Table of Contents
- 1 Acid Rain – Acidification
- 2 Ocean Acidification
- Acid rain refers to any precipitation (rain, fog, mist, snow) that is more acidic than normal (pH of less than 5.6. [pH below 7 = acidic]).
- Acid rain is caused by atmospheric pollution from acidic gases such as sulphur dioxide and oxides of nitrogen emitted from burning of fossil fuels.
- It is also recognized that acidic smog, fog, mist, move out of the atmosphere and settle on dust particles which in turn accumulate on vegetation as acid depositions. When rain falls, the acid from these depositions leak and form acid dews.
- The pH scale is a measure of how acidic or basic (alkaline) a solution is.
- It ranges from 0 to 14. A pH of 7 is neutral.
- A pH less than 7 is acidic, and a pH greater than 7 is basic.
- It is based on hydrogen ion concentration in an aqueous solution.
- pH values decreases as hydrogen ion levels increases.
- A solution with pH 4 is ten times more acidic than solution with pH 5, and a hundred times more acidic than solution with pH 6.
- Whilst the pH range is usually given as 0 to 14, lower and higher values are theoretically possible.
|CO2 (Carbon dioxide)||Fossil fuel burning, industrial process, respiration.|
|CH4 (Methane)||Paddy fields, wetlands, gas drilling, landfills, decomposition of animals wastes and carcasses.|
|CO (Carbon monoxide)||Biomass burning, Industrial sources: smelting of iron ore, Biogenesis, Plant isoprene’s.|
|SO. (Sulphur oxides)||Fossil fuel burning, power plants, smelting of metal sulfide ores, industrial sources, industrial production of sulfuric acid in metallurgical, chemical and fertilizer industries volcanoes, seas and oceans, decomposition of organic matter.|
|NO. (nitrogen oxides)||Fossil fuel burning, lightening, biomass burning, forest fires, oceans, power plants.|
|Formic acid (HCOOH)||
|Carbonic acid (H2CO3)||Carbon monoxide and carbon dioxide dissolve in water (water vapor) to form carbonic acid.|
Q1. Acid rain is caused by the pollution of environment by
- carbon dioxide and nitrogen
- carbon monoxide and carbon dioxide
- ozone and carbon dioxide
- nitrous oxide and sulphur dioxide
Carbon dioxide, carbon monoxide, oxides of nitrogen and oxides of sulphur in atmosphere can cause acid rains.
Both b) and d) are correct. But d) has more influence than b)
Answer: d) nitrous oxide and sulphur dioxide
- “Acid rain” is a broad term referring to a mixture of wet and dry deposition (form of deposition material) from the atmosphere.
- If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to the ground in the form of rain, snow, fog, or mist.
- As this acidic water flows over and through the ground, it affects a variety of plants and animals.
- In areas where the weather is dry, the acid chemicals may become incorporated into dust or smoke and fall to the ground through dry deposition, sticking to the ground, buildings, vegetation, cars, etc.
- Dry deposited gases and particles can be washed from these surfaces by rainstorms, through runoff. This runoff water makes the resulting mixture more acidic.
- About half of the acidity in the atmosphere falls back to earth through dry deposition.
Six basic steps are involved in the formation of acid rain:
- The atmosphere receives oxides of sulfur and nitrogen from natural and man-made sources.
- Some of these oxides fall back directly to the ground as dry deposition, either close to the place of origin or some distance away.
- Sunlight stimulates the formation of photo-oxidants (such as ozone) in the atmosphere.
- These photo-oxidants interact with the oxides of sulfur and nitrogen and other gases (like NH3) to produce H2SO4 (sulphuric acid) and HNO3 (nitric acid) by oxidation.
- Acid rain containing ions of sulfate, nitrate, ammonium and hydrogen falls as wet deposition.
- Acid precipitation affects both aquatic and terrestrial organisms. It also damages buildings and monuments.
Effects on humans
- Acid rain affects human health is a number of ways.
