Unit 4 - Acid Rain

What is acid rain?

Any form of precipitation with acidic components such as high levels of nitric and sulphuric acid which fall to the ground in wet (rain, snow) and dry (dust) forms. These acidic components have been formed by pollutant gases which mix with water vapour in the atmosphere to form acidic particles.

What is the usual pH of acid rain?

Usually, acid rain has a pH of between 4.2 and 4.4 but it can be any precipitation which is more acidic than pH 5.6

What reactions might sulphur dioxide undergo to form acid rain?

• In the atmosphere, sulphur dioxide reacts with water, oxygen and other substances to form sulphuric acid

• Sulphur dioxide dissolves in water to produce sulphurous acid

• Sulphur dioxide can be oxidised in the atmosphere by gases like ozone, which produces sulphur trioxide. Sulphur trioxide dissolves to form sulphuric acid - this is a much more more powerful acid than sulphurous acid

What reactions might nitrogen oxides undergo to form acid rain?

• In the atmosphere, nitrogen oxides react with water, oxygen and other substances to form nitric acid

• Nitrogen oxides dissolve to produce nitrous and nitric acids

Which other gases are involved in the formation of acid rain?

• Hydrogen chloride dissolves to form hydrochloric acid

• Ozone is involved in the oxidation of sulphur dioxide to sulphur trioxide and it also harms plants directly

How does burning fossil fuels contribute to the formation of acid rain - which gases are released? What are the main sources which burn fossil fuels?

Burning fossil fuels releases sulphur dioxides and nitrogen oxides into the atmosphere. Major sources that use and burn fossil fuels include power plants such as coal-fired power stations; vehicle exhausts; wood burning stoves; refineries.

What processes release sulphur dioxide - give some examples of sources?

Sulphur dioxide is released from the combustion or oxidation of materials which contain sulphur, particularly burning coal and smelting sulfide ores

What process releases nitrogen oxides? What are some sources of this?

High temperature combustion - such as in power stations and petrol and diesel internal combustion engines

What are the sources of hydrogen chloride?

The combustion of coal and incineration of wastes that contain chlorine such as PVC plastic waste release hydrogen chloride

What kind of pollution is acid rain and how does this influence its impacts?

It is a transboundary form of pollution meaning that winds can spread the acidic solutions over hundreds of miles, meaning that the impacts are wide-ranging and very severe

What are the main impacts of acid rain on non-living things?

• Acid deposition corrodes metals, which causes damage to human structures like railway lines, metal railings, water pipes, pylons and overhead powerlines

• Limestone structures such as buildings and statues can be damaged by acid rain as the acids dissolve the surface layers and weaken the structure of the porous limestone - an example of this is the pollution damage to York Minster

How does acid rain directly affect living organisms?

• Acids are harmful to all living organisms, though the range of tolerance differs between species - conditions may move out of a species range of tolerance. For example, lichens are very sensitive to acidic conditions

• Low pH’s denature proteins in cell membranes and can inhibit enzyme action

• Acidic solutions can enter water systems as run-off and sinks into the ground, which can make water toxic to crayfish, clams, fish and other aquatic animals. The acidification of lakes and streams makes water highly acidic and inhospitable to most species.

• Invertebrates with exoskeletons might die as the acids dissolve the calcium compounds which form the skeleton - freshwater crayfish are particularly vulnerable - this impacts the rest of the food chain

• Tissues with living cells which are exposed to the environment are most likely to be damaged by acid rain e.g. the cells inside leaf stomata, plant root hair cells, germinating seeds and fish eggs and fish gills so can have a particularly damaging effect on forests and water sources

• Sulphur dioxide in acid rain can create breathing difficulties and increase the frequency of respiratory problems like asthma

Using examples and knowledge of the direct impacts of acid rain on living organisms, how might the impacts differ between species? Which kinds of species are most vulnerable?

