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                                           TOPIC 6.4: ACID DEPOSITION

Acid rain is a broad term referring to a mixture of wet and dry deposition (deposited material) from the atmosphere containing higher than normal amounts of nitric and sulfuric acids. The precursors, or chemical forerunners, of acid rain formation result from both natural sources, such as volcanoes and decaying vegetation, and man-made sources, primarily emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) resulting from fossil fuel combustion.

When sulfur dioxide and nitrogen oxide emitted by cars and factories combine with moisture in the air, acid rain is formed. Acid rain, which often falls far from the source of pollution, kills trees, makes lakes unfit for fish, and even dissolves the stone in buildings and monuments. Rocky areas with thin topsoil are particularly apt to be damaged by acid rain. 

In this unit we will look at the formation of acid rain, its effects on the ecosystem and strategies to reduce acid rain formation.

​This unit is a minimum of 2.5 hours.


Significant Ideas:

  • Acid deposition can impact living systems and the built environment.

  • The pollution management of acid deposition often involves cross-border issues.

Big questions:

  • To what extent have the solutions emerging form this topic been directed at preventing environmental impacts, limiting the extent of the environmental impacts, or restoring systems in which environmental impacts have already occurred?

  • How are the issues addressed in this topic of relevance to sustainability or sustainable development?

  • In what ways might the solutions explored in this topic alter your predictions for the state of human societies and the biosphere some decades from now?

  • To what extent is acidification yesterdays problem? Why has acidification declined in certain regions?

  • Examine the relationship between acidification and sustainability

  • In what ways is acidification likely to change over the next decades?


Knowledge and Understanding

6.4.U1 The combustion of fossil fuels produces sulfur dioxide and oxides of nitrogen as primary pollutants. These gases may be converted into secondary pollutants of dry deposition (such as ash and dry particles) or wet deposition (such as rain and snow).
[The use of chemical symbols, formula or equations is not required]

Refer to the conversion of sulfur dioxide and oxides of nitrogen (NOx) into the sulfates and nitrates of dry deposition and the sulfuric and nitric acids of wet deposition. Knowledge of chemical equations is not required.
Acid deposition can be either wet or dry:

  • Wet deposition - acidic rain, snow, or other precipitation

  • Dry deposition - acidic gas or dry particles, not mixed with water

Primary pollutants - those directly emitted by a factory or automobile, such as...

  • SO2 - sulfur dioxide

  • NO and NO2, usually identified as NOx

Secondary pollutants - primary pollutants react with other substances in the atmosphere and create different pollutants, such as...

  • H2SO3 - sulfurous acid

  • H2SO4 - sulfuric acid

  • HNO3 - nitric acid


6.4.U2 The possible effects of acid deposition on soil, water and living organisms include:

  • direct effect—for example, acid on aquatic organisms and coniferous forests

  • indirect toxic effect—for example, increased solubility of metal (such as​ aluminium ions) on fish

  • indirect nutrient effect—for example, leaching of plant nutrients.

Acid rain directly affects the chemical and pH balances in ground water. The excess aluminum created by acid rain makes aquatic environments such as the sea, lakes, and streams, toxic. The animals that can withstand the imbalance of the water's natural minerals might survive, but quickly lose their food source as the weaker creatures die off.  

Acid rain leaches calcium out of the soil when it is absorbed by the earth. This directly affects the mineral levels of the soil and the creatures, such as snails, that rely on that calcium for shell growth. Consequently, snails die off and birds, which eat them for calcium, lay eggs with shells that are weak and brittle and therefore fail to hatch.

Acid rain directly impacts forest ecosystems and their inhabitants. Acid rain damages leaves as it falls. Acid rain runoff from the trees and forest floors infiltrates the forest's water supplies; runoff that doesn't enter the water supply is absorbed by the soil.

Acid rain is dangerous to humans. The same sulphate and nitrate particles that directly affect the soil and water pH balances can cause serious damage to the respiratory system if inhaled deeply. A damaged respiratory system means decreased oxygen in the blood supply, which eventually damages the heart.

6.4.U3 The impacts of acid deposition may be limited to areas downwind of major industrial regions but these areas may not be in the same country as the source of emissions.
[Possible case studies of intergovernmental situations involving acid deposition to consider include the USA Midwest and Eastern Canada interaction, as well as the impact of industrial Britain, Germany and Poland on Sweden.]

Refer to areas downwind of major industrial regions that are adversely affected by acid rain and link them to sources of sulfur dioxide and nitrogen dioxide emissions. Consider the effect of geology (rocks and soils) on water acidity
through buffering.

  • Acid precipitation falls back to Earth rather than entering stratospheric jet stream

  • most areas are downwind of pollution sources

  • Canadian forests damaged by coal-fired power plants in USA

  • Scandinavian and German forests damaged by British coal plants

6.4.U4 Pollution management strategies for acid deposition could include:

  • altering human activity—for example, through reducing use, or using alternatives to, fossil fuels; international agreements and national governments may work to reduce pollutant production through lobbying

  • regulating and monitoring the release of pollutants—for example, through the use of scrubbers or catalytic converters that may remove sulfur dioxide and oxides of nitrogen from coal-burning powerplants and cars.

  • Reducing use of fossil fuels

  • Reduce the number of cars

  • Switch to low sulfur fuel

  • Remove sulfur before combustion

  • Remove sulfur from waste gases

  • Wet scrubbing

  • Dry scrubbing


6.4.U5 Clean-up and restoration measures may include spreading ground limestone in acidified lakes or recolonization of damaged systems—but the scope of these measures is limited.

