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Home , Acid rain

The North American Mosaic: An Overview of Key

Pollutants Deposition

Key Findings Acid deposition primarily results from the transformation of air such as

n Acid deposition (commonly called dioxide and into secondary pollutants such as , ammonium acid ) degrades the quality nitrate and . Acidic particles and vapors can be deposited on the ’s surface of , coastal , as acid (wet deposition) or via particles such as fly ash, , nitrates, and lakes and ; harms wildlife; gases (dry deposition). and corrodes building materials. Acidifying emissions can cross national and provincial or state What Is the Environmental Issue? 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 (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 and 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 ural sources such as volcanoes, fires and such as metal smelting and fossil to be deposited in ecosystems hundreds and . 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 deposition in the eastern United States and 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 regions previously affected by high and levels of acid deposition are not Adapted from: Michael Pidwirny, Physical Geography.net—Fundamentals of Physical Geography, recovering as expected. .

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. Clean rainwater is slightly 5-ye ar (15-99ye0ar - 1(199994)0 Me- 19an94) Su Melpanha Sute Wlpetha Dteep Wositiet Doepn ositi(k g/haon/ yr)(k g /ha /yr) 5 -ye ar (25-00ye0ar - 2(200004)0 Me- 20an04) Su Melpanha Sute Wlpetha Dteep Wositiet Doepn ositi on 5-5ye-yearar (1 (199990 -0mean 1- 91994)94) Me sulfate Meanan Su Sulp lhwetpahtaet Wedeposition Wetet D epDepositiositiono n(k (gk/gha/ha/yr)/yr) 5-ye armean (2 000 -sulfate 2004) Me wetan Su depositionlphate W et Depositi on acidic, with a pH of about 5.6, because it 5-ye ar (2 000 - 200(kg4) Me/haan/yr) (kgSu /lhaph/ayr)te W et Depositi on (kg(kg/ha/ha/yr)/yr) contains dissolved dioxide from the air. Acidic pollution has a pH lower than this, normally ranging between 4 and 5. A decrease in a single pH unit represents a ten- fold increase in acidity. Thus rainwater with a pH of 4.2 is about 25 times more acidic than clean rain. By making and water more acidic, acid deposition harms plants, 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep ositi on (k g/ha /yr) 5animals-ye ar (2 and00 ecosystem0 - 200 4) integrity Me an in Su affectedlph ate W et Dep ositi on 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep ositi on (k g/ha /yr) 5-ye ar (2areas.000 It- also200 damages4) Me anbuildings, Su(kglp monumentsh/haate/yr) W et Dep ositi on 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep ositi on (k g/ha /yr) 5-ye ar (2 000 - 2and004) painted Me an surfaces. Su (kglp h/haate/yr) W et Dep ositi on 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep ositi on (k g/ha /yr) 5-ye ar (2 000 - 2004) Me an SuIn a(kg l lake,/haa acidte/yr) W deposition et Dep createsositi o a n cas - 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep ositi on (k g/ha /yr) 5-ye ar (2 000 - 2004) Me an Su (kglpcadeh/haa ofte/ effectsyr) W et thatDep reduceositi fishon populations and 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep ositi on (k g/ha /yr) 5-ye ar (2 000 - 2004) Me an Su lphate W et Dep ositi on 5-year (1990–1994) 5-year (2000–2004) (kg /ha /yr)may even completely eliminate a fish species 5-ye ar (1 990 - 1994) Me an Su lphate W et Dep5ositi-5ye-yearar (1 (199o990n -0 1-( 91k994)9g4) Me/ Mehaanan Nitrate/ Nitrateyr) W Wetet De Depopsitiositio5no- n(kye (g/k g/ha/yr)arha/yr) (2 005-5ye-0year ar -(2 (20020000 -0 0 2- 04)2004)04) MeMe Meanan Nitratean Nitrate Su W Wetlet Dep Dehpopasitiosititoeno n(Wk (g/k g/heta/yr)ha/yr) D ep ositi on 5-ye ar (1 990 - 1994) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye armean (2 000 nitrate- 2004) Me wetan Nitratedeposition(kg W/haet De/yr)positi on (k g/ha/yr)from a water body. 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 available in aquatic systems. In certain lakes (kg/ha/yr) in Canada and the United States that have a low pH, the , 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. In forest soils, excess acid deposition

