Project Atmosphere Canada

Module 14 / Acid Rain

Project Atmosphere Canada

Project Atmosphere Canada (PAC) is a collaborative initiative of Environment Canada and the Canadian Meteorological and Oceanographic Society (CMOS) directed towards teachers in the primary and secondary schools across Canada. It is designed to promote an interest in meteorology amongst young people, and to encourage and foster the teaching of the atmospheric sciences and related topics in Canada in grades K-12.

Material in the Project Atmosphere Canada Teacher's Guide has been duplicated or adapted with the permission of the American Meteorological Society (AMS) from its Project ATMOSPHERE teacher guides.

Acknowledgements

The Meteorological Service of Canada and the Canadian Meteorological and Oceanographic Society gratefully acknowledge the support and assistance of the American Meteorological Society in the preparation of this material.

Projects like PAC don't just happen. The task of transferring the hard copy AMS material into electronic format, editing, re-writing, reviewing, translating, creating new graphics and finally format-ting the final documents required days, weeks, and for some months of dedicated effort. I would like to acknowledge the significant contributions made by Environment Canada staff and CMOS members across the country and those from across the global science community who granted permission for their material to be included in the PAC Teacher's Guide.

Eldon J. Oja Project Leader Project Atmosphere Canada On behalf of Environment Canada and the Canadian Meteorological and Oceanographic Society

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher.
Permission is hereby granted for the reproduction, without alteration, of materials contained in this publication for non-commercial use in schools or in other teacher enhancement activities on the condition their source is acknowledged. This permission does not extend to delivery by electronic means.

Published by Environment Canada
© Her Majesty the Queen in Right of Canada, 2001

Cat. no. En56-172/2001E-IN
ISBN 0-662-31474-3

Contents

Introduction

Basic understandings

Activity

Answer Key

Glossary

Introduction

In order to understand acid rain, we need to understand the life cycle of atmospheric pollutants. There are three basic components to this cycle: a) emission, b) transport and transformation, and c) deposition.

The emission component includes natural and anthropogenic (human-made) emissions from various sources.

The transport and transformation component includes everything that happens to the pollutants while they are resident in the atmosphere: dispersion, diffusion, transport, and chemical and physical transformation. In other words, the atmosphere is the "reaction vessel" and the delivery system for the pollutants.

The deposition component includes wet and dry removal processes, as well as effects of the pollutants on the aquatic and terrestrial environment and human health.

Basic understandings

Figure 1 - pH scale

pH AND ACID RAIN

1. The pH scale is commonly used by chemists to indicate the "strength" of an acid (or of an "alkali" or a base, the chemical opposite of acid).
   
   
2. Decreasing pH corresponds to increasing acidity, but in a non-linear (logarithmic) way. For example, a pH of 4 is ten times more acidic than a pH of 5, and a hundred times more acidic than a pH of 6.
   
   
3. Water that is absolutely pure has a pH of 7 (neutral). Water that is left standing for a long time in contact with non-polluted air, however, becomes slightly acidic with a pH near 5.6. This acidity is derived from absorption of carbon dioxide from the air to produce a weak acid called carbonic acid.
4. Even in remote locations in the Southern Hemisphere, precipitation has a pH of about 5 due to acidifying compounds (such as sulphuric acid) in the atmosphere, derived from both anthropogenic and natural sources (such as volcanoes).
   
   
5. When we talk about acid "rain", we are really referring to any liquid, freezing or frozen precipitation that is more acidic than about pH 5. The average rain in eastern Canada is about pH 4.5. Extreme acid rains can have a pH as low as 3.
   
   

CAUSES OF ACID RAIN

6. The most common acidifying chemicals found in precipitation are sulphuric acid (H₂SO₄) and nitric acid (HNO₃). They are formed from sulphur dioxide (SO₂) and nitrogen oxides (NO_x=NO+NO₂), respectively, that are emitted from anthropogenic sources such as power plants, smelters and vehicles.
   
