What is Acid Rain?
Acid rain refers to a mixture of deposited material, both wet and dry, coming from the atmosphere containing more than normal amounts of nitric and sulfuric acids. Simply put, it means rain that is acidic in nature due to the presence of certain pollutants in the air due to cars and industrial processes. It is easily defined as rain, fog, sleet or snow that has been made acidic by pollutants in the air as a result of fossil fuel and industrial combustions that mostly emits Nitrogen Oxides (NOx) and Sulfur Dioxide (SO2). Acidity is determined on the basis of the pH level of the water droplets. Normal rain water is slightly acidic with a pH range of 5.3-6.0, because carbon dioxide and water present in the air react together to form carbonic acid, which is a weak acid. When the pH level of rain water falls below this range, it becomes acid rain.
When these gases react with water molecules and oxygen among other chemicals found in the atmosphere, mild acidic chemical compounds such as sulfuric and nitric acid are formed resulting to acid rain. Acid rain generally leads to weathering of buildings, corrosion of metals, and peeling of paints on surfaces. Erupting volcanoes contains some chemicals that can cause acid rain. Apart from this, burning of fossil fuels, running of factories and automobiles due to human activities are few other reasons behind this activity.
Presently, large amounts of acid deposition is witnessed in the southeastern Canada, northeastern United States and most of Europe, including portions of Sweden, Norway, and Germany. In addition, some amount of acid deposition is found in parts of South Asia, South Africa, Sri Lanka, and Southern India.
Forms of Acid Rain
There are two forms in which acid deposition occurs – wet and dry. Both are discussed below:
- Wet Deposition: When the wind blows the acidic chemicals in the air to the areas where the weather is wet, the acids fall to the ground in the form of rain, sleet, fog, snow or mist. It removes acid from the atmosphere and deposit them on the earth’s surface. When this acid flows through the ground, it affects large number of plants, animals and aquatic life. The water from drain flows into rivers and canals which is them mixed up with sea water, thereby affecting marine habitats.
- Dry Deposition: If the wind blows the acidic chemicals in the air to the areas where the weather is dry, the acidic pollutants slip into dust or smoke and fall to the ground as dry particles. These stick to the ground and other surfaces such as cars, houses, trees and buildings. Almost 50% of the acidic pollutants in the atmosphere fall back through dry deposition. These acidic pollutants can be washed away from earth surface by rainstorms.
It was discovered way back in 1800s during the Industrial Revolution. A Scottish chemist, Robert Angus Smith, was first to discover this phenomenon in 1852 as a relationship between acid rain and atmospheric pollution in Manchester, England. But it gained public attention mainly in 1960s. The term was coined in 1972 when the NY Times published reports about the climate change effects which started arising due to the occurrence of acid rain in the Hubbard Brook Experimental Forest in New Hampshire.
Causes of Acid Rain
Both natural and man-made sources are known to play a role in the formation of acid rain. But, it is mainly caused by combustion of fossil fuels which results in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx).
1. Natural Sources
The major natural causal agent for acid rain is volcanic emissions. Volcanoes emit acid producing gases to create higher than normal amounts of acid rain or any other form of precipitation such as fog and snow to an extent of affecting vegetation cover and health of residents within the surrounding. Decaying vegetation, wildfires and biological processes within the environment also generate the acid rain forming gases. Dimethly sulfide is a typical example of a major biological contributor to sulfur containing elements into the atmosphere. Lighting strikes also naturally produces nitric oxides that react with water molecules via electrical activity to produce nitric acid, thereby forming acid rain.
2. Man-made sources
Human activities leading to chemical gas emissions such as sulfur and nitrogen are the primary contributors to acid rain. The activities include air pollution sources emitting sulfur and nitrogen gases like factories, power generations facilities, and automobiles. In particular, use of coal for electrical power generation is the biggest contributor to gaseous emissions leading to acid rain. Automobiles and factories also release high scores of gaseous emissions on daily basis into the air, especially in highly industrialized areas and urban regions with large numbers of car traffic. These gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds such as sulfuric acid, ammonium nitrate, and nitric acid. As a result, these areas experience exceedingly high amounts of acid rain.
The existing winds blow these acidic compounds over large areas across borders and they fall back to the ground in the form of acid rain or other forms of precipitation. Upon reaching the earth, it flows across the surface, absorbs into the soil and enters into lakes and rivers and finally gets mixed up with sea water.
The gases i.e. i.e. sulfur dioxide (SO2) and nitrogen oxides (NOx) are primarily gases occurring from electric power generation by burning coal and responsible for acid rain.
Effects of Acid Rain
Acid rain has significant effects on the world environment and public health.
- Effect on Aquatic Environment: Acid rain either falls directly on aquatic bodies or gets run off the forests, roads and fields to flow into streams, rivers and lakes. Over a period of time, acids get accumulated in the water and lower the overall pH of the water body. The aquatic plants and animals need a particular pH level of about 4.8 to survive. If the pH level falls below that the conditions become hostile for the survival of aquatic life. Acid rain tendency of altering pH and aluminum concentrations greatly affects pH concentration levels in surface water, thereby affecting fish as well as other aquatic life-forms. At pH levels below 5, most fish eggs cannot hatch. Lower pHs can also kill adult fish. Acid rain runoff from catchment areas into rivers and lakes has also reduced biodiversity as rivers and lakes become more acidic. Species including fish, plant and insect types in some lakes, rivers and brooks have been reduced and some even completely eliminated owing to excess acid rain flowing into the waters.