increases the susceptibility of forests to stresses from pests, pathogens and climate change, resulting in poorer forest health, lower timber Air emissions of SO2 Air emissions of NOx yields and eventual changes in the composi- 5-ye ar (130,000990 - 1994) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye artion (2 of00 forest0 - 2species.004) AcidMe anrain Nitrate weakens W et De positi on (k g/ha/yr) 5-ye ar (1 990 - 1994) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye ar (2 000 by- 2damaging004) Me theiran leaves,Nitrate limiting W et the De nutripo- siti on (k g/ha/yr) 5-ye ar (1 990 - 1994) Me25,000an Nitrate W et De positi on (k g/ha/yr) 5-ye ar (2 000 - 2004) ents Me availablean Nitrate to them W et or exposingDe positi themon to ( k g/ha/yr) 5-ye ar (1 990 - 1994) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye ar (2 000 - 2004) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye ar (1 990 - 1994) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye ar (2 000 - 2004) Me an Nitrate Wtoxicet substancesDe positi slowlyon (releasedk g/ha/yr) from the soil. 5-ye ar (1 990 - 1994) Me an Nitrate W et De po20,000siti on (k g/ha/yr) 5-ye ar (2 000 - 2004) Me an Nitrate W et DeQuitepositi often,on these(k g/ heffectsa/yr) of acid rain, in com- 5-ye ar (1 990 - 1994) Me an Nitrate W et De positi on (k g/ha/yr) 5-ye ar (2 000 - 2004) Me an Nitrate W et De positi obinationn (k g/ withha/yr) one or more additional threats, 15,000 injure or kill trees. Finally, the pollutants that cause acid Thousand tonnes 10,000 rain are harmful to human health. In the air, they join with other chemicals to pro- 5,000 duce , which can irritate the lungs and make breathing difficult, especially for

0 1990 2005 1990 2005 people suffering from , or other respiratory conditions. Fine particu- Canada Mexico United States late matter, containing sulfate derived from

Sources: Environment Canada, Instituto Nacional de Ecología (latest data from Mexico from 2002, not 2005), US Environmental Protection Agency. SO2, is thought to be especially damaging to the lungs.

2 Commission for Environmental Cooperation Why Is This Issue Important to Case Study – Acidification at El Tajín, Mexico North America?

The effects of acid deposition across North America can be addressed only in cooperation with the neighboring jurisdictions that contrib- ute to acidifying emissions. The issue of acid rain first caught the public’s attention in the late 1970s and early 1980s when its devastating impacts on ecosystems in eastern North America were pub- licized. In 1980 Canada and the United States began working together to address this trans- boundary issue. The two countries signed the Canada-US Air Quality Agreement in 1991 to promote scientific understanding and pollution reduction in both countries.

Efforts to Reduce Emissions Canada and the United States gave priority to

SO2 emissions because lowering these emis- sions was understood to be most important in lessening damage to sensitive ecosystems.

Since 1990, SO2 air emissions in North America have declined by almost one-third (see graph). At present, electric power generation accounts for the largest emissions of SO2 in the United States, while in Canada the dominant emitting sector is base metal smelting. Over the same period, air emissions of

NOx have declined by just over one-fifth (see graph). Mobile sources such as cars and trucks are the most significant sources of NOx emis- sions in North America, with the remainder coming from power plants and other sources. Pyramid of the Niches, El Tajín, Veracruz, Mexico. Source: Luiz Castro.

Results Located in the present-day municipality of Papantla de Olarte in Veracruz, Mexico, El Tajín In response to emission reductions, levels of was one of the most important cities in the Mesoamerican Gulf zone. Its archaeological sulfate deposition in the eastern United States zone contains constructions dating back to 100 A.D. From 600 to 1150 A.D., the city reached and Canada decreased substantially over the its maximum size and influence. period 1990–2004, whereas changes in levels Humberto Bravo Álvarez and the section on environmental contamination at the of nitrate deposition have been less dramatic Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, are (see maps). studying the effects of acid rain on archaeological and historical sites at El Tajín. From 18 August 2002 to 9 April 2003, they collected 40 rain samples at the El Tajín archaeo- Affected Areas logical site and applied atmospheric trajectory analysis to each precipitation sample to Many of the water and soil systems in eastern determine air pathways corresponding to the precipitation events. Trajectory North America cannot neutralize acid natur- models are useful in identifying upwind regions likely to contribute to the ally. As a result, these areas are sensitive to burden at downwind receptors. acid deposition. To understand the capacity The analyses indicated that 85 percent of the precipitation events sampled at El of ecosystems to absorb acid deposition, sci- entists have developed the concept of “criti- Tajín were acidic (pH < 5.62). The back trajectory analysis of these acidic events showed cal load”—that is, an estimate of the amount a great variation, indicating there was no apparent directional preference for trans- of deposition that a particular can port during these events and suggesting the importance of local sources. The El Tajín receive below which no harmful effects occur. archaeological zone is surrounded by possible sources of acid rain precursors in the The critical load depends on the quantity of form of industries burning fuel oil with a high sulfur content (such as electric power acid-neutralizing bases, such as calcium and plants and refineries). Thus both these sources and more distant ones may be impor- salts, in a region’s water and in tant contributors to rainfall acidity at El Tajín. the surrounding rocks and soils.

