Air Pollution 5 page Paper

The Formation and Effects of Acid Rain

Student EXAMPLE

CEE 373: Fundamentals of Air Pollution

Term Paper

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TABLE OF CONTENTS

Abstract…………………………………………………………………………………...iii

List of Tables……………………………………………………………………………..iv

List of Figures…………………………………………………………………………….v

1. Introduction……………………………………………………………………………1

2. Natural Sources…….………………………………………………………..……..….1

2.1 Chemical Formation…………………...……………………………………..........1

3. Anthropogenic Sources………………………………………………………………..2

3.1 Chemical Formation……………………………………………………….……...4

4. Environmental Impact………………………….…………...………………..………..7

4.1 Soil………………………...……………………………………………..………..7

4.2 Trees……………………………………………………………………………….8

4.3 Other Plants………………………………………………………………………..9

4.4 Surface Waters…………………………………………………………………….9

4.5 Aquatic Organisms………………………………………………………………...9

4.6 Ecosystem………………………………………………………………………..10

5. Conclusion…………………………………………………………………………...10

6. References……………………………………………………………………………11

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ABSTRACT

Acid rain is rainwater with a larger concentration of hydrogen ions than

hydroxyl ions, thus reducing the pH below 7. Although the natural presence of carbon

dioxide makes unpolluted rainwater slightly acidic, the burning of fossil fuel emits extra

nitrogen oxides and sulfur dioxide, furthering lower the pH of rainwater. When fossil fuels

are burned, the sulfur contained in them reacts with the oxygen in the air, during

combustion, to form sulfur dioxide. Sulfur dioxide then reacts with water, forming sulfuric

acid. Due to strong acidity, sulfuric acid rapidly dissociates in water, decreasing the pH of

rainwater (Casiday & Frey, 1998). A similar process takes place involving nitrogen oxides,

resulting in the formation of nitric acid, thus further lowering the pH of rainwater to

potentially harmful levels.

The acidity of rainwater creates several environmental concerns. Soils soak up

the acid rain, thus dissolving nutrients and helpful minerals to trees. Acid rain also enables

the release of toxic substances, like aluminum, into soil making it available for trees to

uptake. Additionally, leaves and needles are stripped of their nutrients, thus making trees

more susceptible to other environmental factors (Schnabel, 2008). Acid rain is also

deposited into surface waters, such as lakes and streams, causing chemical reactions similar

to those that take place in soil. This harms aquatic life, therefore having the potential to

disrupt the ecosystem as a whole.

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LIST OF TABLES

Table 1: Largest coal-fired power producing states in 2005. ……...……………………...4

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LIST OF FIGURES

Figure 1: NO and SO2 concentrations in polluted versus clean air…………………..........3

Figure 2: Rainwater pH throughout the United States in 2005……...…………………….3

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1. INTRODUCTION

Rain is essential for all life. Plants, which provide animals (including humans) with oxygen

and food, depend on rain to nurture them and grow. Although rainwater is naturally acidic, human

activity pollutes rainwater causing it become more acidic and posing a threat to the environment

(Schnabel, 2008). Industrial and transportation sources emit excessive sulfur dioxide and nitrogen

oxides into the atmosphere. Oxidation and hydrolysis transforms these compounds into sulfuric

acid and nitric acid in the air. Eventually these acids are condensed and deposited onto soil,

affecting trees and other plants, and onto surface waters, harming aquatic life (Kinsman &

Wisniewski, 1982). This widespread environmental impact of acid rain can potentially disrupt the

ecosystem as a whole.

2. NATURAL SOURCES

Unpolluted rainwater is naturally acidic, having a pH of approximately 5.6. This acidity is

due to the natural presence of the following acid-producing gases found in the troposphere: carbon

dioxide, nitric oxide, and sulfur dioxide. Carbon dioxide is produced during the decomposition of

organic material. It is the largest contributor of the acidity in unpolluted rainwater due to its high

atmospheric concentration of roughly 335 parts per million. Nitric oxide is formed by atmospheric

electric discharge like lightning, resulting in an atmospheric concentration of around 0.01 parts per

million. Finally, sulfur dioxide is the smallest contributor of acidity in unpolluted rainwater. The

atmospheric concentration of naturally occurring sulfur dioxide, which is due to volcanic activity,

is in the range of 0 to 0.01 parts per million (Casiday & Frey, 1998).

