Acid Rain

ü Acid rain caused by emission of SO₂ and NOx from various sources to the atmosphere and they dissolve in atmospheric water and produce acids in the rain water.

ü 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.

ü Sulphur dioxide is naturally produced by volcanic eruptions, sea spray, planktons, rotting vegetation and forest fires.

ü 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).

ü NOx is naturally produced by lightening, bacterial action, forest fire and volcanoes, manmade emission are by automobiles (43%) and fertilizer industries, utility plants and other industrial combustion (32%) (‘Causes and Effects of Acid Rain’)

ü Acid rain increases nitrate levels in soil, leading to nitrogen saturation in soils. Nitrate ions remove additional calcium and magnesium from soil, excess nitrogen subsequently lower the frog population as well.

ü Acid rain releases aluminum from the soil into lakes and streams which is toxic to many aquatic organisms. According to natural surface effects of deposition about 75% of the lakes and about 50% of the streams

ü 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.

Acid deposition

Effects on Forest

ü 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

ü In addition, effects such as thinning of annual growth ring and reduction in biomass (due to reduced growth), it also damage the fine root system, affect root mycorrhiza (due to increase in Al and acidity) and decrease the lichens, reduction of soil fertility as potassium leached out of the soil,

ü Phosphorus is also reduced this reduces the fruit production, toxic metals such as zinc and aluminum accumulates, aluminum toxicity retard root growth and causes loss of chlorophyll.

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

ü 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.

ü Essential nutrients such as phosphorus and other limiting minerals get released and thereby planktons and plant productivity get increased.

ü Agricultural lime (CaCO₃) is recommended type generally in use, Dolomite lime (MgCO₃) is impure substance, Quicklime (CaO) and Hydrated lime (Ca(OH)₂) are caustic, used in acidified lakes in little quantities and if applied in high amount results fish kill and Soda ash (Na₂CO₃) also usable but it is less preferred due to its high cost.

ü In addition factors such as hardness of the water, existing pH, alkalinity, chemistry and acidity of bottom mud, temperature and water quality, density and type of aquatic plants, targeted pH, type, type of limestone (purity and particle size) amount and flushing rate to be considered in liming applications.

ü Treatment can be extended in several additional applications if required, limestone is applied by boat or barge (flushing lime at high pressure) and by snow mobile tractor or spreader on ice covered lakes (lime dissolved on surface ice and subsequently reaches the waters).

Health Effects

ü The causing agents of acid rain SO₂, SO₃ and NOx may affect the health particularly SO₂ & SO₃ effect on asthma and emphysema patients and increase the incidence, 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.

Reduce Acid Rain

ü 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.

ü Use solar batteries, fuel cells, natural gas and electric motor vehicles. EPAs energy star program, reduce carpool by using public transportation, maintain the vehicle for low NOx emission and factory boilers such as clean the stacks and exhaust pipes.

ü Use energy efficient boilers and using filters or scrubbers to catch the oxides of sulphur and Nitrogen in industrial effluents and vehicles, defining the right stack height was 150-300m common in smelters and thermal electric generating plants.

ü Acid rain is one of the world’s major environmental problems since 19th century. Coal burning is the major cause of SO₂ production and also vehicle emission and various fossil fuel based power generation emits NOx. Both SO₂ and NOx produces sulphuric and nitric acid

ü 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.

ü Acid rain also 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.

ü Acid rain deteriorates the marble, stone monuments and architectures, corrode metal structures and fading paints. Liming is used to neutralize the acidity in soil and aquatic bodies.

ü Several methods are used to reduce the emission of SO₂ and NOx such as reducing the sulphur content in fuels, using scrubbers such as flue gas desulphurization (FGS) lime injection multi stage burning (LIMB) or fluidized bed combustion (FBC or circulation dry scrubber).