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Important Topics related to Acid Rain
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Types of Acid Deposition
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The term “acid rain” broadly refers to a mix of wet and dry forms of acidic deposits falling from the atmosphere.
A. Wet Deposition:
- When acidic compounds present in the air are transported into areas experiencing wet weather, they can return to the ground through precipitation—such as rain, snow, fog, or mist.
- As this acidic water moves over and through the ground, it impacts a variety of plants and animals.
- The effects’ intensity depends on factors such as water acidity, soil chemistry, buffering capacity, and the reliance of living organisms on the water.
- Precipitation removes atmospheric gases and particles through rain-out (particles incorporated into cloud drops) and washout (materials swept down by rain or snow).
B. Dry Deposition
- In dry-weather areas, acid chemicals may mix with dust or smoke and fall to the ground via dry deposition, adhering to surfaces like the ground, buildings, vegetation, and vehicles.
- Rainstorms can wash dry-deposited gases and particles from surfaces through runoff, making the mixture more acidic.
- Approximately half of atmospheric acidity returns to Earth through dry deposition.
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The pH Scale
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- The pH scale gauges a solution’s acidity or basicity (alkalinity), ranging from 0 to 14.
- A pH of 7 is neutral, below 7 is acidic, and above 7 is basic.
- Devised in 1909, it’s a logarithmic index for hydrogen ion concentration.
- pH values decrease as hydrogen ion levels increase.
- A solution with pH 4 is ten times more acidic than pH 5, and a hundred times more acidic than pH 6.
- Although the pH range is typically stated as 0 to 14, theoretically, lower and higher values are possible.
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Sources of Compounds Causing Acid Rain
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A. Sulphur
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(i) Natural Sources:
- Seas and oceans.
- Volcanic eruptions.
- Biological processes in the soil, such as the decomposition of organic matter.
(ii) Man-made Sources:
- Burning of coal (contributing to 60% of SO) and petroleum products (30% of SO).
- Smelting of metal sulfide ores for obtaining pure metals.
- Industrial production of sulfuric acid in metallurgical, chemical, and fertilizer industries.
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B. Nitrogen Natural Sources:
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- Lightning.
- Volcanic eruptions.
- Biological activity.
Anthropogenic Sources:
- Forest fires.
- Combustion of oil, coal, and gas.
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C. Formic Acid
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- Biomass burning during forest fires releases formic acid (HCOOH) and formaldehyde (HCHO) into the atmosphere.
- A significant portion of formaldehyde undergoes photo-oxidation, forming formic acid.
Other Acids
- Chlorine.
- Phosphoric acid.
- Hydrochloric acid (from smokestacks).
- Carbon monoxide and carbon dioxide from automobiles can become carbonic acid.
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Common Characteristics of Areas Prone to Acid Rain
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- Concentrated in the industrialized belt of the northern hemisphere.
- Often upland and/or mountainous, well-watered by rain and snow.
- Abundance of water, with numerous lakes, streams, and extensive vegetation-covered land.
- Upland areas often have thin soils and glaciated bedrock.
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World Scenario
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- Regions in Scandinavia, Canada, the North and Northeast United States, and Northern Europe, including parts of West Germany and upland Britain, share these characteristics.
- Acid rain hot spots across the Atlantic include Nova Scotia, Southern Ontario, Quebec in Canada, the Adirondack Mountains in New York, the Great Smoky Mountains, parts of Wisconsin, Minnesota, and the Colorado Rockies in the US.
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In India
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- Bombay reported the first instances of acid rain in 1974.
- Acid rain cases are now reported in metropolitan cities.
- Annual SO2 emissions in India have nearly doubled in the last decade due to increased fossil fuel consumption.
- Lowering of soil pH is noted in northeastern India, coastal Karnataka and Kerala, parts of Orissa, West Bengal, and Bihar.
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Chemistry of Acid Rain
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Six fundamental steps contribute to the formation of acid rain:
- The atmosphere receives sulfur and nitrogen oxides from both natural and man-made sources.