2.1 CHEMICAL FORMATION

When organic matter decomposes, carbon dioxide is released into the atmosphere. This

dissolves into water vapor, forming carbonic acid as follows:

CO2(g) + H2O(l)  H2CO3(aq)

Carbon acid quickly dissociates, yielding a hydrogen ion:

H2CO3(aq)  H +

(aq) + HCO3 - (aq)

During lightning storms, nitrogen and oxygen react to form nitric oxide:

N2(g) + O2(g) + lightning  2NO(g)

This is then oxidized in the air, forming nitrogen dioxide (Casiday & Frey, 1998):

NO(g) + ½ O2(g) -> NO2(g)

Nitrogen dioxide dissolves into water vapor, producing both nitric and nitrous acid (Chemical

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Formula, 2011):

2NO2(g) + H2O(l)  HNO3(aq) + HNO2(g)

Just like carbonic acid, nitric acid rapidly dissociates, again resulting in the production of

hydrogen ions (Casiday & Frey, 1998):

HNO3(aq)  H +

(aq) + NO3 - (aq)

3. ANTHROPOGENIC SOURCES

As previously discussed, unpolluted rainwater is naturally acidic; however, human activity

produces large amounts of the aforementioned acid-forming compounds (carbon dioxide, nitric

oxide, and sulfur dioxide), thus polluting rainwater and producing excess acidity. Although the

carbon dioxide concentration in polluted air is significantly greater than both the nitric oxide and

sulfur dioxide concentrations, carbon dioxide does not produce acid to the same degree that the

other two acid-forming compounds do. Therefore, the production of nitric oxide and sulfur dioxide

due to anthropogenic sources is responsible for the excess acidity of rainwater.

The anthropogenic source of nitric oxide is mainly high-temperature air combustion

(usually over 1300 C), such as in car engines and power plants (Chemical Formula, 2011). High-

temperature air combustion increases the atmospheric concentration of nitric oxide from 0.01 parts

per million to about 0.2 parts per million. This accounts for about 25% of the acidity of polluted

rainwater.

The anthropogenic source of sulfur dioxide is the combustion of sulfur-containing fossil

fuels, such as in power plants. Fossil fuel combustion is responsible for 80% of the all sulfur

dioxide in polluted air, increasing the atmospheric concentration of sulfur dioxide up to 200 times.

This contributes to about 75% of the acidity of polluted rainwater (Casiday & Frey, 1998). Figure

1 shows the increase in sulfur dioxide as well as nitric oxide atmospheric concentrations due to

anthropogenic sources.

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Figure 1. NO and SO2 concentrations in polluted versus clean air. From Casiday & Frey (1998).

In areas of highly industrialized and urbanized areas, the average annual pH of precipitation

is around 4.0 to 4.5 and has historically dropped below 3.0 (which is 1000 times more acidic than

unpolluted rainwater) during individual rain events (Acid Rain, 2011). Figure 2 shows the average

pH of rainwater throughout the United States in 2005.

Figure 2. Rainwater pH throughout the United States in 2005. From NADP (2011).

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The areas of highest precipitation acidity (or lowest pH) correspond to areas within and

downwind of heavy industrialization and urbanization (Schnabel, 2008). In fact, about two-

thirds of all sulfur dioxide and one-quarter of all nitrogen oxide in the United States comes coal-

fired electric power generation (USEPA “What is Acid Rain?”, 2007). Table 1 lists the top ten

coal-fired power producing states in 2005.

Table 1. Largest coal-fired power producing states in 2005. From U.S. EIA (2011).

Rank State # of Plants Total Capacity

(MW)

Power Production

(GWh)

1 Texas 20 21,238 148,759

2 Ohio 35 23,823 137,457

3 Indiana 31 21,551 123,985

4 Pennsylvania 40 20,475 122,093

5 Illinois 32 17,565 92,772

6 Kentucky 21 16,510 92,613

7 West Virginia 19 15,372 91,601

8 Georgia 16 14,594 87,624

9 North Carolina 25 13,279 78,854

10 Missouri 24 11,810 77,714

When comparing Table 1 to Figure 2, notice that the precipitation is most acidic downwind of

the high concentration of coal-fired power plants in the Ohio Valley. This is because when

sulfur dioxides and nitrogen oxides are released into the atmosphere from sources like power

plants, wind can blow these compounds over hundreds of miles (USEPA “What is Acid Rain?”,

2007). This allows for acid deposition to occur hundreds to thousands of kilometers downwind

of emissions sources (Acid Rain, 2011).

