Acid rain on plants

Effects of Acid Rain

On this Page:

  • Effects of Acid Rain on Ecosystems
  • Effects of Acid Rain on Materials
  • Other Effects of SO2 and NOX
    • Visibility
    • Human Health

The Effects of Acid Rain on Ecosystems

This figure illustrates the pH level at which key organisms may be lost as their environment becomes more acidic. Not all fish, shellfish, or the insects that they eat can tolerate the same amount of acid.

An ecosystem is a community of plants, animals and other organisms along with their environment including the air, water and soil. Everything in an ecosystem is connected. If something harms one part of an ecosystem – one species of plant or animal, the soil or the water – it can have an impact on everything else.

Effects of Acid Rain on Fish and Wildlife

The ecological effects of acid rain are most clearly seen in aquatic environments, such as streams, lakes, and marshes where it can be harmful to fish and other wildlife. As it flows through the soil, acidic rain water can leach aluminum from soil clay particles and then flow into streams and lakes. The more acid that is introduced to the ecosystem, the more aluminum is released.

Some types of plants and animals are able to tolerate acidic waters and moderate amounts of aluminum. Others, however, are acid-sensitive and will be lost as the pH declines. Generally, the young of most species are more sensitive to environmental conditions than adults. At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die. Some acidic lakes have no fish. Even if a species of fish or animal can tolerate moderately acidic water, the animals or plants it eats might not. For example, frogs have a critical pH around 4, but the mayflies they eat are more sensitive and may not survive pH below 5.5.

Effects of Acid Rain on Plants and Trees

Dead or dying trees are a common sight in areas effected by acid rain. Acid rain leaches aluminum from the soil. That aluminum may be harmful to plants as well as animals. Acid rain also removes minerals and nutrients from the soil that trees need to grow.

At high elevations, acidic fog and clouds might strip nutrients from trees’ foliage, leaving them with brown or dead leaves and needles. The trees are then less able to absorb sunlight, which makes them weak and less able to withstand freezing temperatures.

Buffering Capacity

Many forests, streams, and lakes that experience acid rain don’t suffer effects because the soil in those areas can buffer the acid rain by neutralizing the acidity in the rainwater flowing through it. This capacity depends on the thickness and composition of the soil and the type of bedrock underneath it. In areas such as mountainous parts of the Northeast United States, the soil is thin and lacks the ability to adequately neutralize the acid in the rain water. As a result, these areas are particularly vulnerable and the acid and aluminum can accumulate in the soil, streams, or lakes.

Episodic Acidification

Melting snow and heavy rain downpours can result in what is known as episodic acidification. Lakes that do not normally have a high level of acidity may temporarily experience effects of acid rain when the melting snow or downpour brings greater amounts of acidic deposition and the soil can’t buffer it. This short duration of higher acidity (i.e., lower pH) can result in a short-term stress on the ecosystem where a variety of organisms or species may be injured or killed.

Nitrogen Pollution

It’s not just the acidity of acid rain that can cause problems. Acid rain also contains nitrogen, and this can have an impact on some ecosystems. For example, nitrogen pollution in our coastal waters is partially responsible for declining fish and shellfish populations in some areas. In addition to agriculture and wastewater, much of the nitrogen produced by human activity that reaches coastal waters comes from the atmosphere.

  • Learn more about Nitrogen Pollution
  • EPA’s Chesapeake Bay Program Office

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Effects of Acid Rain on Materials

Not all acidic deposition is wet. Sometimes dust particles can become acidic as well, and this is called dry deposition. When acid rain and dry acidic particles fall to earth, the nitric and sulfuric acid that make the particles acidic can land on statues, buildings, and other manmade structures, and damage their surfaces. The acidic particles corrode metal and cause paint and stone to deteriorate more quickly. They also dirty the surfaces of buildings and other structures such as monuments.

The consequences of this damage can be costly:

  • damaged materials that need to be repaired or replaced,
  • increased maintenance costs, and
  • loss of detail on stone and metal statues, monuments and tombstones.

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Other Effects of SO2 and NOX


In the atmosphere, SO2 and NOX gases can be transformed into sulfate and nitrate particles, while some NOX can also react with other pollutants to form ozone. These particles and ozone make the air hazy and difficult to see through. This affects our enjoyment of national parks that we visit for the scenic view such as Shenandoah and the Great Smoky Mountains.

  • Learn more about Visibility and Regional Haze

Human Health

Walking in acid rain, or even swimming in a lake affected by acid rain, is no more dangerous to humans than walking in normal rain or swimming in non-acidic lakes. However, when the pollutants that cause acid rain —SO2 and NOX, as well as sulfate and nitrate particles— are in the air, they can be harmful to humans.

SO2 and NOX react in the atmosphere to form fine sulfate and nitrate particles that people can inhale into their lungs. Many scientific studies have shown a relationship between these particles and effects on heart function, such as heart attacks resulting in death for people with increased heart disease risk, and effects on lung function, such as breathing difficulties for people with asthma.

Learn more about:

  • Sulfur Dioxide
  • Nitrogen Oxides
  • Particulate Matter (PM)
  • Asthma

In addition, NOX emissions also contribute to ground level ozone, which is also harmful to human health.

  • Learn more about Ground Level Ozone

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Toxic Rain: The Effect of Acid Rain on the Environment

Two vacation places that I frequently visit all throughout the year are Southampton, Long Island, and the Adirondacks, New York. These places have become great destinations for me when I want to forget about my worries and relax. Whether I am skiing down Whiteface Mountain or boogie boarding the huge waves at Copper Beach, the placid atmosphere of these two places engulfs me and it is my way of relieving the stress that has accumulated during the past week or so. Even though these two places are far from major cities, I have recently become aware that parts of Long Island and the Adirondacks have been experiencing acid deposition for a couple of decades. This worries me, because if we humans do not do something about this quick, these places might become so polluted that we may never have the chance again to go back to the clean environments that we once enjoyed and treasured. I have known these two places since I was a few months old, and the thought that in future years they might not be the same for my grandchildren and their family is really upsetting.