Species with a low range of tolerance for acidic conditions may be impacted most negatively because of changing acidic conditions, which may move out of their range of tolerance e.g. lichens. Invertebrates are particularly vulnerable as acidic solutions dissolve the calcium compounds that form their skeleton such as crayfish, which has an impact on the entire food chain. Species that have living cells exposed to the environment such as inside leaf stomata and germinating seeds are also particularly vulnerable to damage.

How does acid rain indirectly impact living organisms?

Acid rain causes soil to become more acidic, which moves conditions outside of species range of tolerance, denaturing enzymes and killing entire forests’ trees and vegetation; acid rain being absorbed into soil makes trees more susceptible to viruses, fungi and insect pests; the number of microorganisms present in the soil decreases as the soil becomes more acidic, further depleting the amount of nutrients available to plants as a result of reduced decomposition.

How does acid rain cause metal ions to become more mobile? What is the impact of this on living organisms?

Metal ions normally become more soluble at low pH’s and as acid percolates through soils, it can leach metal ions from the soil. Important plant and soil nutrients such as calcium and magnesium are lost first, which reduces nutrient availability for plants. Affects cation exchange on humus/clay particles which leads to loss/leaching of soil nutrients.

How does acid rain cause toxic ions to become more mobile? What is the impact of this on living organisms?

After these metal ions have been leached, other ions are mobilised that would normally be adsorbed onto the chemistry, including toxic ions like Aluminium and Lead - these toxic ions inhibit enzyme action in plant root hair cells and other soil organisms like detritivores and decomposers. These mobilised toxic ions may be leached into rivers and lakes where they can harm aquatic organisms, they can also impact human health as lead is a neurotocin and aluminium ions may be associated with some neurological disorders.

How else can acid rain impact human health?

Acid rain has profound effects on human health, causing heart and lung disorders such as asthma and bronchitis as well as impairing brain functioning.

How does soil lime content affect the severity of acid rain?

Soils that have a high lime content include Calcium Carbonate which neutralises acids and reduces the impact of acids on soil pH, therefore reducing acid rain’s severity.

How does the presence of long periods of fog influence the severity of acid rain?

In areas where there are long periods of fog, the acidic water droplets may be in contact with tree leaves for longer while usually acid rain may only have brief contact with the leaves of vegetation. This increases the damage that is caused by the acids.

How does the severity of acid rain differ based on whether the precipitation is rain or snow?

Where all precipitation is rain, the impact of the acids may be spread over long periods of time. However, areas which have lot of snow might accumulate acidic snow and if this melts quickly, the pH of the meltwater may be very low, harming life in the soil and rivers.

Control of acid rain - What pollutants need to be controlled to control the formation of acid rain?

The primary pollutants - emissions of SOx and NOx

Control of acid rain - How does natural gas desulphurisation work to control the production of sulphur dioxide?

Natural gas desulphurisation involves removing hydrogen sulphide from natural gas after it has been extracted by dissolving it in an amine solution or reacting it with iron particles. This prevents the production of sulphur dioxide (and prevents corrosion damage to refinery and pipeline equipment).

Control of acid rain - How does crude oil desulphurisation work?

Sulphur compounds are removed from crude oil during distillation using molybdenum catalysts in the process of hydrodesulphurisation. This does produce hydrogen sulfide, which must be managed and disposed of safely.

Control of acid rain - How does coal desulphurisation work?

Most of the sulphur in coal is present as solid iron pyrites, which can be removed through washing and steaming. The coal is crushed and then put in a stream of flowing water where the flow rate is fast enough to carry the coal away but leaves the denser pyrites behind.

What might some of the disadvantages of coal desulphurisation be?

Requires a lot of water. The process also takes a long time and just seperates out - it doesn’t deal with completely.

What is flue-gas desulphurisation?

After fuel combustion, several processes can be used to remove sulphur dioxide from power station emissions

Acid rain control - How does dry flue-gas desulphurisation work?

Flue gases are passed through a bed of crushed calcium carbonate and the sulphur dioxide then reacts with the calcium carbonate and this produces solid calcium sulphur.

