Acid rain - chemistry, causes and effects

Acid rain is the term used to describe polluted rainfall (acid pollution). Acid rain is directly linked air pollution. Acid rain is becoming a global environmental problem. Acid rain is a transboundary pollution problem.The main acid-forming pollutants are sulphur dioxide (SO2) and oxides of nitrogen (NOx).

Origin of the term:

‘acid rain’ - In 1872, Robert Angus Smith coined the term ‘acid rain’ in his book ‘Air and Rain: The beginning of a Chemical        Climatology’. Smith first found acid rain in heavily industrialized Manchester, England in 1852. He proved that acid rain has connections to pollution in atmosphere.

Driving factors of acid rain

The main driving factors are industrialization and urbanization. The rise in world’s energy demands leads to large scale burning of coal and fossil fuels. This results in more sulphur dioxide and nitrogen oxide emissions. These acidic pollutants contribute acidic atmospheric pollution.

Forms of acid rain

Acid deposition is of two types i.e. wet and dry depositions. Wet deposition includes acid rain, acid snow, acid fog, acid frost, acid dew and acid hail whereas dry deposition consists of acid gases and acid dust or particles.

Chemistry of acid rain

Acid rain has a pH below 5.6 due mainly to the reaction of water vapor with sulfur dioxide and the oxides of nitrogen. Acid rain often  has a pH as low as 2.4

2SO₂ + O₂ + 2H₂O ®  2H₂SO₄

4NO₂ + O₂ + 2H₂O  ® HNO₃

Pure water has a pH of 7. The rain from an unpolluted atmosphere has a pH close to 6.0 (slightly acidic).

Composition of acid rain

Acid rain is a cocktail mixture of 65% sulphuric acid, 30% nitric acid and 5% hydrochloric acid. Nearly 95% of the acidity comes from atmospheric sulfur dioxide and nitrogen oxides. The contribution of sulfur gases to acid rain is 60-70%.

Sources of acid gases

Natural sources include forest fires, volcanoes, lightening, organic decay from wet lands, and bacterial action in soil. Anthropogenic sources consist of power generation, factories, smelting of metals and combustion of oil, coal and gas.

Types of acid deposition

Sulfuric acid deposition

•         About 75% of acidity is caused by sulfuric acid in rain water.

•         combustion of fossil fuels, volcanic eruptions.

Nitric acid deposition

•         About 25% of the acidity is caused by oxides of nitrogen.

•         Very reactive in the atmosphere.

Episodic acidification-’Acid shocks’

•         Short intense acidic events

•         Winter snowmelt, heavy rains

•         Suddenly changes the pH levels of rivers and lakes.

Chronic acidification

•         Low, stable acidification

•         Loss of acid neutralizing capacity of streams, lakes and soil ecosystems.

Areas affected by acid rain include north eastern United States, south eastern Canada, parts of Europe-parts of Sweden, Norway, Germany, parts of south Asia, South Africa, Sri Lanka and southern India.

Damage of world monuments 

e.g. Taj Mahal in India, temples, murals and ancient inscriptions in Southern India, Acropolis, Parthenon in Greece, Renaissance buildings in Italy, West minister Abbey in England, Mayan pyramids in Mexico, several churches and cathedrals.

Global Acid deposition - from trans-boundary air pollution

Acid rain does not respect political boundaries. Emissions from one country may be carried by wind to a neighbouring country.

Acid rain in India

Sources-Coal consumption, road traffic,fertilizer plants, refineries, thermal power plants, petro-chemical industries and metal smelting industries.

pH of acid rain in Bombay and Trombay=4.45; pH of acid rain in Kolkata = 5.85;pH of acid rain in Chennai= 8.85; pH of acid rain in Delhi = 6.21.

Effects on aquatic ecosystems – acid rain affect the soil buffering capacity of watersheds, restrict flora of aquatic macrophytes and phytoplankton, reduce fish diversity and abundance, cause biotic depletion, implicate metal pollution, limit aquatic productivity and retard recycling of cations from sediments.

Effects on terrestrial ecosystems

1. Decrease the acid neutralizing capacity (ANC) of soils 
2. Leach macro- and micro-nutrients from the soil. 
3. Mobilize aluminium leaching into the soil water. 
4. Increase the accumulation of sulphur and nitrogen in soil. 
5. Harms the microbes of the soil. 
6. Reduces fertility of the soil.

Effects on trees and forest ecosystems

• Directly harm leaf tissues: cuticle, waxy layer and needles. 
• Damages roots and bark. 
• Reduce forest canopies at high elevations. 
• Poison tree with toxic substances and make trees vulnerable to diseases. 
• Reduce tree growth and increase tree mortality.

Effects on plants

·         Cause gradual yellowing (chlorosis) of leaves.

·         Burn leaf tissues and cause depigmentation.

·         Disrupt the process of photosynthesis.

·         Affect the water retaining ability of plants.

·         Leach nutrients from plant tissues.

·         Reduce plant germination and reproduction.

Effects on fish communities

·         Cause extensive gill damage.

·         Increase fish morbidity(sickness) and mortality(death).

·         Reduce age distribution and size.

·         Reduce growth rate and condition factor.

·         Induce skeletal deformities in young ones.

·         Enhance uptake of heavy metals.

·         Cause reproductive failure.

Effects on human health -

Direct effects

·         Increased morbidity (sickness).

·         Asthma, bronchitis

·         Dry coughs

·         Headaches

·         Eye, nose and throat irritations.

·         Mortality from lung diseases.

         
Indirect effects

·         Pain

·         Discomfort

·         Aggravate heart disease

Materials damage

·         Cause physical damage by corrosion of  limestone, marble, carbon-steel, zinc, nickel, paint and some plastics.

·         Reduce life usefulness and aesthetic appeal of structures.

·         Acid smoke solids soiling building materials.

·         Accelerate chemical degradation of metals, paints stone cement etc.

Other negative impacts 

·         Sulfate aerosols diminish visibility and augment  global warming.

·         Nitrogen oxides contribute to the formation of ozone.

·         Mercury contamination in fish.

·         Over-fertilization of coastal waters 9 coastal eutrophication).

·         Decrease bacterial flora and other beneficial micro-organisms.

·         Offset mineral balance in forests, rivers, fields and lakes.

Beneficial effects of acid rain

• Stimulates growth of fine range grasses in iron deficient calcareous soils 
• Improve water penetration in sodic soils.
• Increase micro-nutrient availability in calcareous soils.

"Acid rain is a short-hand term that covers a set of highly complex

  and controversial environmental problems." -Basil John Mason.