   
7. These compounds are mixed into the atmosphere, and carried downwind for hundreds or thousands of kilometres while being chemically transformed into acids. Eventually they fall to earth either as wet deposition in precipitation or as dry deposition (particles or gases), where they can impact on sensitive ecosystems.
   
   
8. Regions that receive acid rain are areas with and downwind of industrialization (eastern North America, western Europe, Japan).
   
   
9. Acid rain is a problem in the Atlantic Provinces and other parts of eastern Canada and the northeastern U.S. because of factors related to the three components of the air pollution cycle:
   Firstly, the major SO₂ and NO_x sources are in the eastern U.S. and southern Ontario and Québec.
   Secondly, weather systems (prevailing winds) transport this pollution towards Atlantic Canada.
   Thirdly, much of eastern Canada, including the Atlantic Provinces, is very sensitive to inputs of acidity.
   
   
10. Neutralization can occur in the soils and waters of the receiving environment if it has the buffering capacity, which is dependent on the availability of substances such as limestone.
    Unfortunately, the bedrock of acid sensitive regions is composed of granite, quartzite and slate, and associated soils are generally thin and deficient in chemicals which can neutralize acidity.
    
    

THE CONCEPT OF “CRITICAL LOADS”

11. The critical load is a measure of how much pollution an ecosystem can tolerate; in other words, the threshold above which the pollutant load, acid deposition in this case, harms the environment. Ecosystems that can tolerate acidic pollution have high critical loads, while sensitive ecosystems have low critical loads.
    
    
12. Critical loads vary across Canada. They depend on the ability of each particular ecosystem to neutralize acids. Scientists have defined the critical load for aquatic ecosystems as the amount of wet sulphate (which makes sulphuric acid) deposition which will maintain a pH of 6 or more in at least 95% of the lakes in a region.
    
    
13. Much of Atlantic Canada and portions of Quebec and Ontario have critical loads of less than 8 kilograms of wet sulphate per hectare per year (kg/ha/yr). (100 ha = 1 km²) Unfortunately, the critical load is being exceeded over much of this region, and environmental damage is occurring.
    
    

ECOSYSTEM IMPACTS

14. What impacts have been seen from this excessive acidic deposition? Many aquatic organisms are very sensitive to acidic waters, so the aquatic environment was the first to show noticeable impacts of acid rain.
    
    
15. Many species of amphibians are very sensitive to acid water, with high mortality occurring in the egg stage. Damage starts at pH less than 6.5.
    
    
16. Many species of fish begin to disappear below pH 6. Changes in blood chemistry are observed in fish, as well as retardation of egg development.
    
    
17. One species endangered by acid rain is the Atlantic salmon. Most of the salmon cannot survive when the water pH drops to 5. When it drops to about 4.7, the salmon disappear from the river. This has occurred in 14 rivers in the southern part of Nova Scotia. In addition, 35 more rivers in this area have been adversely impacted by acid rain.
    
    
18. Research has shown that acid rain has direct and indirect effects on forests. The main concern is the indirect effect that acid rain has through its impact on soil chemistry, through both losses of soil nutrients and increases in metals that are toxic to roots (see 23, 24). In this process, acid rain leaches essential nutrients (e.g., calcium, magnesium) from forest soils, thus causing nutrient deficiencies and imbalances for the forest.
    
    
19. Some tree species are less tolerant than others. For example, since sugar maples need more calcium than most tree species, they are among the first affected.
    
    
20. Acid rain also has direct effects on the forest; in particular, it affects the cuticle (the waxy covering on the upper surface of the leaf). Cuticle damage accelerates the natural ageing process of the leaves, which impairs the ability of the tree to cope with other stresses, such as other pollutants, drought, insect infestations, disease and increased ultraviolet radiation due to a thinner ozone layer.
    
    
21. Until recently, sulphuric acid has been our chief acidification concern. The impact of nitric acid is also important, but has been perceived as a lesser problem because nitrogen acts as a fertilizer, taken up by plants rather than appearing in soils and surface waters as acid.
    