So, what is acid deposition?

Acid deposition, also called acid rain, is rain or gases that have been polluted by high amounts of chemicals and acids in the atmosphere. It can result from decaying plants and animals or natural cataclysms, such as volcanoes, but the major cause of acid rain is the releasing of chemicals by humans. The main gases that lead to acid rain are sulfur dioxide and nitrogen dioxide. When they come into contact with water and oxygen they turn into acids. Acid Deposition can be in the form of precipitation, which is called wet deposition, or it could be in the form of gases and microscopic particles floating the air, which is called dry deposition.

Scientists can measure how much acid is in rain or a body of water by using the pH scale. There are 14 numbers on it, ranging from 0 through 14. If a lake has a low pH, that tells us that there is a high amount of acid in the lake. If a lake has a pH 8 or above, it is alkaline, which means there is not a lot of acid in it. When a body of water has a pH of 7, it is neutral, since it is in the middle. New York State’s rain pH level is between 4 and 4.5. That is 30 times more acidic than the normal level!

Remember: All bodies of water have acid in it, but the problem with acid rain is that too much acid is accumulating, and the effects are harmful.

Where does Sulfur dioxide and nitrogen dioxide come from?

One of the central sources of sulfur dioxide and nitrogen oxide come from power plants. When power plants generate electricity, they are burning the fossil fuel, coal. Coal is sometimes dubbed as the dirty fuel source because when it is burned, it lets out sulfur, nitrogen, and other gases. The more coal we use, the more sulfur and nitrogen we are admitting into our atmosphere. Fumes and emissions from cars and other vehicles are also another source of sulfur dioxide and nitrogen oxide.

Harmful effects of acid deposition

Acid deposition is very dangerous for trees and forests because it rids the soil of very important nutrients trees need to survive, like magnesium and calcium. Without these vital nutrients, the trees are more vulnerable to infections and damage by cold weather and insects. Acid rain also allows aluminum to seep into the soil, and with too much aluminum in the soil, the trees have a very hard time collecting water. Acid rain is even thought to destroy leafs’ outer-coat and when it finally wears down, the acid can make its way into the tree, which prevents photosynthesis from taking place. Photosynthesis makes food and energy for the plant, and without it, the plant or tree dies.

Not only are plants affected by acid deposition, but humans are too. If we breathe in the infinitesimal acid particles, we are prone to getting lung and respiratory problems and diseases such as asthma, chronic bronchitis (long-term), and pneumonia. Just in the United States and Canada alone, there were 1520 visits to the emergency room because of dry deposition. Yet, if you swim in a body of water with a high acidity level, nothing will happen to your body.

Acid Rain proposes a very harmful affect on the ecosystems as well. The acidity in the water can cause many fish and sea life to die, and that can throw off the whole food-chain. A test was done and the results, which were published in 1990, showed that most of the lakes in the Adirondack area had low pH levels and that the lakes with these low levels had no fish.

What is the United States doing to help the issues of acid disposition?

In 1985, the Clean Coal Technology Program was established to help make the burning of coal “cleaner.” Four billion dollars have been donated by the coal industry and two billion dollars by the federal government to help with this goal. There are many ways coal can become cleaner, such as crushing it and washing it before using it, because by doing so some of the sulfur is being removed. Companies also install flue gas desulfurization systems, otherwise known as a scrubber, which have the potential to remove about 90% of the sulfur dioxide before its gets emptied out into the atmosphere. This system works by spraying a limestone and water mixture on the pipe where the smoke from the coal is released. When the lime meets the smoke, with the sulfur in it, the smoke is absorbed into it and becomes a gooey liquid or powder and the most of the sulfur is trapped. You can then recycle the liquid or powder to make objects such as concrete blocks. These are just some of the ways coal can cause less pollution, and there are many more ways.

In 1990, Congress passed the Clean Air Act. This act stated that the EPA, Environmental Protection Agency, should do their part and help protect the air we breathe, so the Acid Rain Program was initiated. This program strives hard to achieve both environmental and health satisfaction by limiting the amount of sulfur dioxide and nitrogen oxide admitted into the air by power plants.

If we reduce air pollution, acid rain might become a thing of the past! Think of a place that you really love to go and picture it polluted in future years. Not a nice thought, right? This is why we have to try our best to protect the air that we live and breathe every day!

Image source: Brian Adams (via Flickr)

“1990 Interpretive Report: Executive Summary.” Adirondack Lakes Survey Corporation.

“Acid Rain.” NYS Department of Environmental Conservation.

“Acid Rain.” Oracle Education Foundation.

“All About Coal.” American Coal Foundation.

“Clean Air Act.” United States Environmental Protection Agency.

“What Causes Acid Rain?” United States Environmental Protection Agency.

“What is Acid Rain?” United States Environmental Protection Agency.

“Why is Acid Rain Harmful?” United States Environmental Protection Agency.


Effects of simulated acid rain on leaf anatomy and micromorphology of Genipa americana L. (Rubiaceae)

Bruno Francisco Sant’Anna-Santos; Luzimar Campos da Silva; Aristéa Alves Azevedo; Rosane Aguiar


Experiments were conducted in order to characterize the injuries on leaf structure and micromorphology of G. americana and evaluate the degree of susceptibility of this species to simulated acid rain. Plants were exposed to acid rain (pH 3.0) for ten consecutive days. Control plants were submitted only to distilled water (pH 6.0). Leaf tissue was sampled and fixed for light and scanning electron microscopy. Necrotic interveinal spots on the leaf blade occurred. Epidermis and mesophyll cells collapse, hypertrophy of spongy parenchyma cells, accumulation of phenolic compounds and starch grains were observed in leaves exposed to acid rain. The micromorphological analysis showed, in necrotic areas, plasmolized guard cells and cuticle rupture. Epidermal and mesophyll cells alterations occurred before symptoms were visualized in the leaves. These results showed the importance of anatomical data for precocious diagnosis injury and to determine the sensitivity of G. americana to acid rain.