3.1 CHEMICAL FORMATION

As previously discussed, 25% of the acidity of polluted rainwater is due to the nitrogen

monoxide which mainly results from high-temperature air combustion. At high-temperatures,

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oxygen thermolyzes and combines with diatomic nitrogen, resulting in a series of free-radical

reactions that produce nitrogen monoxide. Thermal nitrogen monoxide formation is governed by

the Zeldovich model, as shown below (Fox, 2011):

N2(g) + O(g)  NO(g) + N(g)

N(g) + O2  NO(g) + O(g)

N(g) + OH(g)  NO(g) + H(g)

The emitted nitrogen monoxide then slowly combines with atmospheric oxygen, forming soluble

nitrogen dioxide:

NO(g) + ½ O2(g)  NO2(g)

Sulfur-containing fossil fuel combustion contributes the 75% of the acidity of polluted

rainwater. When these fossil fuels are burned, sulfur is combusted with oxygen releasing sulfur

dioxide into the air, as shown below (Casiday & Frey, 1998):

S(s) + O2(g)  SO2(g)

Once the primary pollutants (nitric oxide and sulfur dioxide) are released in the air, they

rise into the atmosphere and undergo oxidation in the clouds to from more stable compounds. This

can occur in a variety ways, but usually involves hydrogen peroxide or ozone due to the

effectiveness as oxidizing agents. These oxidizing agents arise from photochemical reactions,

which usually involve the primary pollutants themselves. In the warm sector of mid-latitude

cyclones, primary pollutants and oxidants are trapped in stagnant air, thus making the conditions

favorable for photochemical reactions to initiate oxidation of both nitrogen monoxide and sulfur

dioxide (Schnabel, 2008).

Under these conditions, sulfur dioxide oxidizes to sulfur trioxide by initially slowly

reacting with atmospheric hydroxyl radicals, produced by photodecomposition of ozone:

SO2(g) + OH(g)  HSO3(g)

This further reacts with oxygen, producing a perhydroxyl radical:

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HSO3(g) + O2(g)  SO3(g) + HO2(g)

The resulting sulfur trioxide combines with water vapor to produce sulfuric acid:

SO3(g) + H2O(l)  H2SO4(aq)

Similarly, ozone or perhydroxyl radicals oxidize nitrogen monoxide in the atmosphere to

form nitrogen dioxide. The oxidation of nitrogen monoxide with ozone in the presence of sunlight

forms nitrogen dioxide and oxygen:

NO(g) + O3(g) + sunlight  NO2(g) + O2(g)

On the other hand, the oxidation of nitrogen monoxide with perhydroxyl radicals produces

nitrogen dioxide and a hydroxyl radical:

NO(g) + HO2(g)  NO2(g) + OH(g)

Nitrogen dioxide can then undergo oxidation by hydroxyl radicals in the atmosphere:

NO2(g) + OH(g)  HNO3(g)

This reaction is common during the day. At night, nitrogen dioxide reacts with ozone producing

a nitrate radical (Jacob, 1999):

NO2(g) + O3(g)  NO3(g) + O2(g)

Further atmospheric oxidation of sulfur dioxide and nitrogen monoxide takes place in cloud

and rain droplets. This occurs when a cold front approaches the warm sector, putting the

atmospheric acids and related radicals into contact with moist air and integrating them with frontal

cloud systems. Next, the pollutants attach to individual cloud particles by various microphysical

processes. Since most atmospheric sulfates are very soluble, they act as good nuclei for cloud drop

formation. Soluble gases are absorbed by cloud water as the cloud drops grow by condensation

(Schnabel, 2008).

This allows sulfur dioxide to combine with water to produce hydrogen sulfite:

SO2(g)  SO2H2O(aq)

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SO2H2O(aq)  HSO3 - (aq) + H

+ (aq)

The hydrogen sulfite ion is immediately oxidized by previously formed hydrogen peroxide

(produced from self-reaction of perhydroxyl radicals) in the aqueous phase:

H2O2(g)  H2O2(aq)

HSO3 - (aq) + H2O2(aq) + H

+ (aq)  SO4

2- (aq) + 2H

+ (aq) + H2O(l)

The cloud water also allows the previously formed nitrate radical to further react with

nitrogen dioxide, producing an intermediate that combines with the water (Jacob, 1999):

NO2 (g) + NO3 (g)  N2O5(g)

N2O5(g) + H2O(l)  2HNO3(aq)

Then, precipitation forms within the clouds, ultimately precipitating these acids out of the

atmosphere. The formation of ice particles in the cold, upper reaches of the clouds initiates most

precipitation in midlatitude storms. These ice particles melt and collide with concentrated cloud

drops in the lower portions of the storm clouds, thus acidifying the precipitation.

