The big picture

You may have seen images of the effects of acid deposition. They are often used to represent how pollution can devastate habitats.

Figure 1. Effects of acid rain in Germany - such images are commonly used to illustrate the dramatic effects of pollution.

Theory of Knowledge

Images of forest destroyed by acid rain can be considered to be iconic. How do differing cultural and moral outlooks influence this perception of iconic status.

In this subtopic we will be looking at what contributes to formation of acid deposition, impacts of acid deposition and how it may be managed.

Acid deposition is an example of transboundary pollution, where pollution formed in one country can cause damage in another country. Meteorological and topographical factors determine where acid deposition occurs. Acid deposition is considered a regional problem as it often affects neighbouring or nearby countries downwind to the source of pollution. For example the Black Forest in Germany was affected by acid deposition that originated in neighbouring Poland and the former Czechoslovakia (now the Czech Republic and Slovakia).

Figure 2. Westerly winds transported pollutants from Poland, Czech Republic and Slovakia into Germany that contributed to acid deposition.

What is acidity?

You may find the following reminder of what we mean by acidity useful.

Acidity refers to the level of hydrogen ions present in a substance and is expressed as pH. Key aspects of pH are:

• The pH is defined as the negative logarithm of hydrogen ions: pH = -log[H+].
• The pH scale used ranges from 0 to 14.
• pH of 7 indicates a neutral solution, (where the number of hydrogen ions are equivalent to hydroxide ions).
• pH of less than 7 represents an acidic solution whereas a pH greater than 7 represents an alkaline solution.
• A change in scale by one unit is equivalent to a ten fold difference. Hence, a change from pH 7 to pH 5, (i.e. by 2 units) represents an increase in hydrogen ions by 102, a hundred fold increase.

Figure 3. The pH scale.

Rain usually has a pH of between five and six due to the presence of carbonic acid formed from carbon dioxide reacting with water in the atmosphere.

CO2 + H2O ⇌ H2CO3

carbon dioxide + water ⇌ carbonic acid

When precipitation has a pH lower than five it is referred to as acid deposition. The pH of precipitation has been lowered in many areas around the world by emissions produced from human activities.

Acid deposition

The pH of natural deposition is usually acidic at between pH 5 and 6. This is due to the presence of carbon dioxide in the atmosphere which forms carbonic acid. Emissions from human activity can further lower the pH of any deposition.

This acid deposition occurs as a result of primary pollutants of sulphur dioxide and nitrogen oxides reacting in the atmosphere to form secondary pollutants of sulphuric acid and nitric acid respectively. This results in the pH declining to below five.

SO2 + H2O = H2SO4

sulphur dioxide + water = sulphuric acid

NOx + H2O = HNO3

nitrogen oxides + water = nitric acid

The main human based source of the primary pollutants, sulphur dioxide and nitrogen oxides is the combustion of fossil fuels.

Natural sources of sulphur dioxide include volcano emissions, hot springs and biodegradation of dead organic matter. Natural sources of nitrogen oxides include lightning and biodegradation of dead organic matter.

Deposition can occur as:

• Wet deposition e.g. when pollutants are incorporated into the clouds or falling raindrops and result in acidified rain or snow.
• Dry deposition, when atmospheric pollutants are removed by gravity or direct contact under dry conditions e.g. when emissions of ash or dry particles from power stations are absorbed directly onto plants and buildings.

Figure 1. Acid deposition.

Dispersion and distribution of acid deposition

When sulphur dioxide and nitrogen oxides are emitted into the air, they can be transported long distances by prevailing winds. In the presence of strong winds the pollutants can be dispersed over a larger area compared to light winds that allow pollutants to accumulate near the source and can result in more acute pollution.

Tall smoke stacks have often been used to reduce local pollution by increasing dispersion of the pollutants. Wind speeds are lower at ground level due to friction from the ground, vegetation and buildings. Higher up there is less friction and wind speeds are greater. Prevailing winds can carry pollutants long distances to areas otherwise unpolluted. For example, the majority of acid deposition in that occurs in Norway originates in other countries such as Germany, UK and Poland.

Figure 2. Tall industrial chimney used to disperse pollutants further downwind.

The topography downwind of the source of pollution can also effect the distribution of acidic deposition. In mountainous areas, moist air masses are forced to rise, which causes cooling and condensation resulting in precipitation potentially increasing acidification in the area.

International-mindedness

The source of acid deposition is often different to where the impacts of acid deposition occur. The pollutants can travel large distances crossing national borders. Hence cooperation between nations is required to effectively reduce acid deposition.

Impacts of acid deposition

The impact of acid deposition on aquatic and terrestrial ecosystems also depends on the capacity of the environment to neutralize the acidic input. The presence of alkaline calcium and magnesium compounds increases the buffering capacity of the soil and water and reduces the effect of acid deposition. E.g. calcium carbonate also referred to as limestone has a high buffering capacity.

Figure 1. Limestone, an alkaline can neutralize acid deposition.

Aquatic ecosystems

Acid deposition can enter aquatic ecosystems either directly (e.g. precipitation as rain) or indirectly as run-off. It can lower the pH of the aquatic environment beyond the ability of some organisms to survive. Species of phytoplankton, invertebrate (e.g. crayfish) and fish (e.g. trout and salmon) can be sensitive to changes in pH. Loss of some species can cause a knock on effect through the food chain adversely affecting other organisms (e.g. due to lack of prey). Some species may suffer from reproductive failure and many fish eggs do not hatch at pH below five.

The increase in soil pH also releases aluminium ions which are then leached into the aquatic system. Fish exposed to aluminium ions secrete excess mucus around the gills preventing oxygen update and leading to death by asphyxiation.