Key words: Genipa americana, simulated acid rain, leaf anatomy, leaf micromorphology


Experimentos foram conduzidos para avaliar o grau de susceptibilidade e determinar as injúrias causadas pela chuva ácida simulada na anatomia e micromorfologia foliar de Genipa americana. Plantas foram expostas à chuva com pH 3,0 durante 10 dias consecutivos. No tratamento controle utilizou-se apenas água destilada (pH 6,0). Amostras foliares foram coletadas e fixadas para microscopia de luz e eletrônica de varredura. Foram observados nas folhas expostas à chuva ácida: necroses pontuais intervenais, colapso das células do mesofilo e da epiderme; hipertrofia do parênquima lacunoso e acúmulo de compostos fenólicos e grãos de amido. A análise micromorfológica evidenciou, nas áreas necrosadas, plasmólise das células-guarda e ruptura da cutícula e da crista estomática. Alterações anatômicas ocorreram antes que sintomas visuais fossem observados nas folhas. Estes resultados comprovam a importância de dados anatômicos na diagnose precoce da injúria e na determinação da sensibilidade de G. americana à chuva ácida.


The sources of atmospheric deposition can be categorized as either natural or anthropogenic. Unlike the case of fluoride that is emitted by a fewer industries such as the aluminum ones, there are many anthropogenic sources that acidify rain water (Horner and Bell, 1995). Nitrogen and sulphur oxides are the major sources of atmospheric acidity; both are products of combustion, and both are converted in the atmosphere to strong acids, mainly nitric and sulphuric acids that acidify the rain water (Cowling and Linthurst, 1981). Rain that presents a concentration of H+ ions greater than 2.5 µeq-1 and pH values lower than 5.6 is considered acid (Evans, 1984).

Urban air pollution is a major environmental problem, mainly in the developing countries (Mage et al., 1996). In Brazil, cities such as Piracicaba, São Paulo, Cubatão and Rio de Janeiro experience the impact of atmospheric pollution and acid rain (Klumpp et al., 1996, 1999; Lara et al., 2001). Acid precipitation with pH values below 5.0 was registered in different regions in Brazil (Santos and Souza, 1988; Mello, 2001; Mello and Almeida, 2004).

Several experiments have been carried out in the field and in greenhouses to investigate the effects of acid rain on plants (Paparozzi and Tukey, 1983; Percy and Baker, 1987; Turunen and Huttunen, 1991; Nouchi, 1992; Temple et al., 1992; Gabara et al., 2003; Silva et al., 2005a; 2005b). Some species are more sensitive to acid rain than others (Evans, 1984; Silva et al., 2005a; 2005b). The incidence and severity of leaf injury to acid rain is associated to many variables as plant species, age of tissue and plants, foliar wettability, leaf pubescence and environmental factors (Dickison, 2000).

In the tropical regions there are few studies on pollution using plant species (Klumpp et al., 1996; Klumpp et al., 1999). Forest ecosystems in southeastern Brazil are potentially susceptible to problems related to acid deposition because of increases in the consumption of oil products, natural gas, and coal used to produce energy for the different economic sectors of the region (Mello and Almeida, 2004).

The Rio Doce State Park (RDSP) is considered the largest area of tropical semidecidous forest under legal protection in Minas Gerais State (IEF, 1994). Located in a region known as “Steel Valley”, the RDSP has an area of approximately 36,000 hectares and there are charcoal burners in the south and large-sized steel industries in the north.

Preliminary precipitation chemistry analysis from 1985 to 1986 in RDSP detected pH values between 3.47 to 7.62 (Castro et al., 1987). The minimum value observed has large potential to cause damaging effects to the ecosystem (Castro et al., 1987). Jordão et al. (1996) observed that plant populations were under pollution impact occasioned by anthropogenic activities in this region. Genipa americana occurs in the RDSP and Silva et al. (2000) showed evidence of air pollution impacts on this species.

G. americana, popularly known as ‘Jenipapeiro’, is a fast growing tree species found throughout Brazil. It has single, subcoriaceous, and glabrous leaves (Lorenzi, 2002). The wood is used for construction and for cabinet-work. The fruits are edible and provide a delicious liqueur that is much consumed. In some regions of Brazil, the juice is used as a blue colorant (Lorenzi, 2002). The aim of this study was to characterize the injuries on leaf structure and micromorphology of G. americana and evaluate the degree of susceptibility of this species to simulated acid rain.


Seedlings and saplings of Genipa americana were obtained from germinated seeds and supplied by the RDSP nursery. The seedlings (about two months old with cotyledons and two nodes with leaves) were supplied in 290 cm3 containers (53 mm internal diameter, 190 mm height, presenting a circular section with 8 vertical grooves) filled with soil. Saplings (about four months old) were transferred from plastic bags to plastic pots (2,000 ml capacity) filled with sand. The plantlets were acclimatized in a covered area for 15 to 20 days. The plants received Hoagland nutritive solution (1/4 strength) every five days throughout the experiment and were daily submitted to simulated acid rain (SAR) event of twenty minutes for 10 consecutive days. The simulated rain was applied using a chamber constructed and adapted by Alves (1988) from the model proposed by Evans et al. (1977). Before and after each simulation, the plants were maintained under a luminous panel, consisting of eight incandescent high pressure mercury lamps (E-27, 220-230 Volts, 250 Watts) for fifteen minutes under radiant flow density of 95 W.m-2 and remained the rest of the time under ambient light conditions (28.8/9.20C average day/night temperatures, relative humidity 79.1%, precipitation 0.0 mm).

The SAR was prepared by mixing 1N sulphuric acid and distilled water to reduce the pH to a value equal to 3.0. The pH value of the SAR used in this study was chosen based on the data of Silva et al. (2005a; 2005b). This value of pH was approximately similar to the minimum value registered in ‘Steel Valley’ by Castro et al. (1987). Distilled water alone was used in the control treatment (pH 6.0). Treatments were applied in a completely randomized design with two treatments (pH 3.0 and pH 6.0) and ten plants (5 seedlings and 5 saplings) per treatment.