4. ENVIRONMENTAL IMPACTS

Once the acids in the air condense and fall to the ground as rain, several inorganic and

biochemical reactions are initiated. This occurs in the soil, negatively affecting trees and other

plants, and in surface waters, harming aquatic ecosystems. The strength of effects is dependent on

the acidity of the rainwater; the chemistry and buffering capacities of the soils that come in contact

with the rainwater; and the type of fish, trees, and other living things that rely on the rainwater

(USEPA “What is Acid Rain?”, 2007).

4.1 SOIL

Acid rain soaks into soil and triggers a number or acid-consuming reactions within it in

attempt to lessen the adverse effects of additional acid. Carbonates within soil react with the acidic

rainwater allowing the pH of soil to be maintained while raising the pH of the soil drainage waters.

Additionally, the cation exchange capacity of the soil can neutralize the acid rain, but degrades

soil minerals (Schnabel, 2008).

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The extent to which the acidity is neutralized is dependent on the soils buffering capacity,

or its ability to resist acidity. The buffering capacity of forest soils is dependent on the thickness,

composition, and underlying bedrock of the soil (USEPA ”Effects of Acid Rain – Forests”, 2007).

Neutral soil, having a pH of 6 to 8, exchanges the cations calcium and magnesium with

hydrogen ions in the rainwater. This decreases the soil drainage water acidity; however, it

increases the soil acidity. Once the carbonates and exchangeable cations are depleted, other soil

minerals react with the acidic rainwater. As time progresses, the soil becomes less fertile and more

acidic. Eventually, when all soil minerals are exhausted (around a soil pH of 5.2 or less), metal

ions are solubilized and washed away in runoff. For example, a substantial amount of aluminum

exists in clay minerals and coatings on soil particles (Schnabel, 2008). When soil pH is reduced

to around 5, aluminum solubilizes and is mobilized by runoff (Casiday & Frey, 1998).

4.2 TREES

The reactions triggered in soil due to acid rain have damaging effects on trees. The transfer

of soil minerals to soil drainage water washes away important minerals, like calcium, necessary

for tree growth before trees can use them. Furthermore, acid rain solubilizes metal ions like

aluminum, thus exposing trees to toxic substances (USEPA “Effects of Acid Rain – Forests”,

2007).

Acid rain can also damage the foliage of trees. The acidity causes trees to lose the nutrients

contained in their leaves and needles, therefore turning the foliage brown and sometimes causing

it to fall off. As a result of the loss of nutrients in their foliage, trees are more vulnerable to damage

by environmental factors like winter weather (USEPA “Effects of Acid Rain – Forests”, 2007).

This visible damage to trees only occurs from prolonged exposure to extremely acidic precipitation

(around a pH of 3), making soil acidification mainly responsible for the damaging effects of acid

rain on trees (Schnabel, 2008).

In forests, soil acidification from acid rain in conjunction with other stressors (such as

competition for light, water, and nutrients; disease organisms; climate extremes; and atmospheric

pollutants) has led to an increased decline and mortality of sensitive tree species and a widespread

reduction in tree growth. Since soils vary in sensitivity to acidification, forest decline does not

directly correlate with acid deposition. Instead, forest decline is dependent upon both the acid

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deposition and the buffering capacity of the soil in the area, making it hard to establish a concrete

relationship between acid deposition and forest decline. At the same time, acid rain has been

implicated in forest and soil degradation in many areas, especially in the eastern United States

where the buffering capacity of soil is low (for example, there has been injury to white pine trees

here) (Schnabel, 2008).

4.3 OTHER PLANTS

Like trees, other plants are negatively affected by soil acidity. The reduced pH of the soil

causes less-active populations of soil microorganisms, thus slowing the decomposition of plant

residues and cycling of plant nutrients. Additionally, nutrients like phosphorus are less available

for plants to uptake due to the solubilization of metal ions, which causes phosphorus to take the

form of mostly aluminum and iron phosphate. Meanwhile, the increase in trace metals can reach

phytotoxic levels (Schnabel, 2008).

4.4 SURFACE WATERS

Acid rain deposited directly into surface waters causes reactions analogous to those in the

soils. In lakes and streams, the carbonate-bicarbonate system buffers the water, similar to the

buffering capacity of soils. Once enough acid has been deposited into lakes and streams to reach

a pH of 4.5, bicarbonate is mostly depleted. Any additional acid then solubilizes the metals in

suspended solids and the lake or stream bed (Schnabel, 2008).

Soil drainage and runoff from acid-susceptible soils also directly deposits acid into lakes

and streams. The soil controls the water quality of the soil drainage and runoff, which supplies

most of the water to the aquatic system. Although the soil drainage and runoff of acid-sensitive

soils contain carbonate, it does not provide enough carbonate to buffer the aquatic system

(Schnabel, 2008).