- Effect on Forests: It makes trees vulnerable to disease, extreme weather, and insects by destroying their leaves, damaging the bark and arresting their growth. Forest damage due to acid rain is most evident in Eastern Europe – especially Germany, Poland and Switzerland.
- Effect on Soil: Acid rain highly impacts on soil chemistry and biology. It means, soil microbes and biological activity as well as soil chemical compositions such as soil pH are damaged or reversed due to the effects of acid rain. The soil needs to maintain an optimum pH level for the continuity of biological activity. When acid rains seep into the soil, it means higher soil pH, which damages or reverses soil biological and chemical activities. Hence, sensitive soil microorganisms that cannot adapt to changes in pH are killed. High soil acidity also denatures enzymes for the soil microbes. On the same breadth, hydrogen ions of acid rain leach away vital minerals and nutrients such as calcium and magnesium.
- Vegetation Cover and Plantations: The damaging effects of acid rain on soil and high levels of dry depositions have endlessly damaged high altitude forests and vegetation cover since they are mostly encircled by acidic fogs and clouds. Besides, the widespread effects of acid rain on ecological harmony have lead to stunted growth and even death of some forests and vegetation cover.
- Effect on Architecture and Buildings: Acid rain on buildings, especially those constructed with limestone, react with the minerals and corrode them away. This leaves the building weak and susceptible to decay. Modern buildings, cars, airplanes, steel bridges and pipes are all affected by acid rain. Irreplaceable damage can be caused to the old heritage buildings.
- Effect on Public Health: When in atmosphere, sulfur dioxide and nitrogen oxide gases and their particulate matter derivatives like sulfates and nitrates, degrades visibility and can cause accidents, leading to injuries and deaths. Human health is not directly affected by acid rain because acid rain water is too dilute to cause serious health problems. However, the dry depositions also known as gaseous particulates in the air which in this case are nitrogen oxides and sulfur dioxide can cause serious health problems when inhaled. Intensified levels of acid depositions in dry form in the air can cause lung and heart problems such as bronchitis and asthma.
- Other Effects: Acid rain leads to weathering of buildings, corrosion of metals, and peeling of paints on surfaces. Buildings and structures made of marble and limestone are the ones especially damaged by acid rain due to the reactivity of the acids in the rain and the calcium compounds in the structures. The effects are commonly seen on statues, old grave stones, historic monuments, and damaged buildings. Acid rain also corrodes metals like steel, bronze, copper, and iron.
Solutions to Acid Rain
- Cleaning up Exhaust Pipes and Smokestacks
Most of the electric power supporting the modern-day energy requirements comes from combusting fossil fuels such as oil, natural gas, and coal that generate nitrogen oxides (NOx) and sulfur dioxide (SO2) as the chief contributors to acid rain. Burning coal largely accounts for SO2 emissions while NOx emissions are mostly from fossil fuel combustions.
Washing coal, use of coal comprised of low sulfur, and use of devices known as “scrubbers” can provide technical solution to SO2 emissions. “Scrubbing” also called flue-gas desulfurization (FGD) typically work to chemically eliminate SO2 from the gases leaving smokestacks. It can eliminate up to 95% of SO2 gases. Power generation facilities can also shift to using fuels that emit much less SO2 such as natural gas instead of burning coal. These methods are simply called emission reduction strategies.
Similarly, NOx emissions from automobile fossil fuel combustions are mitigated upon by use of catalytic converters. Catalytic converters are fixed on the exhaust pipe system to reduce NOx emission. Improvement of gasoline that combusts cleaner is also a strategy for reducing emission of NOx gases.
- Restoring Damaged Environments
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.
- Alternative Energy Sources
Besides fossil fuels, there is a wide range of alternative energy sources that can generate electrical power. These include wind energy, geothermal energy, solar energy, hydropower, and nuclear power. Harnessing these energy sources can offer effective electrical power alternatives instead of using fossil fuels. Fuel cells, natural gas, and batteries can also substitute use of fossil fuel as cleaner energy sources. As of today, all energy sources have environmental and economic costs as well as benefits. The only solution is using sustainable energy that can protect the future.
- Individual, National/State, and International Actions
Millions of people directly and indirectly contribute to SO2 and NOx emissions. Mitigation of this challenge requires individuals to be more informed about energy conservation and ways of reducing emissions such as: turning off lights or electrical appliances when not using them; use public transport; use energy efficient electrical appliances; and use of hybrid vehicles or those with low NOx emissions.
Image Credit: agnaldo_pereira_miguel , numbphoto
Rinkesh is passionate about clean and green energy. He is running this site since 2009 and writes on various environmental and renewable energy related topics. He lives a green lifestyle and is often looking for ways to improve the environment around him.
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topic 6.4: acid depositionimage from acidrainisbad.webs.com
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.
- Acid deposition can impact living systems and the built environment.
- The pollution management of acid deposition often involves cross-border issues.
- 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?
[The use of chemical symbols, formula or 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
- 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 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.
- 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
- 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
[Reference to Figure 3 Pollution Management]
- 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
- 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
- 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?