- The obvious ones are bad smells, reduced visibility; irritation of the skin, eyes and the respiratory tract.
- Some direct effects include chronic bronchitis, pulmonary emphysema and cancer.
- Some indirect effects include food poisoning vis a vis drinking water and food.
- An increase in the levels of toxic heavy-metals like manganese, copper, cadmium and aluminium also contribute to the detrimental effects on human health.
Effects on soil
- The exchange between hydrogen ions and the nutrient cations like potassium and magnesium in the soil cause leaching of the nutrients, making the soil infertile. This is accompanied by a decrease in the respiration of soil organisms.
- An increase in ammonia in the soil due to a decrease in other nutrients decrease the rate of decomposition. The nitrate level of the soil is also found to decrease.
- The impact of acid rain on soil is less in India; because Indian soils are mostly alkaline, with good buffering ability.
Effects on aquatic life
- The pH of the medium is very important for metabolic processes of aquatic organisms.
- Eggs or sperms of fish, frogs and other aquatic organisms are sensitive to pH changes.
- Acid rain kills their gametes affecting the life cycles and productivity. Death or their inability to increase in numbers causes severe ecosystem imbalances.
- Acidic lake waters may kill bacteria/microbes/planktons and the acidic lakes become unproductive and life less. Such acidic and lifeless ponds/lakes adversely affect fisheries and livelihood.
Naturally acidic lakes
Anthropogenically acidified lakes
|Brown to yellow colour caused by humic substances||Very clear water caused by reduced primary productivity|
|Concentrations of dissolved organic carbon are high while transparency is low||Dissolved organic carbon concentrations are low. Whereas the transparency is high.|
|Low pH but well buffered.||Poorly buffered|
|Abound with aquatic life.||Some of the more sensitive taxa, such as blue-green algae, some bacteria, snails, mussels crustaceans, mayflies and fish either decrease or /are eliminated.|
Effect on terrestrial life
- Acid rain damage cuticle of plant leaves resulting etiolation (pale and weak) of foliage. This in turn reduces photosynthesis.
- Reduced photosynthesis accompanied by leaf fall reduces plant and crop productivity.
- Acidic medium promotes leaching of heavy metals like aluminum, lead and mercury. Such metals when percolate into ground water affect soil micro flora/fauna.
- Absorption of these toxic metal ions by plants/microorganisms affect their metabolism.
- Acid rain can directly affect the eggs and tadpoles of frogs and salamanders that breed in small forest ponds.
- It has been postulated that acid rain can indirectly affect wildlife by allowing metals bound on soils and sediments to be released into the aquatic environment, where toxic substances may be ingested by animals, like birds, that feed in such an environment.
- Other indirect effects of acid rain on wildlife are loss or alteration of food and habitat resources.
Effects on micro organisms
- pH determines the proliferation of any microbial species in a particular environment and the rate at which it can produce.
- The optimum pH of most bacteria and protozoa is near neutrality; most fungi prefer an acidic environment, most blue-green bacteria prefer an alkaline environment.
- So after a long run of acid rain, microbial species in the soil and water shift from bacteria-bound to fungi-bound and cause an imbalance in the micro flora.
- This causes a delay in the decomposition of soil organic material, and an increase in fungal disease in aquatic life and forests.
Effects on forests
- Acid rains kill vegetation and causes severe damage to the forest landscape.
Effect on buildings, monuments and materials
- Many old, historic, ancient buildings and works of art/textile etc. are adversely affected by acid rain.
- Limestone and marble are destroyed by acid rain. Smoke and soot cover such objects. They slowly dissolve/flake away the surfaces because of acid fumes in the air. Many buildings/monuments such as Taj Mahal in Agra have suffered from acid rain.