    
22. However, extra nitrogen-based atmospheric compounds may lead to nitrogen saturation and subsequent release of nitric acid into surface waters. Nitrogen-based acidification has occurred for a large number of lakes in south central Ontario and southwestern Quebec.
    
    
23. The same emission sources that discharge sulphur and other acidifying substances also discharge toxic metals in trace amounts. In addition, acid rain itself impacts soil and water systems through its effect on metals. The solubility of trace metals, whether from emissions or already present in the aquatic and terrestrial environment, increases with increasing acidity.
    
    

HUMAN HEALTH AND OTHER IMPACTS

24. Dissolved metals are not only toxic to fish, trees and other organisms, but metals such as aluminum in drinking water present a serious human health hazard if levels are too high. Cadmium has already reached dangerous levels in wildlife in acid-sensitive areas, thus necessitating a ban on organ meats for human consumption in some areas.
    
    
25. Another health-related component of the acid rain phenomenon is the aspect of acidic aerosols, or tiny solid or liquid atmospheric particles with a high acid content. When these particles are inhaled, they can be associated with increased hospital admissions, respiratory diseases (bronchitis, asthma and emphysema) and premature death.
    
    

Figure 2 - Air pollution health effects pyramid

26. Another issue related to acid rain is the reduction in visibility in eastern North America due to sulphate aerosols. Impaired visibility is a safety issue for airport traffic control and can affect tourism as well.
    
    
27. In addition, acid rain and acidifying pollutants can accelerate corrosion of building materials such as stone, brick, concrete and metal.
    
    

EMISSION CONTROLS

28. In 1985 the governments of Canada and the seven eastern provinces joined forces to take action on reducing sulphur dioxide, the major contributor to acid rain. They launched a program to cut sulphur dioxide emissions in the eastern provinces in half by 1994, which was more than successful. By 1994, sulphur dioxide emissions in eastern Canada were 54% lower than 1980 levels.
    
    
29. Since about half of the acid rain in eastern Canada comes from American sources, the co-operation of the U.S. was also needed. In 1990, the U.S. acted to reduce emissions of sulphur dioxide by amending its Clean Air Act and, in 1991, by signing the Canada-U.S. Air Quality Agreement.
    
    
30. By 1996, U.S. sulphur dioxide emissions had declined to 27% lower than they were in 1980, and by 2010, they should decrease by a total of about 40%. With decreasing emissions, the amount of acid rain has also decreased.
    
    

OUTSTANDING CONCERNS

31. Despite this progress, an acid rain science assessment conducted in the mid-1990's showed that some serious problems remained. More action was needed to fully protect Canada's ecosystems.
    
    
32. Without further emission reductions beyond those required under the 1991 Air Quality Agreement, scientists estimated that an area of some 800,000 square kilometres, extending from central Ontario through southern Quebec and across much of Atlantic Canada, would still be receiving more sulphate than its natural systems could tolerate.
    
    

Figure 3 - Predicted wet sulphate deposition in excess of critical loads in 2010, without further controls (in kg/ha/yr)

33. Atmospheric modelling conducted as part of the science assessment showed that a further 75% reduction in sulphur dioxide emissions (beyond current commitments) in targeted regions of eastern Canada and the U.S. would be necessary to protect all of the 95,000 lakes in this area.
    
    
34. Some acidified lakes showed signs of recovery, but many more did not. The greatest improvements were seen in the Sudbury area, where lakes had been very badly damaged. Here, fish populations rebounded and fish-eating birds such as loons increased.
    
    
35. However, no substantial wildlife recovery has been seen beyond the Sudbury area. The least improvement was seen in Atlantic Canada, although lakes in this region were never as highly acidified as those in some parts of Ontario and Quebec.
    