Figure 2. Acid deposition can reduce biodiversity within an aquatic ecosystem which can become dominated by acid tolerant species such as Sphagnum moss.

Terrestrial ecosystems

Acid deposition on land can increase soil acidity. This lowering of soil pH can result in:

• Leaching of plant nutrients such as calcium, magnesium and potassium. This reduces the nutrients available for plant uptake.
• Mobilization of aluminum ions that can damage plant root systems and can also be leached into nearby watercourses adversely affecting fish, as discussed above.
• Mobilization of other toxic metals from the soil such as cadmium, lead and mercury which can then be leached into aquatic ecosystems adversely affecting aquatic organisms and potentially contaminating drinking water supplies.

Exposure of plants to acid deposition also results in:

• Damage to the cuticle wax found on leaves which reduces plant photosynthesis.
• Lower tolerance to pests, disease and low temperatures.

Overall these effects result in:

• Reduction in crop yield in agricultural areas.
• Loss of biodiversity and reduction in forest areas.

Materials

Acid deposition increases the rate of stone erosion and metal corrosion.

Stone erosion

Figure 3. Acid rain erodes some stone materials.

Nino Barbieri / Wikimedia Commons / CC BY-SA 3.0

Buildings and statues made of limestone and marble are particularly vulnerable. Acid deposition reacts with calcium carbonate within the limestone and marble forming gypsum (as shown in the chemical equation below), which can easily flake off.

CaCO3 + H2SO4 ⇌ CaSO4.H2O + CO2

calcium carbonate + sulphuric acid ⇌ gypsum + carbon dioxide

Acid deposition can also penetrate beyond the stone surface through the pores, where crystals of gypsum can grow causing cracks to appear and the stone to crumble.

Metal corrosion

Acid deposition increases corrosion of metals such as iron and steel which could weaken the structure of buildings.

Figure 4. Acid deposition can damage paint and increase corrosion rate of metal.

The impact of acid deposition on the Taj Mahal in India includes:

• Damage to the external marble stone of the building.
• Corrosion of the iron lugs and dowels that join the marble slab together.

Figure 5. Taj Mahal a UNESCO world heritage site.

Watch the following video ‘Acid rain’ by the Fuse School which provides an overview of acid deposition and some impacts:

Management of acid deposition

The main sources of sulphur dioxide and nitrogen oxides, the primary pollutants that result in formation of acid rain include:

• Stationary sources (i.e. fixed point sources) such as power station, industry and domestic boilers.
• Mobile sources which are predominately vehicles.

Methods to reduced emissions of air pollutants considered in the previous subtopic, Photochemical smog, also apply to the reduction of acid deposition.

International and regional cooperation

Acid deposition is a transboundary issue, where the impacts may occur hundreds of miles downwind of the source of pollution. Acid deposition frequently occurs in neighbouring or nearby countries. Hence acid deposition can be considered as a regional rather than global problem.

Figure 1. Risk of acid deposition in Europe.

UN Geneva Convention

Following growing recognition of the problem of acid deposition in 1972 The United Nations Conference on Human Environment in Stockholm recommended that international effort was required to reduce acid deposition.

This was followed by the Geneva Convention of Long Range Transboundary Air Pollution in 1979 focused on reducing air pollution which included nitrogen oxide and sulphur dioxide. The Geneva Convention has been extended over the years through Protocols and associated amendments. The latest amendment was in 2012 in which the 1999 Gothenburg Protocol to abate acidification, eutrophication and ground level ozone was reviewed and more stringent emission standards set.

The Geneva Convention provides a platform for parties to share knowledge and information and the latest scientific information is used to set and update emission targets.

Regional air quality agreement

The Canada-United States air quality agreement in 1991 recognized the problem of transboundary pollution between the two countries and agreed to work together to reduce nitrogen oxides and sulphur dioxide emissions. Emission levels of precursors to acid deposition have fallen significantly in both countries.

Figure 2. Reduction in sulphur dioxide levels in the USA (ARP – Acid rain program, part of the Canada-US air quality agreement; CAIR – Clean air interstate rule)

Source: EPA, 2014

Cap and trade system

The US use cap-and-trade to encourage energy utilities to reduce emission levels. Cap-and-trade involves allocating the utilities with permits which allow them to produce a set amount of emissions (e.g. one tonne of emissions during the period of compliance). The utilities are then free to buy and sell emission permits to each other. Those that do not use their emission permits can gain economically by selling their surplus to others.

In-situ clean up and restoration

Clean up methods have primarily focused around neutralizing the acidic water by adding limestone (calcium carbonate) a process also referred to as liming. In Sweden where thousands of lakes were affected by acid deposition, liming has been used to restore the natural pH of about 7,500 lakes. This has allowed some lakes to be restocked successfully with fish.

Figure 3. Swedish lake that has been restored.

"Näsbyholmssjön, Skåne" by jorchr - (Own Work). CC BY-SA 3.0 via Wikimedia Commons

Raising the pH of water by liming causes aluminium ions to precipitate out of the water column. However, if acid deposition continues the pH will fall again. At a low pH the aluminium ions are released back into the water column which can have a detrimental effect on fish stocks.

In Sweden, limestone is added to some lakes every year to maintain a suitable pH for fish stocks. However, liming should be considered as only a temporary solution while the source of pollution is being reduced The majority of the acid deposition that affects the lakes in Sweden originates in other countries such as Germany, Poland and the UK, therefore cooperation between nations is necessary to reduce acid deposition at source.

Terrestrial systems can also be limed to increase the soil pH and immobilize toxic metals. On agricultural land, fertilizers may also be added to replace loss nutrients.

Watch the following video ‘Reducing acid rain or its effects ‘by the Fuse School:

Theory of Knowledge

Can pollution be ethically and morally justified if you know it will have an impact on others?