Type of Impact
Principal Air Pollutants
|Metals||Corrosion, tarnishing||Sulphur Oxides and other acid gases|
|Paints and organic coatings||Surface erosion, discoloration, soiling||Sulphur dioxides, hydrogen sulphide|
|Textiles||Fading, colour change||Nitrogen oxides, ozone|
|Leather||Weakening, powdered surface||Sulphur oxides|
- A low pH of the rainwater and subsequent increased acidity in the environment can trigger off or aggravate the effects of certain harmful pollutants.
- Methyl mercury and related short chain alkyl mercurial compounds are most dangerous to humans, as they accumulate in edible fish tissue.
- Although acid deposition may not increase the production of methyl mercury, it may increase the partitioning of methyl mercury into the water column.
- The use of lime has helped in reducing the mercury levels in fish.
- Acidified waters are known to leach substantial amounts of aluminium from watersheds.
- Even at relatively low levels, aluminium has been implicated in dialysis dementia, a disorder of the central nervous system, which may be toxic to individuals with impaired kidney function.
- Cadmium can enter the drinking water supply through corrosion of galvanized pipe or from the copper-zinc solder used in the distribution systems.
- A decrease in water pH from 6.5 to 4.5 can result in a fivefold increase in cadmium and could cause renal tubular damage.
- Foetuses and infants are highly susceptible to drinking water lead contamination.
- High blood lead levels in children (>30 mug/ Ml) are believed to induce biochemical and neurophysiological dysfunction.
- However, lower than normal blood levels of lead can cause mental deficiencies and behavioural problems.
- Asbestos in natural rock can be released by acidic waters.
- They are concentrated in the industrialized belt of the northern hemisphere.
- They are often upland and/or mountainous areas, which are well-watered by rain and snow.
- Due to the abundance of water, they possess numerous lakes and streams and also have more land covered with vegetation. Being upland, they often have thin soils and glaciated bedrock.
- Many parts of Scandinavia, Canada, the North and Northeast United States and Northern Europe (particularly West Germany and upland Britain) share these features.
- Across the Atlantic there are number of acid rain hot spots including Nova Scotia, Southern Ontario and Quebec in Canada, the Adirondack Mountains in New York, Great Smoky mountains, parts of Wisconsin, Minnesota, and the Colorado Rockies of the US.
- In India, the first report of acid rain came from Bombay in 1974. Instances of acid rain are being reported from metropolitan cities.
- In India, the annual SO2 emission has almost doubled in the last decade due to increased fossil fuel consumption.
- Lowering of soil pH is reported from north-eastern India, coastal Karnataka and Kerala, parts of Orissa, West Bengal and Bihar.
- Lichens serve as good bio-indicators for air pollution.
- In the variety of pH around 6.0, several animals, those are important food items for fish decline. These include the freshwater shrimp, crayfish, snails and some small mussels.
- Any procedure that shall reduce, minimize, or halt emission of sulphur and nitrogen oxides into the atmosphere shall control acid rain.
- Use of low sulphur fuel or natural gas or washed coal (chemical washing of pulverized coal) in thermal plants can reduce incidences of acid rain.
- Buffering – the practice of adding a neutralizing agent to the acidified water to increase the pH is one of the important control measures. Usually lime in the form of calcium oxide and calcium carbonate is used.
- Ocean acidification has been called the “evil twin of global warming” and “the other CO2 problem”.
- Ocean acidification is the ongoing decrease in the pH of the Earth’s oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere.
- An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes.
- To achieve chemical equilibrium, some of it reacts with the water to form carbonic acid.
- Some of these extra carbonic acid molecules react with a water molecule to give a bicarbonate ion and a hydronium ion, thus increasing ocean acidity (H+ ion concentration).
- Checking CO and CO2 emissions and controlling pollution are the only means to reduce ocean acidification.
- Acid rain can have a pH between 1 and 6 and has impact on surface ocean chemistry. It has major effect on ocean acidification locally and regionally but very small globally.
- Eutrophication leads to large plankton blooms, and when these blooms collapse and sink to the sea bed the subsequent respiration of bacteria decomposing the algae leads to a decrease in sea water oxygen and an increase in CO2 (a decline in pH).
