds of SO2 and a quarter 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 and not just those living near the emission sources (US EPA, 2019).
References
NASA, 2014. . Accessed 24 March 2021.
UNECE, 1979. . Accessed 7 October 2020.
NASA, 2019. . Accessed 18 November 2019.
US EPA, 2019. . Accessed 18 November 2019.
Pawar, P., no date. . Accessed 24 March 2021.
Government of Canada, 2021. . Accessed 13 May 2021.