What are the advantages of dry flue-gas desulphurisation?

If the effluent gases have been cleaned to remove smoke particles, the calcium sulfate can be used to make gypsum building plaster. Energy consumption is relatively low and the product is easy to handle - the equipment is simple.

What are the disadvantages of dry flue-gas desulphurisation?

Slow reaction speed; low desulphurisation rate; desulphurisation efficiency is low; service life is short

Acid rain control - How does wet flue-gas desulphurisation work?

Flue gases are bubbled through sodium sulphite solution in a process known as sodium sulphite scrubbing and this produces sodium hydrogen sulphite. The sodium hydrogen sulphite can then be heated and it breaks down to produce sodium sulphite and water, which are reused, and then the concentrated pure sulphur dioxide can be converted to solid sulphur or sulphuric acid.

What are the advantages and disadvantages of wet flue-gas desulphurisation?

Fast reaction speed and high desulphurisation efficiency, usually higher than 90%. It is also a mature technology and widely implemented. However, it produces liquid or sludge which can be difficult to handle; energy consumption is high; water consumption is high; it is generally more suitable for large-scale power plants

Control of acid rain - How can low temperature combustion help to control nitrous oxide concentrations?

If lower temperature combustion techniques are used, less nitrous oxide is produced. An example is fluidised bed combustion where an increased surface area for combustion maintains rapid combustion without the need for high temperatures.

Control of acid rain - How do catalytic converters work to reduce nitrous oxide emissions?

Catalytic converters are a chamber in exhaust pipes of cars which contain a catalyst like platinum, which chemically reduces NOx back to nitrogen and oxygen gases, which are less harmful pollutants.

How effective are catalytic converters? By how much have they reduced hydrocarbon, carbon monoxide and nitrous oxide emissions from exhausts?

They are highly effective, having reduced hydrocarbon emissions by 87%, carbon monoxide by 85% and nitrous oxide by 62%. They are widely implemented and it is illegal in the UK for petrol vehicles not to be fitted with them.

How do urea sprays work to reduce NOx concentrations?

NOx can be removed by reacting it with urea. Urea contains ammonia, which reacts with the NOx gas to produce Nitrogen gas, Carbon Dioxide and Water. The urea must be handled carefully as it is an irritant and can be corrosive to the skin.

How much have sulphur dioxide emissions in the UK been reduced since the UK’s coal burning peak in 1970?

In 2020, compared to 1970, sulphur dioxide emissions have decreased by 98%

Control example USA/Canada - What was the 1985 Eastern Canada Acid Rain Programme?

It was an Eastern Canada cap of 2.3 million tonnes of sulphur dioxide in the seven easternmost provinces. This was met by 1994 and maintained until 2000.

Control Example USA/Canada - What was the 1970 US Clean Air Act and what ammendments have since been made to it?

It introduced regulations on air pollutants for the first time. Amendments in 1990 introduced a cap-and-trade system for reducing sulphur dioxide emissions at the source. This involved the government imposing limits on the total amount of sulphur dioxide the electric power sector could emit each year and then they divided the permitted emissions into a fixed number of ‘allowances’ distributed to each power plant - this meant that costs could be kept low while also limiting pollution, forcing every utility to reducing their pollution in the long-term.

Identify the name of the agreement between the US and Canada which recognised the transboundary nature of acid rain pollution.

The Canada-United States Air Quality Agreement 1991

What is a piece of evidence/statistic that reflects the success of these agreements?

By 2020, sulphur dioxide emissions in Canada and the US had decreased by 78% and 92% respectively from 1990 emission levels.

Outline three reasons why there has been more success in controlling acid rain pollution than greenhouse gas emissions

• The severity of environmental and human health impacts, which governments quickly responded to

• Control strategies easier to implement without damaging the economy, doesn’t require significant behavioural changes

• The impact of climate change is far less immediate and visible from the point of emission to impact