Use of limestone or lime, a process called liming, is a practice that people can do to repair the damage caused by acid rain to lakes, rivers and brooks. Adding lime into acidic surface waters balances the acidity. It’s a process that has extensively been used, for instance in Sweden, to keep the water pH at optimum. Even though, liming is an expensive method and has to be done repeatedly. Furthermore, it only offers a short-term solution at the expense of solving the broader challenges of SO2 and NOx emissions and risks to human health. Nevertheless, it helps to restore and allow the survival of aquatic life forms by improving chronically acidified surface waters.


Application and Skill

6.4.A1 Evaluate pollution management strategies for acid deposition.
[Reference to Figure 3 Pollution Management]​

Measures to reduce fossil fuel combustion should be considered, for example, reducing demand for electricity and private cars and switching to renewable energy. Refer to clean-up measures at “end of pipe” locations (points of emission). Consider the role of international agreements in effecting change. The cost-effectiveness of spreading ground limestone in Swedish lakes in the early 1980s provides a good case study.


  • switch to renewable energy sources (reduce fossil fuel use)

  • increase energy efficiency (better light bulbs and appliances)

  • more public transportation (fewer automobiles on the road)

  • use low-sulfur fuels


  • install ‘scrubbers’ on smokestacks of coal-fired power plants to remove SO2

  • catalytic converters installed on automobiles (required by law in the US, Canada, and Europe)


  • add lime to acidified lakes and streams

  • add lime to forestry plantations (why not natural forests?)

  • UN Convention on Long-Range Transboundary Air Pollutants (LRTAP) - 1979; subsequently amended and modified by US, Canada, and Europe


Thing to consider when evaluating

  • Acid deposition travels with wind and water vapor in the atmosphere

  • The additional environmental impacts of cleaning up emissions e.g. mining, baking and transporting of limestone

  • Monitoring and identify sources may be difficult, as they are often non-point

  • Intergovernmental agreements often require proof and appropriate compensation

Classroom Material
6.4 Acid Deposition ppt
6.4 Acid Deposition notes
Acid Rain activity
Acid Rain lab
Leaves and Air Pollution activity

Case Studies
Intergovernmental situations involving acid deposition

Canada and the Unites States

Useful Links
EPA animation outlining the formation and consequences of acid rain
Acid Rain Facts - National Geographic
Animation depicting the formation of acid rain and its consequences. From
Flash animation about acid rain from
Australian site for acid rain. - Asuetute
What is Acid Rain - EPA
Ocean Acidification - National Geographic
Liming Acid Lakes - Virginia Cooperative Extension
1979 Long Range Transboundry Air Pollution

In The News
Environmental Global Issues: Global Warming, Acid Rain, Depletion of Ozone Layer Effects - For The Future 13 Jan 2013
Schneiderman creates $400G Program to help reclaim acid rain-damaged waters of the Adirondacks - Environmental Headlines 23 Jan 2013
Organic carbon suggests Swedish lakes were less acidified - Aug 01, 2011 Organic carbon suggests Swedish lakes were less acidified


  • The polluting country and the polluted country are often not the same: acid deposition affects regions far from its source. Therefore, solving this issue requires international cooperation.


  • To what extent does the recognition of the ethical responsibility of knowledge influence the further production or acquisition of knowledge?

Most cases of non‑point source pollution exemplify well the intractable ethical problem of the “tragedy of the commons”. That is to say, an individual polluting a common resource suffers little themselves from their own pollution and yet may benefit considerably in other ways. Therefore, those that do not pollute are doubly penalized—they suffer the pollution, and yet gain no benefit from polluting the resource themselves. There is thus a net advantage for any individual who does pollute. Ultimately, as many individuals adopt the most advantageous attitude, this leads to a great deal of suffering for all. It is exactly this conundrum that underlies much of the difficulty in managing non‑point source pollution of shared resources on both a local (for example, a river) and an international (for example, the atmosphere) scale. Indeed, that one nation may gain considerably from non‑compliance, especially while others comply, underlies much of the hesitancy in reaching international agreements on pollution strategies. Consideration and comparison of how both deontological and utilitarian approaches to ethics address this issue may make for interesting debate. In addition, the role of international legislation compared to increasing public awareness in tackling the problem could arguably be seen as a directly parallel debate. That is, is a system of rules, or appealing to the general good, the most effective way forward?


Video Clips

A fun, engaging and relevant programme, inspiring the viewer to consider the science within and how scientific process can be used to test ideas and develop theories, rather than just looking for a given answer to a known question.



















Learn the basics about Acid Rain. What causes acid rain?


















This groundbreaking NRDC documentary explores the startling phenomenon of ocean acidification, which may soon challenge marine life on a scale not seen for tens of millions of years


















Clip from National Geographic's Appalachian Trai





















Learn the basics about reducing acid rain or its effects. How can we reducing acid rain and its effects?


















Rob Dunbar hunts for data on our climate from 12,000 years ago, finding clues inside ancient seabeds and corals. His work is vital in setting baselines for fixing our current climate -- and, scarily, in tracking the rise of deadly ocean acidification.

















Showing how Liming can help neutralize the effects of acid rain on lakes














Acid Deposition/ Acid Rain (6.4)

What is Acid Rain?
​Causes and Effects
Acid Rain- Causes, Effects, and Solutions- Energy conservation
Chemistry of Acid Rain
​Ecological Affects of Acid Deposition

Management Strategies
What's being done?- EPA
Proposed Recovery solutions (Technical) - about 1/2 way down

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