- Some of these oxides precipitate directly to the ground as dry deposition, either near the source or at a distance.
- Sunlight induces the formation of photooxidants, including ozone, in the atmosphere.
- These photo-oxidants react with sulfur and nitrogen oxides, resulting in the production of H2SO4 and HNO3 through oxidation.
- The sulfur and nitrogen oxides, photo-oxidants, and other gases (such as NH3) are involved in the process.
- Acid rain, containing ions of sulfate, nitrate, ammonium, and hydrogen, falls as wet deposition.
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Impact of Acid Rain
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A. Soil
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- Exchange between hydrogen ions and nutrient cations like potassium and magnesium causes the leaching of nutrients, leading to soil infertility.
- Decreased respiration of soil organisms is observed.
- An increase in ammonia due to decreased nutrients reduces the rate of decomposition.
- The nitrate level in the soil is found to decrease.
- Acid rain impact on soil is less in India due to predominantly alkaline soils with good buffering capacity.
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B. Vegetation
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- Acid rain affects trees and undergrowth, causing reduced or abnormal growth.
- Symptoms include discoloration and loss of foliar biomass, loss of feeder-root biomass (especially in conifers), premature aging of older needles in conifers, increased susceptibility to damage, death of herbaceous vegetation under affected trees, and prolific lichen production on affected trees.
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C. Microorganisms
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- pH influences the proliferation of microbial species, leading to a shift from bacteria-bound to fungi-bound microflora.
- This shift causes a delay in the decomposition of soil organic material and an increase in fungal diseases in aquatic life and forests.
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D. Wildlife
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- Effects on wildlife are not always evident but can impact productivity and survival.
- Direct effects include damage to eggs and tadpoles of breeding frogs and salamanders.
- Indirect effects involve the release of metals from soils into the aquatic environment, where they may be ingested by animals, leading to potential toxic effects.
- Other indirect effects include loss or alteration of food and habitat resources.
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E. Humans
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- Acid rain affects human health in various ways.
- Obvious effects include bad smells, reduced visibility, and irritation of the skin, eyes, and respiratory tract.
- Direct effects encompass chronic bronchitis, pulmonary emphysema, and cancer.
- Indirect effects involve food poisoning through contaminated drinking water and food.
- Increased levels of toxic heavy metals like manganese, copper, cadmium, and aluminum contribute to adverse effects on human health.
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F. Acid rain damage on Materials
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- Acid rain and the dry deposition of acidic particles contribute to the corrosion of metals (such as bronze) and the deterioration of paint and stone (such as marble and limestone).
- These effects seriously reduce the value to society of buildings, bridges, cultural objects (such as statues, monuments, and tombstones), and cars.
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G. Socio-economic Impacts of Acid Rain:
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- The detrimental effects of acid rain on agriculture and fishing contribute to the degradation of key life quality indicators, including Gross National Product (GNP) and per capita income. This impact is particularly pronounced in predominantly agricultural and developing nations such as India.
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Trigger Effect of Acid Rain on Pollutants
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- The Ministry of Environment, Forest and Climate Change (MoEFCC) has devised criteria for categorizing industrial sectors into Red, Orange, Green, and White categories.
- The categorization is based on the Pollution Index, a numerical value (0 to 100) derived from factors such as emissions (air pollutants), effluents (water pollutants), hazardous waste generation, and resource consumption.
- The Pollution Index (PI) signifies the degree of pollution load, with a higher PI indicating increased pollution from the industrial sector.
- The re-categorization process is a scientific exercise aimed at addressing challenges in the old categorization system, which did not accurately reflect industries’ pollution levels.
- The newly introduced White category, representing practically non-polluting industries, does not require Environmental Clearance (EC) and Consent. This facilitates obtaining financing from lending institutions.
- No industries in the Red category, indicating higher pollution levels, will typically be permitted in ecologically fragile or protected areas
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