The appearance and location of leaf necroses related to SAR were recorded daily. For the anatomical and micromorphological characterization of injuries, leaf blades samples of five representative seedlings (n = 5) and saplings (n = 5), one leaf of each plant, were collected 24 hours after the last rain application. Leaf material came from the middle region of the expanding leaf at the second node from shoot apex.

For light microscopy, the samples were fixed in Craft III (Berlyn and Miksche, 1976) dehydrated in ethyl/butyl series, and embedded in histological paraffin. Transversal sections (10 mm thick) were obtained using a rotatory microtome (model Spencer 820, American Optical Corporation, New York, USA). The sections were stained with 1% alcoholic safranin, for sixty minutes, and 1% astra blue, for 3 minutes, and further were mounted in synthetic Canada balsam. A test with lugol was performed to confirm the presence of starch grains. The photographic documentation was made using a photonic microscope (model AX70TRF, Olympus Optical, Tokyo, Japan) with a U-Photo system.

For scanning electron microscopy (SEM), the samples were fixed in glutaraldehyde (2%), followed by thiosemicarbazide (1%) as mordent, and post fixed in 1% osmium tetroxide (Silveira, 1989). The material was dehydrated in ethyl series and submitted to critical point drying using liquid CO2 in Balzers equipment (model CPD 020, Bal-Tec, Balzers, Liechtenstein). The material was covered in gold using the catodic spraying process in a Sputter Coater (model FDU010, Bal-Tec, Balzers, Liechtenstein). The photographic documentation was performed using a scanning electron microscope (model JSM-T200, Jeol Co., Tokyo, Japan).


G. americana had dorsiventral and hypostomatic leaves with uniseriate epidermis and mesophyll formed by 1-2 layers of palisade parenchyma and 2-3 layers of spongy parenchyma (Fig. 1). Interveinal necroses spots with a blackened appearance were the visual symptoms observed in response to SAR. Seedlings and saplings showed similar foliar symptoms.

The onset of these symptoms occurred after the first rain application preferentially on the adaxial surface of expanding leaves; in sequence cellular damage develops internally. In young leaves, some necroses began from the abaxial epidermal cells.

The structural analysis of the injuries caused by SAR showed the occurrence of epidermal cells presenting dark contents and hypertrophy in spongy parenchyma cells that had a more compact arrangement (Fig. 2). The lesion development resulted in total collapse of leaf tissues in affected areas (Fig. 3). The cells near necrotic areas showed alterations in the differentiation pattern (Fig. 3) without a typical palisade and spongy mesophyll. Rounded chloroplasts with large starch grains were observed in region between necrotic and healthy tissues. In this region, a cicatrization tissue consisting of large cells with suberized walls was formed as a result of the mesophyll cell division (Fig. 4), in some necrosed leaves. Epidermal and mesophyll cells with alterations in shape and contents were identified in leaves submitted to acid rain but without visible necrosis (Fig. 5).

In the control treatment, the micromorphological analyses showed the presence of turgid epidermal cells with distinct contours (Fig. 6). In the leaves submitted to SAR, adaxial epidermis presented altered contours, plasmolised aspect and rupture of several cell groups in necrotic areas (Fig. 7). The rupture of the epidermis cells exposed the parenchyma cells to acid rain treatment (Fig. 8). The abaxial epidermis with stomata is shown in Figure 9. The plants submitted to SAR presented stomata with deformed aperture (Fig. 10) as a consequence of rupture of overlying cuticle and plasmolysis of guard cells, leading to the formation of depressions.


Sensitive plants to pollutants can present changes in their morphology, anatomy, physiology and biochemistry (Neufeld et al., 1985; Azevedo, 1995; Hara, 2000; Moraes et al., 2000; Chaves et al., 2002; Gabara et al., 2003; Reig-Armiñana et al., 2004; Silva et al., 2005b). The presence of necrotic spots, observed on G. americana leaves exposed to low-pH rain were also related by Silva et al. (2005a) who classified this species (based on morphological changes), in comparation with other four tree species, as moderately injuried when exposed to SAR. The structural and micromorphological analyses of the injuries caused by acid rain clearly showed that G. americana was seriously injuried when exposed to SAR (pH 3.0).

The anatomical analysis of injuries caused by pollutants on plant species has been used in various studies to assess the real damage caused by pollutants (Azevedo, 1995; Hara, 2000; Chaves et al., 2002; Reig-Armiñana et al., 2004; Silva et al., 2005a; 2005b). Several authors have related the deleterious effects of acid rain on the anatomy and ultrastructural leaf characteristics (Paparozzi and Tukey, 1983; Percy and Baker, 1987; Gabara et al., 2003; Silva et al., 2005a; 2005b), however, there are relatively few studies in tropical species as G. americana.

Leaf orientation appears to be a critical factor in determining lesion development in response to acid rain (Knittel and Pell, 1991). Most of the necroses caused by SAR started from the adaxial leaf surface because they were directly exposed to the pollutant (Silva et al., 2000). However, in G. americana, young leaves were vertically oriented and both leaf surfaces were exposed to the acid rain water and manifested the onset of necroses formations.

In G. americana the foliar epidermis were the first tissue to be injuried like observed in other plants species treated with acid rain (Evans and Curry, 1979; Rathier and Frink, 1984). The rupture of the epidermis intensified the effects of SAR on leaf internal tissues of G. americana.

The reduction in turgidity of the subsidiary cells observed in G. americana could induce alterations in the guard cells permeability (Kozlowski, 1980) and interfere with the gas exchange rates (Evans, 1984). Generally, the damage caused to the stomata impaired the plant growth and yield, because it reduced the photosynthetic rates. In some species of Liriodendron, there were reductions in biomass due to acid rain decreases in photosynthetic capacity (Neufeld et al., 1985).