4.4 AQUATIC ORANISMS

Aquatic organisms are harmed due to low pH and high metal concentrations in acidified

waters. Lowering of the pH causes lesions to form on gills of fish and erode the tissue, adversely

effecting respiration, excretion, and liver function. As pH decreases and the aluminum

concentration increases, severe acidosis occurs causing the discharge of excessive mucus from fish

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gills and making gill membranes more porous. This inhibits the diffusion of oxygen through the

gills (negatively affecting respiration) and increases the loss of sodium. In fact, the improper salt

balance at pH 4 to 5 causes the death of fish. Due to metal mobilization, metals like aluminum

tend to accumulate on fish gills (further inhibiting respiration) and iron accumulates on the gut

membranes (inhibiting food absorption) (Schnabel, 2008).

Acidified waters also impair fish reproduction. For normal ovary formation and egg laying,

female fish require high serum calcium levels; female fish in acidified waters maintain lower levels

than normal. Also, eggs and fry are more sensitive to lower pH. Consequently, egg laying is

hindered, many eggs and fry after rainfall, and skin cell corrosion kills embryo. As a result, some

fish may find water at a more preferable pH. These areas may not be ideal for egg development,

thus decreasing the number of offspring. Failure to reproduce has decreased the number and

diversity of fish (Schnabel, 2008).

4.5 ECOSYSTEM

The reduction in quantity and quality of fish and plants can trigger a chain reaction throughout the

food chain. For example, a reduction in the amount of algae reduces the number of herbivous

invertebrates, which reduces the amount of the predatory invertebrates and fish. Reducing the

number of predatory dominant fish species, which are required to control the number of

invertebrates, compromises the stability of the ecosystem as a whole (Schnabel, 2008).

5. CONCLUSION

The resulting environmental impacts have called attention to the issue of acid rain. As a

result the Acid Rain Program was established. This program aims to reduce sulfur dioxide and

nitrogen oxide emissions. It does this by promoting pollution prevention and more energy efficient

strategies and technologies. To date, the program has lessened the acidity of rainwater and in turn,

the health of plants and animals has started to increase (USEPA “Acid Rain Program”, 2009).

10. REFERENCES

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Acid rain. (2011). In Encyclopædia Britannica. Retrieved from

http://www.britannica.com.ezproxy.lib.lehigh.edu/EBchecked/topic/3761ac

id-rain

Casiday, Rachel & Frey, Regina. (1998). Acid Rain. Retrieved from Washington

University website:

http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/acidrain.html

Chemical Formula. (2011). Acid Rain. Retrieved from

http://www.chemicalformula.org/acid-rain

Fox, John T. (2011). “NOx Introduction.” Powerpoint Presentation. Lehigh University,

Bethlehem, Pa.

Jacob, Daniel J. (1999). Acid Rain, Introduction to Atmospheric Chemistry (pp. 247 – 255).

Princeton, NJ: Princeton: University Press.

Kinsman, John D. & Wisniewski, Joe. (1982). An Overview of Acid Rain Monitoring

Activities in North America. American Meteorological Society, 63(6), 598 – 618.

National Atmospheric Deposition Program. (2011). Hydrogen Ion Concentration as pH from

Measurements Made at the Central Analytical Laboratory, 2005. Retrieved from

http://nadp.sws.uiuc.edu/maps/Default.aspx

Schnabel, Ronald R. et al. (2008). Acid Rain. AccessScience. McGraw-Hill Companies.

Retrieved from

http://www.accessscience.com.ezproxy.lib.lehigh.edu/content.aspx?searchStr=Acid+Rain

&id=004760

U.S. Energy Information Administration. (2011). Existing Electric Generating Units in the

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United States, 2005. Retrieved from

http://www.eia.gov/cneaf/electricity/page/capacity/capacity.html

U.S. Environmental Protection Agency. (2009). Acid Rain Program. Retrieved from

http://www.epa.gov/airmarkets/progsregs/arp/index.html

U.S. Environmental Protection Agency. (2007). Effects of Acid Rain – Forests.

Retrieved from http://www.epa.gov/acidrain/effects/forests.html

U.S. Environmental Protection Agency. (2008). Effects of Acid Rain – Surface Waters

and Aquatic Animals. Retrieved from

http://www.epa.gov/acidrain/effects/surface_water.html

U.S. Environmental Protection Agency. (2007). What is Acid Rain?

Retrieved from http://www.epa.goc/acidrain/what/index.html

Xie, Shaodong et al. (2004). Investigation of the Effects of Acid Rain on the

Deterioration of Cement Concrete Using Accelerated Tests Established in

Laboratory. Atmosphere Environment, 38, 4457-4466.