    
36. Reducing nitrogen oxides is becoming more important. If nitrate deposition continues at current levels, its contribution to acidification will eventually erode the benefits gained from the reductions in sulphur dioxide.
    
    
37. Because nitrogen oxides also contribute to ground-level ozone, the main ingredient in smog, reducing these emissions would also help to improve air quality.
    
    

FUTURE ENDEAVOURS

38. On October 19, 1998, federal, provincial, and territorial Energy and Environment Ministers signed The Canada-wide Acid Rain Strategy for Post-2000. The Strategy laid the framework for how Canada would manage acid rain in the future. The primary long-term goal of The Strategy is to achieve critical loads (or the threshold level) for acid deposition across Canada.
    
    
39. Current priorities under The Strategy include formalizing the new targets for emission reductions in eastern Canada through federal-provincial agreements, pursuing further U.S. emission reduction commitments, and conducting further scientific work on the role of nitrogen in acidification.
    
    

WHAT YOU CAN DO

40. Sulphur dioxide and nitrogen oxides are the main pollutants that cause acid rain. These pollutants are emitted largely by the combustion of fossil fuels. Reducing the use of fossil fuels therefore, including the use of electricity generated by coal- and oil-fired power plants, will help reduce acid rain-causing emissions.
    
    
41. For more specific suggestions on what you, as an individual, can do, please see: http://www.ec.gc.ca/acidrain/done-you.html
    Here are a few examples:
    
    In the home
    • Install a low-flow showerhead.
      
      
    • Run the dishwasher and washing machine only with a full load.
      
      
    • Hang-dry the laundry.
      
      
    • Buy energy-efficient appliances.
      
      
    Transportation
    • Walk, ride your bike or take a bus to work.
      
      
    • Have your engine tuned at least once every six months.
      
      
    • Check your car tire pressure regularly.
      
      
    • Drive at moderate speeds.
    
    
    

Activity

Activity 1 - Measuring pH

After completing this activity, you should be able to:

• measure the approximate pH of chemicals in water using a pH indicator
  
  
• acquire a basic understanding of the pH scale
  
  
• learn the pH of some common substances
  
  

Introduction

A pH scale is used to measure the amount of acid in a liquid (like water). Because acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as "pH." This scale is used to measure the acidity of rain samples.

The smaller the number on the pH scale, the more acidic the substance is. (The larger the number, the less acidic, or more alkaline or basic.) Rain measuring between 0 and 5 on the pH scale is acidic and therefore called "acid rain." Small number changes on the pH scale actually mean large changes in acidity.

For example, a change in just one unit from pH 6.0 to pH 5.0 would indicate a tenfold increase in acidity. Clean rain has a pH of 5.6. It is slightly acidic because of carbon dioxide which is naturally present in the atmosphere.

Here the pH of some common substances, as well as natural water, will be measured.

If pH indicator paper is not available in your science lab, it may be purchased at some beer- and wine-making supply stores, pharmacies and hair salons. If you do not have distilled water in your lab, you can find it in some pharmacies, or grocery, department, hardware or speciality stores.

Figure 4 - pH scale

Materials Needed

• pH paper and colour chart (pH range 3 to 12)
  
  
• distilled water
  
  
• white vinegar
  
  
• household ammonia (or baking soda)
  
  
• 4 small, clear cups or glasses
  
  
• 4 stirring spoons
  
  
• measuring cups and spoons (1/2 cup; 1/8 and 1/2 teaspoon)
  
  
• notebook and pencil
  
  
• labels
  
  

Procedure

Rinse one cup with distilled water, shake out excess water, and label "natural water". Locate a stream, river, lake, or pond. Scoop some of the surface water into this cup. (A plastic collection bottle can be used to transfer the water back to your lab, but this must be first rinsed with distilled water as well.)

Rinse each of the other three cups with distilled water as above. Label one cup "vinegar", one "ammonia" (or "baking soda"), and one "distilled water". Pour 1/2 cup distilled water into each of these 3 cups. Add 1/2 teaspoon white vinegar to the vinegar cup and stir with a clean spoon. Add 1/2 teaspoon ammonia to the ammonia cup (or 1/2 tsp. baking soda) and stir with another clean spoon. Do not add anything to either of the water cups.