It has been shown that biotic and abiotic stresses such as atmospheric pollutants can induce an increase in the amount of phenolic compounds in plants (Zobel, 1996). In G. americana the dark contents observed in leaf tissues, especially in epidermal cells probably were phenolic compounds. Zobel and Nighswander (1991) reported that the accumulation of these compounds was generally followed by cytoplasm degradation and vacuolar content release that led to cell death.

Leaves exposed to low-pH showed hypertrophy and hyperplasia of the mesophyll cells (Dickison, 2000; Silva et al., 2005b). In G. americana only hypertrophy occurred. Leaf wrinkling and curling usually associated to hypertrophy and hyperplasia (Evans and Curry, 1979) were not registered.

The occurrence of cells with abnormal quantities of starch observed near necrotic areas in G. americana was probably related with the inhibiting effect of pollutants on the translocation of carbohydrates (Rennenberg et al., 1996). The accumulation of large starch grains in the chloroplasts was also observed in mesophyll cells of Clusia hilariana and Lycopersicon esculentum mesophyll cells after exposure to acid rain (Gabara et al., 2003; Silva et al., 2005b).

The structures of the epidermal and mesophyll cells within a lesion were indistinguishable but healthy cells occurred immediately adjacent to collapsed necrotic region (Knittel and Pell, 1991; Silva et al., 2000; Silva et al., 2005a; 2005b).

The differentiation of a cicatrization tissue (from the parenchyma cells adjacent to the necrosis) on the G. americana leaves was similar to cicatrization observed on C. hilariana leaves attacked by fungi (Schneider, 1985) and submitted to SAR (Silva et al., 2005b). This cicatrization tissue functions as a barrier, preventing the progress of the necrosis to other regions of the leaf and results of the ability of plants to form new tissues from the proliferative capabilities of parenchyma cells (Dickison, 2000).

On the analysis of damage caused by SAR at structural and micromorphological levels, the present study led to conclude that visual assessments alone were not sufficient to determine the real effects of SAR. The occurrence of cellular injury in the absence of visual macroscopic symptoms in G. americana showed that anatomical investigations allowed a more precise injury diagnosis caused by pollutants as observed in soybean (Azevedo, 1995). It would be important to emphasize that a more detailed physiological assessment should be made to evaluate the potential of this species as bioindicator of polluted environments, since G. americana presented considerable structural and micromorphological alterations in response to SAR.


We thank the CNPq (Brazil) for a scholarship and Professor Wagner C. Otoni (UFV) for suggestions in writing this paper.

Dickison, W. C. (2000), Integrative Plant Anatomy. Massachusetts: Harcourt/Academic Press.

Evans, L. S. (1984), Botanical aspects of acidic precipitation. The Botanical Review, 50, 449-490.

Hara, S. (2000), Alterações estruturais em folhas de Panicum maximum Jacq. submetidas à chuva simulada com flúor. M.Sc. Thesis, Universidade Federal de Viçosa, Viçosa, Brasil.

IEF – Instituto Estadual de Florestas. (1994), Pesquisas Prioritárias para o Parque Estadual do Rio Doce, Brasil, Belo Horizonte.

Kozlowski, T. T. (1980), Impacts of Air Pollution on Forest Ecosystems. BioScience, 30, 88-93.

Lorenzi, H. (2002), Árvores brasileiras: manual de identificação e cultivo de plantas arbóreas nativas do Brasil. Nova Odessa: Instituto Plantarum de Estudos da Flora Ltda. v. 1.

Paparozzi, E. T. and Tukey, H. B. J. (1983), Developmental and anatomical changes in leaves of Yellow Birch and Red Kidney Bean exposed to simulated acid precipitation. Journal of the American Society of Horticultural Science, 108, 890-898.

Percy, K. E. and Baker, E. A. (1987), Effects of simulated acid rain on production, morphology and composition of epicuticular wax and on cuticular membrane development. The New Phytologist, 107, 577-589.

Schneider, S. Z. (1985). Anatomia foliar de Clusia hilariana Schlechtendal e Clusia spiritu-sanctensis G. Mariz et Weinberg (Guttiferae) ocorrentes no estado do Espírito Santo. M.Sc. Thesis, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil.

Turunen, M. and Huttunen, S. (1991), Effect of simulated acid rain on the epicuticular wax of Scots pine needles under northerly conditions. Canadian Journal of Botany, 69, 412-419.

Zobel, A. and Nighswander, J. E. (1991), Accumulation of phenolic compounds in the necrotic areas of austrian and Red Pine needles after spraying with sulphuric acid: a possible bioindicator of air pollution. The New Phytologist, 117, 565-574.

Received: September 17, 2004;
Revised: April 18, 2005;
Accepted: January 04, 2006.

Author for correspondence

Acid rain, or acid deposition, is a broad term that includes any form of precipitation that contains acidic components, such as sulfuric acid or nitric acid, according to the Environmental Protection Agency (EPA).

The precipitation is not necessarily wet or liquid; the definition includes dust, gasses, rain, snow, fog and hail. The type of acid rain that contains water is called wet deposition. Acid rain formed with dust or gasses is called dry deposition.


The term acid rain was coined in 1852 by Scottish chemist Robert Angus Smith, according to the Royal Society of Chemistry, which calls him the “father of acid rain.” Smith decided on the term while examining rainwater chemistry near industrial cities in England and Scotland. He wrote about his findings in 1872 in the book “Air and Rain: The Beginnings of a Chemical Climatology.”

In the 1950s, scientists in the United States started studying the phenomenon, and in the 1960s and early 1970s, acid rain became recognized as a regional environmental issue that affected Western Europe and eastern North America.

Though manmade pollutants are currently affecting most acidic precipitation, natural disasters can be a factor as well. For example, volcanoes can cause acid rain by blasting pollutants into the air. These pollutants can be carried around the world in jet streams and turned into acid rain far from the volcano.

After an asteroid supposedly wiped out the dinosaurs 65.5 million years ago, sulfur trioxide was blasted into the air. When it hit the air, it turned into sulfuric acid, generating a downpour of acid rain, according to a paper published in 2014 in the journal Nature Geoscience.