Acid Deposition 3 Figure 1. Forest areas sensitive to acid deposition in the New England states and Eastern Canadian provinces. FigureFigure 1.1. ForestForest areasareas sensitivesensitive toto acidacid depositiondeposition inin ththee NewNew EnglandEngland statesstates andand EasternEastern CanadianCanadian provinces.provinces. FigureFigure(Please(Please 1.note1.note ForestForest discussiondiscussion areasareas ofof sensitive sensitivedatadata variabilityvariability toto acidacid onon pagedepositionpagedeposition 7)7) inin ththee NewNew EnglandEngland statesstates andand EasternEastern CanadianCanadian provinces.provinces. Figure(PleaseFigure(PleaseFigure note1.note1. 1. ForestForest Forest discussiondiscussion areasareas areas ofof datasensitivesensitive datasensitive variabilityvariability toto to acidacid acid onon page pagedepositiondeposition deposition 7)7) inin in thth theee NewNew New EnglandEngland England statesstates states andand and EasternEastern Eastern CanadianCanadian Canadian provinces.provinces. provinces. (Please(Please notenote discussiondiscussion ofof datadata variabilityvariability onon pagepage 7)7) (Please(Please(Please notenote note discussiondiscussion discussion ofof of datadata data variabilityvariability variability onon on pagepage page 7)7) 7)

Despite the progress in reducing acidify-

ing emissions, some ecosystems are making a Case Study – Forest Sensitivity to Sulfur and Nitrogen Deposition slower-than-expected recovery. In the United Although sulfur emissions in the United States and Canada have fallen in response States, acidic surface waters are still found in the upper Midwest, Adirondack Mountains to control programs, the continued emissions of acidifying sulfur and nitrogen com- and northern Appalachian regions. In Canada, pounds present a serious long-term threat to forest health and productivity in parts of the areas receiving depositions higher than northeastern North America. This conclusion was reached by a study of the northeast- their critical loads are in provinces that are ern United States and eastern Canada conducted by the Conference of New England part of the Canadian Precambrian Shield. In Governors and Eastern Canadian Premiers Forest Mapping Group. , Quebec, New Brunswick and Nova According to the Forest Mapping Group’s study, the atmospheric deposition of Scotia, the susceptible hard rock (granite) sulfur and nitrogen from 1999 to 2003 exceeded the critical load in more than one-third areas lack the natural capacity to neutralize or of the study area (see map). In the eastern Canadian provinces, the most sensitive buffer acid rain effectively. Historically, lower forest areas occur in southern Quebec, especially in the Lower-Laurentides north of levels of industrialization combined with the St. Lawrence River, in southeastern Nova Scotia, and in southern Newfoundland. natural factors such as eastwardly moving In New England, the most sensitive forest areas occur in the mountain ranges and patterns and soils with buffering cap- coastal areas where soils are poor and rates low, and where there is greater acity have largely protected western prairie ecosystems in Canada and the United States demand for nutrients due to more intensive harvesting. from the impacts of acid rain. Based on forest monitoring in Quebec and the known effects of acid deposition, Although similar maps are not available the Forest Mapping Group’s study concludes that high exceedances of critical loads for Mexico, the effects of acid deposition are lead directly or indirectly to reduced forest growth and health. evident in national parks near Mexico City where acid rain has damaged forests and soils, Forest areas sensitive to acid deposition in the New England states as well as in the damage to monuments and and eastern Canadian provinces historic buildings in Mexico City and else- where (see case study, previous page).

What Are the Linkages to Other North American Environmental Issues?

Scientists’ initial concerns about the effects of acid deposition on forests and building materi- als have now expanded to include the relation- of acidifying pollution to and Steady-State Exceedance sensitive coastal ecosystems. (eq/ha/yr)

< -600 Biodiversity -599 – -400 The various species that inhabit lakes, rivers -399 – -200 and differ in their abilities to toler- -199 – 0 ate acidity. Acidification primarily reduces +1 – +200 +201 – +400 the variety of life inhabiting a lake and alters +401 – +600 the balance among surviving populations. +601 – +800 Scale Changes in the mix of species inhabiting +801 – +1,000 0 150 300 km > +1,000 water bodies also affect birds and other spe-

cies farther up the food chain, as some kinds Source: Conference of New England Governors and Eastern Canadian Premiers Forest Mapping Group. of food resources become scarcer and others become more abundant. Scientists cannot say whether species that have disappeared 60 percent of estuaries in the United States suf- in the water because of the large amounts of from an acidified lake will ever return—even fer from overenrichment of nitrogen, a condi- algae can kill off submerged aquatic vegetation,

if pH levels return to normal. tion known as . Symptoms of which is an important habitat for many estua- eutrophication include changes in the domi- rine fish and shellfish species. Although a large Coastal Ecosystems nant species of plankton (the primary food number of the most highly eutrophic estuaries Linked to acid deposition is the effect of nitro- source for many kinds of marine life), which are along the US Gulf and Mid-Atlantic coasts, gen deposition on coastal ecosystems, where can cause algal blooms, low levels of overlapping many of the areas with the highest nitrogen is often the limiting nutrient. Higher in the water column, fish and shellfish kills, nitrogen deposition, eutrophic estuaries can levels of nitrogen in coastal waters can cause and cascading population changes up the food be found in every region of the conterminous significant changes to those ecosystems. Some chain. In addition, the higher levels of US coastline.

4 The North American Mosaic: An Overview of Key Environmental Issues

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