Dip an unused, clean strip of pH paper in the vinegar cup for about 2 seconds and immediately compare with the colour chart. Write down the approximate pH value and set the cup aside. Likewise, dip an unused, clean strip of pH paper in the ammonia (or baking soda) cup for about 2 seconds and immediately compare with the colour chart. Write down the approximate pH value and set the cup aside. Repeat this process for the distilled water and natural water cups.

Discussion

1. Is vinegar an acid or a base?   [ANSWER]
   
   
2. What is the approximate pH of vinegar?   [ANSWER]
   
   
3. Is ammonia (or baking soda) an acid or a base?  [ANSWER]
   
   
4. What is the pH of distilled water?  [ANSWER]
   
   
5. What is the pH of your natural water sample?  [ANSWER]
   
   
6. Based on where you live and what you have learned about acid rain, are you surprised by the result?  [ANSWER]
   
   
7. Discuss the findings with your teacher. How does the measured pH compare to the pH levels that affect plants and animals in aquatic habitats? (See the chart explaining how acid rain affects organisms living in the water.)  [ANSWER]
   
   

Figure 5 - Aquatic life forms vary in their tolerance to acidity. Acidification primarily reduces the variety of life inhabiting a lake and alters the balance among the surviving populations.

Activity 2 - Making a Natural pH Indicator

After completing this activity, you should be able to:

• prepare a natural pH indicator
  
  
• determine whether substances are acidic or basic using this indicator
  
  
• acquire a greater understanding of the pH scale and neutralization
  
  

Introduction

In this experiment you will make your own pH indicator from red cabbage. Red cabbage contains a chemical that turns from its natural deep purple colour to red in acids and blue in bases. Litmus paper, another natural pH indicator, also turns red in acids and blue in bases. The red cabbage pH indicator can be obtained by boiling the cabbage.

Materials Needed

• sliced red cabbage
  
  
• stainless steel or enamel pan or microwave casserole dish
  
  
• 1 litre water
  
  
• stove, microwave, or hotplate
  
  
• white vinegar
  
  
• ammonia or baking soda
  
  
• clear, non-cola beverage
  
  
• 3 glass cups (preferably clear)
  
  
• measuring spoons
  
  
• 3 clean teaspoons for stirring
  
  
• measuring cup (1/4 cup)
  
  
• notebook and pencil
  
  

Procedure

Boil cabbage in a covered pan for 30 minutes or microwave for 10 minutes. (Don't let water boil away.) Let cool before removing the cabbage.

Pour about 1/4 cup of cabbage juice into each cup. Add 1/2 teaspoon ammonia or baking soda to one cup and stir with a clean spoon. Add 1/2 teaspoon vinegar to second cup; stir with a clean spoon. Add about 1 teaspoon clear non-cola to the last cup and stir with a clean spoon.

After answering the first two questions for this experiment, pour the contents of the vinegar cup into the ammonia cup.

Discussion

1. What colour change took place when you added vinegar to the cabbage juice? Why?  [ANSWER]
   
   
2. Did the ammonia turn the cabbage juice pH indicator red or blue? Why?  [ANSWER]
   
   
3. What happens to the colour if you pour the contents of the vinegar cup into the ammonia cup?  [ANSWER]
   
   
4. If you were to gradually add vinegar to the cup containing the baking soda (or ammonia) and cabbage juice, what do you think would happen to the colour of the indicator? Try it, stirring constantly.  [ANSWER]
   
   
5. Is the non-cola soft drink acidic or basic?  [ANSWER]
   
   

Answer Key

ACTIVITY 1

1) and 2) Vinegar is an acid, and in this experiment it will display a pH of about 4. Vinegar at pH 4 turns pH paper yellow and most other pH indicators red.