Even before that, over 4 billion years ago, it is suspected that the air may have had 10,000 times as much carbon dioxide as today. Geologists from the University of Wisconsin-Madison backed up this theory be studying rocks and publishing the results in a 2008 issue of the journal Earth and Planetary Science Letters. “At , you would have had vicious acid rain and intense greenhouse . That is a condition that will dissolve rocks,” said study team member John Valley.

Sulfur dioxide (SO2) and nitrogen oxides (NOx) released into the air by fossil-fuel power plants, vehicles and oil refineries are the biggest cause of acid rain today, according to the EPA. Two thirds of sulfur dioxide and one fourth of nitrogen oxide found in the atmosphere come from electric power generators.

A chemical reaction happens when sulfur dioxide and nitrogen oxides mix with water, oxygen and other chemicals in the air. They then become sulfuric and nitric acids that mix with precipitation and fall to the ground. Precipitation is considered acidic when its pH level is about 5.2 or below, according to Encyclopedia Britannica. The normal pH of rain is around 5.6.


Acid rain affects nearly everything. Plants, soil, trees, buildings and even statues can be transformed by the precipitation.

Acid rain has been found to be very hard on trees. It weakens them by washing away the protective film on leaves, and it stunts growth. A paper released in the online version of the journal of Environmental Science and Technology in 2005 showed evidence of acid rain stunting tree growth.

“By providing the only preserved soil in the world collected before the acid rain era, the Russians helped our international team track tree growth for the first time with changes in soil from acid rain,” said Greg Lawrence, a U.S. Geological Survey scientist who headed the effort. “We’ve known that acid rain acidifies surface waters, but this is the first time we’ve been able to compare and track tree growth in forests that include soil changes due to acid rain.”

Acid rain can also change the composition of soil and bodies of water, making them uninhabitable for local animals and plants. For example, healthy lakes have a pH of 6.5 or higher. As acid rain raises the level of acidity, fish tend to die off. Most fish species can’t survive a water pH of below 5. When the pH becomes a 4, the lake is considered dead, according to National Atmospheric Deposition Program.

It can additionally deteriorate limestone and marble buildings and monuments, like gravestones.


There are several solutions to stopping manmade acid rain. Regulating the emissions coming from vehicles and buildings is an important step, according to the EPA. This can be done by restricting the use of fossil fuels and focusing on more sustainable energy sources such as solar and wind power.

Also, each person can do their part by reducing their vehicle use. Using public transportation, walking, riding a bike or carpooling is a good start, according to the EPA. People can also reduce their use of electricity, which is widely created with fossil fuels, or switch to a solar plan. Many electricity companies offer solar packages to their customers that require no installation and low costs.

Additional resources

  • Young Peoples Trust for the Environment: Acid Rain
  • National Geographic Video: What is Acid Rain?
  • World Wildlife Federation: Emissions Reduction

The Canadian government estimates that around 14,000 lakes in eastern Canada are acidic. Water sources can also experience episodic acidification, which is when a heavy downpour or runoff from spring melting causes lakes and streams to become temporarily acidic.

Over the past several decades, Norway has suffered great damage due to the effects of acid rain. While Norway’s sulfur dioxide emissions have decreased significantly since the 1970s and 1980s, and nitrogen oxide emissions have decreased slightly, the damages from acid rain appear to be worsening in southern Norway. This is because it takes years for the ecosystems and the environment to recover from the effects of acidification. The following map shows the state of Norway’s fish stocks over 40 years; the red areas are regions in which fish stocks have been completely lost or damaged as a result of acidification. According to the State of the Environment in Norway, 18 salmon stocks have been lost and 12 are endangered, and salmon have been wiped out of all of the large salmon rivers in southern Norway.

Adding lime to water sources can reduce the acidifcation in lakes and rivers, by increasing the buffering capacity and critical load of an environment. Liming is a temporary solution, and is often used only on the most severely damaged lakes and rivers, so that ecosystems have an opportunity to survive and re-build. According to Norway’s State of the Environment, 90 regions in Norway that used liming were studied, and species diversity was deemed to be satisfactory in 85 to 90 percent of the regions. However, liming is an expensive approach to dealing with acidification. The annual cost to lime rivers and lakes in Norway is more than $18 million (in Canadian dollars).

Can Acid Rain Make Drinking Water Unsafe?

Water that is slightly acidic should not be dangerous, as there are many food that have low pH value; for example, lemon juice has a pH of 2.4. However, a low pH can indicate that there may be other contaminants in the water, because if pollutants have been added to a water source, the pH typically will change.

Water treatment facilities monitor the pH level of the water while they are treating it for municipal use. Acidic or basic water is harder to disinfect than water with a pH that is closer t 7.0. As well, if acidic water was sent through pipes and into homes, there would be a greater danger of pipe corrosion, which could allow metals to dissolve into the drinking water as it flow through the pipes. According to the World Health Organization, a pH less than 8.0 is necessary for effective chlorination. If the pH is too high, water treatment facilities can decrease the acidity in a number of ways. One common method that is used to increase the pH is to send the water through a calcium carbonate filter, which neutralizes the acid and increases the pH of the water. Another common method is to inject a sodium carbonate solution into the water.

What Does Acid Rain Do To Vegetation?

Acid rain can weaken trees by damaging the leaves and limiting the amount of available nutrients. Acid rain dissolves nutrients and minerals and carries them away before the vegetation can use them to grow. Crops are not usually harmed by acid rain, because farmers use fertilizer, which includes the necessary nutrients, or add crushed limestone to their fields. Limestone is an alkaline material, so it increases the buffering capacity of the soil to neutralize acids. The picture below shows the effects that acid rain had on a pine tree. The branch on the left has lost needles and turned yellow, which is the result of acid rain.

Department of Environmental Conservation

Sources and Environmental Impacts of Acid Rain and Acid Deposition

Acid rain is a by-product of our industrialized society. Air pollution combines with water in the atmosphere and falls to the earth as acidic rain or snow. Discussions and reports about acid rain often use the terms acid deposition or atmospheric deposition to describe this return of airborne pollutants to earth. Pollutants can be deposited from the atmosphere in rain or snow (wet deposition) or without precipitation (dry deposition).

