Acid rain

 

Acid rain is one of the world’s major environmental problems since 19th century. Fossil fuels such as coal burning is the major cause of SO₂ production and also vehicle emission and various fossil fuel based power generation emits nitric oxides. Both SO₂ and NO₂ produces sulphuric and nitric acid respectively by reacting with atmospheric water vapour and precipitate as wet deposition such as rain, snow and fog and dry deposition including hazardous particles of PM 2.5 ( Fine (smaller) particles PM_2.5, are more dangerous because they can get into the deep parts of lungs or even into blood.). In dry deposition sulphate and nitrate ions fall as small particles without dissolving in water, about 20-60 % of the total deposition is dry deposition.

Acid rain affects forest trees causes yellowing and leaf fall, acidified rivers and lakes causes fish death, loss of calcareous shell forming species (mollusks). It also affects soil microorganisms causes increased nitrification which also leads to eutrophication in water bodies and changes in the biodiversity. Acid rain destroys the coral reefs. It causes leaching of metal ions including toxic Aluminum and heavy metals such as chromium, cadmium and nickel, which adversely affects the soil micro flora and aquatic biota. Acid rain deteriorates the marble, stone monuments and architectures, corrode metal structures and fading paints.

The effect of acidification has been sighted all over the world such as deleterious ecological effects such as reduced reproduction of aquatic fish species, dieback and stunted growth in plants, accumulation of toxic aluminum and heavy metals in soil and water bodies, biodiversity loss including corals and shellfish, degrade to the manmade structures made up of marble and stone and corrosion of metal structures.

Causes

Acid rain as discussed is caused by emission of SO₂ and NO₂ from various sources to the atmosphere which dissolve in atmospheric water and produce acids in the rain water. SO₂ does not react much in the atmospheric chemicals but it can travel quicker to long distances and when get contact with ozone or hydrogen peroxide it produces SO₃, which is highly soluble in water and form sulphuric acid. Sulphur dioxide is naturally produced by volcanic eruptions, sea spray, planktons, rotting vegetation and forest fires.

Anthropogenic sources - 69.4 % of Sulphur dioxide released from industrial combustion (point sources), house hold heating of fire wood and coal (area or non-point sources) and 3.7% from transportation (mobile sources). Coal burning sources such as coal power plants, coal powered engines in vehicles, smelting of metal ore, production of iron and steel, oil refinery, domestic and industrial boilers, during the manufacture of sulphuric acid using in the production of disinfectants, bleaching agents and fumigants. NO₂ is naturally produced by lightening, bacterial action, forest fire and volcanoes, manmade emission are by automobiles (43%) and fertilizer industries, and other industrial combustion (32%).

Effects of acid rain can either chronic or episodic

Chronic acidification is a longterm effect due to years of acid rain, Episodic acidification is due to heavy rain storms or when snow melts. Acid rain increases nitrate levels in soil, leading to nitrogen saturation in soils. Nitrate ions remove additional calcium and magnesium from soil, excess nitrogen  also leads to eutrophication in water bodies. Trees starve for aluminum and other minerals as aluminum of soil get converted to aluminum nitrate or sulphate when get absorbed by trees cause harmful effects.

Effects on surface waters

Acid rain releases aluminum from the soil into lakes and streams which is toxic to many aquatic organisms. Acidification increases the release of aluminum from granite rocks. Acidic condition together with toxicity of heavy metals such as ions of copper, cadmium, nickel, chromium, cobalt, lead and zinc in the water body reduces the development and growth of the fish. This make the fish less immune, thus become more susceptible to diseases, kills the eggs and larval stages, reduces reproductive success.

Acidification effects shell forming molluscs, shell fish, coral reefs, and juvenile stages of aquatic organisms. In case of shell fish and corals their calcareous shell or skeleton get dissolved in acidic environment. Reduced pH encourages the growth of acid tolerant forms such as some bacteria and protozoa.

Acid rain is not the sole cause of acidification, some swamps, bogs and marshes naturally have low level of pH.

Nitrogen dioxide deposition in water bodies is another major reason for episodic acidification, about 10- 45 % of the nitrogen dioxide reaching water bodies are airborne and they are released to atmosphere mainly from anthropogenic sources.

 

Effects on forest

Acid precipitation on vegetation reduces the photosynthesis and growth also increase the susceptibility to draught and disease, process called ‘dieback’ it causes browning of leaf and fall off. It can cause thinning of annual growth ring and reduction in biomass (due to reduced growth), damage the fine root system, affect root mycorrhiza (due to increase in Aluminium and acidity) and decrease the lichens, reduction of soil fertility and causes loss of chlorophyll. Young seed lings are more susceptible than older plants

Effects to manmade structures

Nitric acid, sulphurus and sulphuric acid concentrated in dew or rain deposited on automotive coating causes fading of the paint, thus the modern vehicle manufactures are coating with acid resistant top paint and modern buildings are painted with acid resistant exterior wall paints. Metal such as bronze and alloy structures get corroded, acid also degrade marble (limestone) architectures.

Visibility impairment

Acid fog particularly particles of suphur dioxide and sulphur trioxide reduces the visibility. SO₂ & SO₃ increase the incidence of asthma and emphysema. Particulate deposition of particles less than PM 2.5 can even reach the blood stream via lungs and cause harmful effects such as lung cancer.

To overcome the effects of acid rain

Liming Lime stone is added to neutralize the acid in the water body; it also facilitates the release of locked nutrients of the acidified mud bottoms by neutralizing the ions. Essential nutrients such as phosphorus and other limiting minerals get released and thereby planktons and plant productivity get increased. In addition, it also reduces the toxic effect of heavy metals which are normally high in acidified waters. The calcium in lime supports the mollusks population in developing their calcareous exoskeleton. As calcium and phosphorus are essential plant nutrients, liming enhances the primary production and subsequently the entire community of the water system.

Soil acidity can be overcome by addition of lime, whereas alkalinity of limestone neutralizes the negative ions in acid

 

Reduce acid rain 

This can be done either fuel switching or scrubbing. Fuel switching includes limiting the use of Sulphur containing fuels such as coal or switching to low sulphur containing coal or oil, switching to alternative energy sources such as using gas boilers instead of coal or oil boilers, nuclear power generation, using renewable energy sources such as wind, air, wave and geothermal energy. Use solar batteries, fuel cells, natural gas and electric motor vehicles. Using public transportation, maintaining the vehicle for low NO₂ emission. Use energy efficient boilers and using filters or scrubbers to catch the oxides of sulphur and Nitrogen in industrial effluents and vehicles.

 Scrubbing includes use of electrostatic precipitators where positively charged sulphur particles are get attracted by negatively charged plate or chemicals