3) Ammonia is a base and in this experiment it will display a pH of about 12. Bases turn most pH indicators blue. (Baking soda is a much weaker base, and should display a pH of about 8.)

4) Your distilled water may not have a neutral pH. PURE distilled water would test neutral, but pure distilled water is not easily obtained because carbon dioxide in the air around us mixes, or dissolves, in the water, making it somewhat acidic.
The pH of distilled water is between 5.6 and 7. To neutralize slightly acidic distilled water, add about 1/8 teaspoon baking soda, or a drop of ammonia, stir well, and check the pH of the water with a pH indicator. If the water is still acidic, repeat the process until pH 7 is reached. Should you accidentally add too much baking soda or ammonia, either start over or add a drop or two of vinegar, stir, and recheck the pH.

5), 6) and 7) Answers will vary.

ACTIVITY 2

1) The vinegar and cabbage juice mixture should change from deep purple to red, indicating that vinegar is an acid.

2) The ammonia and cabbage juice mixture should change from deep purple to blue, because ammonia, like baking soda, is a base, which reacts chemically with the pH indicator, turning it blue.

3) You should find that the acid and base are neutralized, changing the colour from blue or red to purple, which is the original, neutral colour of the cabbage juice.

4) As you add more vinegar, the acid level increases and the colour becomes red.

5) It is acidic and turns the cabbage juice pH indicator red.

Glossary

Acid rain - more properly called acid precipitation, it occurs when sulphur dioxide and nitrogen oxide emissions convert into such pollutants as sulphuric acid and nitric acid. Both dissolve easily in airborne water droplets.

Buffering capacity - the resistance of water or soil to changes in pH.

Critical load - a measure of how much pollution an ecosystem can tolerate.

Deposition - the processes by which chemical constituents move from the atmosphere to the earth's surface, including wet deposition (from precipitation, fog or cloud water) and dry deposition (from particles or gases).

Diffusion - the way in which freely moving particles in liquids and gases spread out to fill all the space available to them.

Dispersion - the process of reducing high concentrations of air pollutants through atmospheric motion.

Emission - a discharge or release of pollutants into the air, such as from a smokestack or automobile engine.

Fossil fuels - coal, oil, and natural gas formed from the remains of ancient plant and animal life.

Ground-level ozone - ozone that is produced at ground level when some of the chemical components of vehicle exhaust and industrial emissions react with sunlight. At ground level, ozone is a powerful and irritating pollutant. In fact, it is the main component of smog. (See also Ozone, Ozone layer.)

Neutralization - the chemical process that produces a solution that is neither acidic nor alkaline.

Nitrogen oxides - (designated NO_x) gases that form when nitrogen and oxygen in the atmosphere are burned with fossil fuels at high temperatures.

Ozone - (chemical formula O₃) a pungent-smelling, slightly bluish gas which is a close chemical cousin to oxygen. About 90 percent of the earth's ozone is located in a natural layer high above the earth in a region of the atmosphere called the stratosphere (see Ozone layer). Ironically, while stratospheric ozone is beneficial to the environment, ground-level ozone is not (see Ground-level ozone).

Ozone layer - the natural layer of ozone located in the stratosphere. Here, it protects the earth from the harmful effects of the sun's ultraviolet radiation by absorbing much of it. (See also Ozone, Ground-level ozone.)

pH - a scale ranging from 0 to 14 which measures how acidic or alkaline (basic) a substance is, defined as the common logarithm of the reciprocal of the hydrogen ion concentration.

Pollution - impurities in air, water or on land, that create an unclean environment.

Prevailing winds - the direction from which the winds blow most frequently during a given period of time. For example, the prevailing winds in Canada come from the west.

Solubility - the amount of a substance that can dissolve in a solution under a given set of conditions.

Stratosphere - the layer of air that extends from about 10 to 50 kilometres above the earth's surface.

Transport - the movement of pollutants with the wind from one region to another.

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