The North American Mosaic: An Overview of Key Environmental Issues Pollutants Acid Deposition Key Findings Acid deposition primarily results from the transformation of air pollutants such as sulfur n Acid deposition (commonly called dioxide and nitrogen oxides into secondary pollutants such as sulfuric acid, ammonium acid rain) degrades the quality nitrate and nitric acid. Acidic particles and vapors can be deposited on the earth’s surface of forests, coastal ecosystems, as acid precipitation (wet deposition) or via particles such as fly ash, sulfates, nitrates, and lakes and soils; harms wildlife; gases (dry deposition). and corrodes building materials. Acidifying emissions can cross national and provincial or state What Is the Environmental Issue? pollution from Asia affects human health and boundaries, affecting ecosystems the environment in North America. hundreds of kilometers away. Acid deposition has already damaged North Air pollutants, particularly emissions of American forests, lakes, soils, buildings and oxides of sulfur and nitrogen, are the precur- n Sulfur dioxide (SO ) and nitrogen 2 historic monuments—in some cases, irre- sors of acid deposition. In North America, sul- oxides (NOx) are acidifying trievably. The air pollutants giving rise to acid fur dioxide (SO2) and nitrogen oxides (NOx) are emissions that contribute to acid deposition affect human health and air qual- emitted by anthropogenic sources such as metal deposition. These chemicals are ity as well. But the problem is not just a North smelting and fossil fuel combustion in electricity emitted mainly by human activities American one. Because acidic pollutants can generation and transportation, as well as by nat- travel great distances through the atmosphere ural sources such as volcanoes, forest fires and such as metal smelting and fossil to be deposited in ecosystems hundreds and lightning. However, the vast majority of SO and fuel combustion in electricity 2 even thousands of kilometers away, acid depo- NOx emissions that contribute to acid deposition generation and transportation. sition is a global problem. Emissions from are a product of human activities (see illustration n North America travel as far as Europe, and of acid deposition process). Since 1990, emissions of SO2 in North America are down by The acid deposition process one-third and those of NOx have declined by just over one-fifth. O3 Sulfate deposition in the eastern United States and Canada has decreased substantially over Oxidation SO2 the last 15 years, whereas the Reaction with moisture NOx reduction in nitrate deposition has in atmosphere been less dramatic. n Despite the considerable progress Natural Ammonia made toward reducing emissions and the deposition of acidifying Dry deposition Wet deposition pollution, many sensitive ecosystems are still receiving levels of acid deposition above the threshold levels that cause long- Dry deposition Emission Wet deposition term damage. Furthermore, some of gases, particles source of dissolved acids regions previously affected by high and aerosols levels of acid deposition are not Adapted from: Michael Pidwirny, Physical Geography.net—Fundamentals of Physical Geography, recovering as expected. <http://www.physicalgeography.net/fundamentals/8h.html>. Note: This publication was prepared by the Secretariat of the Commission for Environmental Cooperation. The views contained herein do not necessarily reflect the views of the governments of Canada, Mexico or the United States of America. Acidification of ecosystems occurs when Wet sulfate and wet nitrate deposition in the United States and Canada the deposition of acidic compounds exceeds the neutralizing capacity of the receiving 5-year (1990–1994) 5-year (2000–2004) environment. 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As acid rain flows through 5-ye ar (1 990 - 1994)mean Me an Nitratenitrate W wetet De depositionpositi on (k g/ha/yr) 5-ye ar (2 000 - 2004) Me an (kgNitrate/ha W et/yr) De po siti on (k g/ha/yr) soils in a watershed, metals such as alumi- num are released into the lakes and streams in that watershed. Both low pH and increased aluminum levels are directly toxic to fish. In addition, they cause chronic stress that, although it may not kill individual fish, does lead to lower body weight and smaller size and makes fish less able to compete for food and habitat. Acidification of lakes and streams can also increase the amount of methyl mercury available in aquatic systems. In certain lakes (kg/ha/yr) in Canada and the United States that have a low pH, the common loon, a duck-like water- < > 5 5 – 10 10 – 15 15 – 20 20 – 25 25 – 30 30 bird, has been found to have elevated blood mercury levels. Source: NAtChem. Maps courtesy of Chul-Un Ro, Environment Canada. 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