- Oceans are an important reservoir for CO2, absorbing a significant quantity of it (one-third) produced by anthropogenic activities and effectively buffering climate change.
- The uptake of atmospheric carbon dioxide is occurring at a rate exceeding the natural buffering capacity of the oceans creating new problems in the form of ocean acidification.
- Increasing acidity depresses metabolic rates and immune responses in some organisms.
- Other chemical reactions are triggered which result in a net decrease in the amount of carbonate ions available. This makes it more difficult for marine calcifying organisms, such as coral (calcareous corals) and some plankton (calcareous plankton), to form biogenic calcium carbonate.
- Commercial fisheries are threatened because acidification harms calcifying organisms which form the base of the Arctic food webs.
- Acidification could damage the Arctic tourism economy and affect the way of life of indigenous peoples. A major pillar of Arctic tourism is the sport fishing and hunting industry.
- Increasing acidity accentuates coral bleaching as corals are very sensitive to changes in water composition.
- Seawater absorbs CO2 to produce carbonic acid, bicarbonate and carbonate ions.
- The carbonate ions are essential to the calcification process that allows certain marine organisms to build their calcium carbonate shells and skeletons (e.g. hard tropical corals, cold water corals, molluscs, crustaceans, sea urchins, certain types of plankton, lobsters, etc.).
- However, increase in atmospheric CO2 levels lead to decrease in pH level, increase in the concentration of carbonic acid and bicarbonate ions, causing a decrease in the concentration of carbonate ions.
- Thus carbonate ions are less available and calcification is therefore harder to achieve, and may be prevented altogether.
- Deep, cold ocean waters are naturally under saturated with carbonate ions causing the shells of most calcifying organisms to dissolve.
- Surface waters are oversaturated with carbonate ions and do not readily dissolve shells of calcifying organisms.
- The saturation horizon is the level below which calcium carbonate minerals undergo dissolution.
- Those organisms that can survive below the saturation horizon do so due to special mechanisms to protect their calcium carbonate from dissolving.
- As ocean acidification causes this horizon to rise vertically (upwelling) in the water column so more and more calcifying organisms will be exposed to under saturated water and thus vulnerable to dissolution of their shells and skeletons.
Impact of Ocean Acidification on Cloud Formation
- The majority of sulfur in the atmosphere is emitted from the ocean, often in the form of dimethylsulfide (DMS) produced by phytoplankton.
- Some of DMS produced by phytoplankton enters the atmosphere and reacts to make sulphuric acid, which clumps into aerosols, or microscopic airborne particles.
- Aerosols seed the formation of clouds, which help cool the Earth by reflecting sunlight.
- But, in acidified ocean water, phytoplankton produce less DMS. This reduction of sulfur may lead to decreased cloud formation, raising global temperatures.
Artificial Cloud seeding
- Cloud seeding is the process of spreading either dry ice, or more commonly, silver iodide aerosols, into the upper part of clouds to try to stimulate the precipitation process and form rain.
- Since most rainfall starts through the growth of ice crystals from super-cooled cloud droplets in the upper parts of clouds, the silver iodide particles are meant to encourage the growth of new ice particles.
Q2. The acidification of oceans is increasing. Why is this phenomenon a cause of concern?
- The growth and survival of calcareous phytoplankton will be adversely affected.
- The growth and survival of coral reefs will be adversely affected.
- The survival of some animals that have phytoplanktonic larvae will be adversely affected.
- The cloud seeding and formation of clouds will be adversely affected.
Which of statements given above is / are correct?
- 1, 2 and 3 only
- 2 only
- 1 and 3 only
- 1, 2, 3 and 4
We have already learnt that ocean acidification decreases the calcifying ability of corals, calcareous plankton, crustaceans etc. It also adversely affects cloud formation and cloud seeding. So Options 1, 2 and 4 are correct.
Answer: d) 1, 2, 3 and 4