While many areas of New York State are not sensitive to acidity because of limestone deposits or soils which neutralize the acid, the Adirondacks, Catskills, Hudson Highlands, Rensselaer Plateau and parts of Long Island are particularly sensitive to acid deposition. The soil and bedrock in these areas are not able to counteract the acid in the rain and snow.

Students and others looking for basic information about acid rain may wish to visit EPA’s Acid Rain information webpage.

Sources of Acid Deposition

The primary emissions responsible for acid deposition are sulfur dioxide (SO2) and oxides of nitrogen (NOx) from the combustion of coal, oil and natural gas. The combustion compounds are transformed into sulfuric and nitric acid and transported downwind before they are deposited.

Some of the conveniences we take for granted everyday also lead to the emissions responsible for acid deposition. The burning of fossil fuels to supply the electricity we use is a source of sulfur dioxide and nitrogen oxides. Another source is the burning of fuels to power cars, trucks, buses and airplanes. However, scientists and engineers are working on new ways to reduce harmful emissions.

Power Plants

Modern power plants use fuel that has had the sulfur reduced before it is burned or the plants use scrubbers in the smokestacks to remove the sulfur from the emissions. NOx emissions are reduced by using specially designed low NOx combustors and selective catalytic reduction (SCR) or non-selective catalytic reduction (NSCR) in the smokestacks.

Automobiles and Other Vehicles

Since the mid-1970s, two important features have been added to automobiles – catalytic converters and electronic fuel injection (EFI). Catalytic converters are located in the exhaust system to remove NOx emissions. EFI controls the formation of NOx emissions during combustion of the fuels.

And, starting in 1996, automobiles have been equipped with onboard diagnostics (OBD) that signal the driver when the various components of emission control are not operating properly. Efforts are also being made to reduce the amount of sulfur in vehicle fuels. Cars using low-sulfur fuels not only emit less SO2, but also less NOx, because catalytic converters work more efficiently with low-sulfur fuel.

Each of us can also take part in reducing acid rain. When we turn off a light, we reduce the demand for electricity from power plants. When we car pool, take public transportation, or walk, we reduce automobile emissions. Many small contributions can together make a significant reduction in emissions that lead to acid deposition.

Environmental Impacts of Acid Deposition

In the early 1970’s, acid deposition was identified as a serious ecological threat to New York State’s waters and forests. The primary emissions responsible for acid deposition are sulfur dioxide (SO2) and oxides of nitrogen (NOx) from the combustion of fossil fuels which are transformed and transported downwind before they are deposited. Acid deposition is of particular concern to New York State because of important and sensitive ecosystems which lie immediately downwind of the largest mid-western utilities burning fossil fuels and emitting SO2 and NOx emissions in North America.

An ecosystem is considered sensitive to acid deposition when it lacks adequate soil buffering capacity to counter the acids deposited to it. Sensitive ecosystems include the Adirondack Mountains, the Catskill Mountains and the Hudson Highlands.

Acid deposition also damages building materials by eroding the ornamental facades, statuary and other vulnerable edifices that are an important part of our heritage. In addition to being the main ingredient in acid rain formation, SO2 also leads to sulfate formation; acidic particles that can cause respiratory problems in humans.

In 1984, the “State Acid Deposition Control Act” (SADCA) required the reduction of SO2 emissions from existing sources and further NOx emission controls on new sources in New York State. SADCA also required the Department of Environmental Conservation (DEC) to set an Environmental Threshold Value (ETV) for wet sulfate deposition. The ETV was set at 20 kilograms per hectare. The Department established the New York State Acid Deposition Network to determine levels of actual deposition in the state for comparison to the ETV and to measure any changes in deposition that might occur as a result of the control program.

Federal Programs

The Acid Rain Program was created under Title IV of the 1990 Clean Air Act Amendments. Its overall goal is to achieve significant environmental and public health benefits through reductions in emissions of SO2 and NOx, the primary causes of acid rain. To achieve this goal at the lowest cost to the public, the program employs both traditional and innovative, market-based approaches for controlling air pollution. Specifically, the program seeks to limit, or “cap,” SO2 emissions from power plants at 8.95 million tons annually starting in 2010, authorizes those plants to trade SO2 allowances, and reduces NOx emission rates. In addition, the program encourages energy efficiency and pollution prevention.

The Clean Air Interstate Rule (CAIR) and the Acid Rain Program (ARP) are both cap-and-trade programs designed to reduce emissions of SO2 and NOx from power plants. EPA’s Cross-State Air Pollution Rule (CSAPR) has replaced CAIR and began implementation on January 1, 2015.

The CSAPR requires a total of 28 states in the eastern half of the U.S. to significantly improve air quality by reducing power plant emissions of SO2 and NOx that cross state lines and contribute to smog (ground-level ozone) and soot (fine particle pollution) in other states.

Acid rain

What do we mean when we talk about acid rain?

In general, the term acid rain is used for precipitation which has a pH of less than about 5. This is more acid than the entirely natural process whereby carbon dioxide in the atmosphere dissolves in riianwater to give a weak solution of carbonic acid. A whole range of substances get into the air as a result of burning coal, petrol and other fossil fuels, forest and other landscape fires, agriculture and also from industrial emissions. All these give off a number of chemicals as among which are oxides of sulphur and nitrogen. There is also a more complex process wherebu ammonia can enter the atmosphere, much of it from agricultural sources, and also be converted to oxides of nitrogen. These oxides react with moisture droplets in the clouds to form a weak solution of sulphuric and nitric acid. The rain thus becomes more acidic than it otherwise would be, and falls as ‘acid rain’.

Since the emissions are carried in the air and the acid rain falls far from the origins of the emissions, the damage often occurs far from the source of the problem. For example, pollution in Britain is carried by the south-westerly prevailing winds and is believed to have caused much damage in the Scandinavian countries.

