Q. Write short notes on: “Acid Rain”. (39 BPSC/1993)

Q. Write short notes on: “Acid Rain”. (39 BPSC/1993)

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure.

Acid deposition:

• Wet deposition: Wet deposition of acids occurs when any form of precipitation (rain, snow, and so on.) removes acids from the atmosphere and delivers it to the Earth’s surface.
• Dry deposition: Acid deposition also occurs via dry deposition in the absence of precipitation. This occurs when particles and gases stick to the ground, plants or other surfaces.

Cause:

• Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids.
• Natural phenomena:
  • The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes.
  • Acid-producing gasses are also created by biological processes that occur on the land, in wetlands, and in the oceans. The major biological source of sulfur compounds is dimethyl sulfide.
  • Nitric acid in rainwater is an important source of fixed nitrogen for plant life, and is also produced by electrical activity in the atmosphere such as lightning.
• Human activity:
  • The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, animal agriculture, factories, and motor vehicles.
  • Electrical power generation using coal is among the greatest contributors to gaseous pollution responsible for acidic rain.
• Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:
  • SO2 (g) + H2O ⇌ SO2·H2OSO2·H2O ⇌ H+ + HSO3−HSO3− ⇌ H+ + SO32−

Impact:

• Acid rain has been shown to have adverse impacts on forests, freshwaters, and soils, killing insect and aquatic life-forms, causing paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues as well as having impacts on human health.
• Surface waters and aquatic animals:
  • Both the lower pH and higher aluminium concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals.
  • At pH lower than 5 most fish eggs will not hatch and lower pH can kill adult fish.
  • As lakes and rivers become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species.
• Soils:
  • Some microbes are unable to tolerate changes to low pH and are killed.
  • The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid.
• Soil acidification:
  • Acidic water enters the plant and causes important plant minerals to dissolve and get carried away; which ultimately causes the plant to die of lack of minerals for nutrition.
  • acid rain that falls on soil and on plant leaves causes drying of the waxy leaf cuticle; which ultimately causes rapid water loss from the plant to the outside atmosphere and results in death of the plant.
  • a plant suffering from soil acidification cannot photosynthesize.
• Ocean acidification:
  • Acid rain can cause the ocean’s pH to fall, known as ocean acidification, making it more difficult for different coastal species to create their exoskeletons that they need to survive.
  • Coral’s limestone skeleton is sensitive to pH drop, because the calcium carbonate, core component of the limestone dissolves in acidic (low pH) solutions.
  • In addition to acidification, excess nitrogen inputs from the atmosphere promote increased growth of phytoplankton and other marine plants which, in turn, may cause more frequent harmful algal blooms and eutrophication.
• Human health effects:
  • Although, the acid in the rainwater is too dilute to have direct adverse effects. The particulates responsible for acid rain (sulfur dioxide and nitrogen oxides) do have an adverse effect. Increased amounts of fine particulate matter in the air contribute to heart and lung problems including asthma and bronchitis.
• * Acid rain can damage buildings, historic monuments, and statues, especially those made of rocks, such as limestone and marble, that contain large amounts of calcium carbonate.
  • e.g. Acid rain corrodes the marble (calcium carbonate) of Taj Mahal.

Prevention:

• Many coal-firing power stations use flue-gas desulfurization (e.g. wet scrubber) to remove sulfur-containing gases from their stack gases. For a typical coal-fired power station, FGD will remove 95% or more of the SO2 in the flue gases.
• Vehicle emissions control reduces emissions of nitrogen oxides from motor vehicles.
  • International treaties on the long-range transport of atmospheric pollutants have been agreed for example, the 1985 Helsinki Protocol on the Reduction of Sulphur Emissions under the Convention on Long-Range Transboundary Air Pollution (total 25 parties, India is not a party to this).
• Emissions trading: Every current polluting facility is given or may purchase on an open market an emissions allowance for each unit of a designated pollutant it emits.

To conclude, we can say that to tackle this issue, both regulatory and market based approaches for controlling air pollution.