What damage does acid rain do?

Acid rain can damage plants, the soil, and lakes and streams into which it enters (either directly or via the soil). Trees, especially conifers with their dense canopies of needle-like leaves, can trap acid rain clouds very effectively. The acid rain droplets are scavenged by the trees and the surfaces of their leaves can be damaged by the acidity which then makes them, and the tree, susceptible to damage by pests and disease. In addition, the concentration of the acid rain drops on the leaf surface can cause the rain to drip down onto the soil below in very large amounts..

The acid enters the soil, can upset the supply of nutrients to the trees and other plants, can seriously damage their roots (especially the fine ones which the plants depend on greatly for absorbing nutrients), and can also make the soil more acid. This does not happen in soils that are naturally alkaline, but soils that are naturally acid can become more so. This can cause acid-sensitive plants to die and can affect the wildlife which feeds on these plants. Thus the overall variety of plants able to grow in a specific area will alter. Plants and trees weakened by the effects of acid rain in this way become more susceptible to disease and early death.

One particular problem associated with acid rain is where the acidity in the rain (typically pH

Acid rain can also cause the mobilisation of potentially toxic metals from the soil – in particular, metals such as Copper and Lead. These metals can have significant health effects for animals and humans.

The nitrogen that is added to the land surface through acid rain can be useful to those planrts that require a good supply for their growth, but in ecosystems that require little nitrogen to support the plant communities then problems can occur. In almost any plant ecosystem, there is competition between the plants. If the supply of nitrogen, for example, increases then those plants that make use of it more effectively can outcompete the neighbouring plants that are less successful in this way. This influx of extra nitrogen can, therefore, disturb ecosystems very greatly. This can have an effect on the composition of plant communities, with those plants able to cope with excess Nitrogen outcompeting those that cannot. Similar problems can also arise with the sulphut from acid rain, although these are usually much less severe than with nitrogen. However, the overall reduction in sulphur deposition to soils as the sources of sulphur emissions have been controlled have resulted in problems for some agricultural crops that received most of their sulphur from acid rain. In some parts of the country, it is now necessary to add sulphur to the soil to make up for the shortfall and achieve reasonable crop yields.

There are also some reports that acid rain may be a contributing factor in flushes of ‘dark water’ – water with a high dissolved organic matter content, although the relationship is far from clear and the subject on ongoping research. Discolouration of water can be a real problem for water companies trying to provide clean drinking water.

Buildings are also affected by acid rain, especially if they are built of limestone, sandstone in which the sand grains are cemented together by lime, or concrete. The materials are alkaline and sas the acid reacts with the surface lime it eats into the surfaces and causes cracks to form, this allows weather erosion to take place more quickly.

What can be done about acid rain?

The worst areas in the world for producing acid rain are North America and Europe and, increasingly, China and India. Efforts are being made to reduce pollution caused by industry and by cars. A reduction in the use of fossil fuels such as coal can help, and many cars today run on diesel fuel which generally emits less nitrogen and sulphur than petrol (biofuels are even better in this respect). However there is still much to be done to find alternatives which cause less damage.

Practical things which we can do include walking and using public transport whenever possible; using less electricity, in ways like turning off unused lights, not leaving TVs and computers on stand-by mode, and by using energy-saving devices when we can – such as long-life light bulbs. Even using less water can help as considerable amounts of energy are used to pump water from place to place and to make it drinkable.

There are techniques to improve ‘dead’ lakes and damaged soil, but these are expensive and will not last for long unless we can reduce the causes of acid rain.

Overall, acid rain is not heard of as much now in the media as it was in the 1980’s and 1990’s. The press seems to have moved on to other issues such as climate change, but this does not mean acid rain has gone away as an issue!

Acid Rain

Acid rain is caused when acid gases rise into the sky and mix with the clouds, this causes the clouds ‘absorb’ the acid gasses and when the clouds produce rain, it falls with a higher than normal level of acidity. Rain is naturally acidic, but acid gasses make it even more acidic. Acid gasses are mainly caused by humans burning fossil fuels like coal and oil; but nature also creates these gasses with volcanoes.

The opposites of acid are alkalis; for example, toothpaste and baking powder are both alkalis. Strong alkalis can also be dangerous, such as ammonia and bleach.

The ph scale is used to measures the strength of acids and alkalis. A low ph number lets us know that a substance is acid; a high number lets us know that a substance is alkali.

Rain is normally a bit acidic, with a ph of around 5.5, if the ph of rain is below 5.5, then the rain is most likely contaminated by acid gases.

Gasses that cause acid rain are sulphur and nitrogen. When these gasses mix with the oxygen and water vapour in the air it causes sulphur dioxide and nitrogen oxide to be formed. Most of the sulphur released into the atmosphere comes from power stations; volcanoes also produce lots of sulphur when they erupt. Most of the nitrogen oxides come from the vehicles people around the world travel in daily, from planes, cars and trucks.

Acid rain is a problem all over the world, when acid gases are released, they go up in the sky, and then they are carried by strong winds. Acid rain in Scandinavian countries is caused by air pollution in Britain and other countries of Europe. In the USA, winds blow the air pollution to certain areas in Canada.

When rain is acidic, it affects trees, lakes, buildings and agricultural land. Sometimes rain is not very acidic and does not cause a lot of problems, but when it is acidic, it can be very harmful to the environment.

The acid in acid rain drains important minerals from the leaves and the soil, and is very bad for plants, trees and agricultural land. If the soil is alkaline; when acid rain falls on it the acid becomes neutral and so the plants are not hugely affected, but it the soil is slightly acidic, it can be disastrous. When sufficient acid rain falls in to lakes and rivers, life can all but die out in a relatively short period of time depending on the mass of water.

Humans are affected when we breathe in air pollution, this can cause breathing problems, and even cancer. Drinking water which has been contaminated with acid rain can cause brain damage over time.

Acid rain also eats into stone and metal, so buildings can be affected by erosion over time, especially sandstone and limestone which are examples of soft stones.

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