Saturday, April 10, 2010

Normally while rain travels through the air, it dissolves floating chemicals and washes down particles that are suspended in air. At the start of its journey raindrops are neutral (pH = 7). In clean air, rain picks up materials that occur naturally such as dust, pollen, some CO2 and other chemicals produced by lightening or volcanic activities. These substances make rain slightly acidic (pH = 6), which is not dangerous. However, when rain falls through polluted air, it comes across chemicals such as gaseous oxides of sulphur (SOx), oxides of nitrogen (NOx), mists of acids such as hydrochloric and phosphoric acid, released from automobile exhausts industrial plants, electric power plants etc.

These substances dissolve in falling rain making it more acidic than normal with pH range between 5.6 -3.5. In some case, it's pH gets lowered to the extent of 2. This leads to acid rain. The term acid rain is used here to describe all types of precipitation, namely, rain, snow, fog and dew more acidic than normal.

Chemistry of acid rain

In the natural processes of volcanic eruptions, forest fires and bacterial decomposition of organic oxides of sulphur and nitrogen, production and reductions of gases naturally tend to an equilibrium. Power plants, smelting plants, industrial plants, burning of coal and automobile exhausts, release additional sulphur dioxide, nitrogen oxides and acidic soot, causing pollution. Sulphur dioxide and nitrogen dioxide interact with water vapours in presence of sunlight to form sulphuric acid and nitric acid mist.

formation of sulphurous acid rain

formation of sulphuric acid rain
formation of nitric and nitrous acid rain

The formed sulphuric acid and nitric acid remain as vapour at high temperatures. These begin to condense as the temperature falls and mix with rain or snow, on the way down to the Earth and make rain sufficiently acidic.

Harmful effects of acid rain

SOx, NOx mixed with water as acid rain causes plant, animal and material damage. Some of the significant ill effects of acid rain are:

Damage to animals

Acid rain chemically strips waterways of necessary nutrients and lowers the pH to levels where plants and animals cannot live. Most of the aquatic animals cannot survive when the pH is less than 4. Some species of fish, such as salmon, die even when the pH is less than 5.5. Certain species of algae and zooplankton are eliminated at pH less than 6. A reduction in the zooplankton and bottom fauna ultimately affects the food availability for the fish population. The problem is most severe downwind of industrial areas where fishing and tourism are major sources of income such as in Norway and Sweden.

Damage to plants

Acidic water is dangerous to plants. Sulphuric and nitric acid rain washes nutrients out of the soil, damages the bark and leaves of trees and harms the fine root hairs of many plants which are needed to absorb water. Leaf pigments are decolorized because acid affects green pigment (chlorophyll) of plants. Agricultural productivity is also decreased. Several non-woody plants, such as barley, cotton and fruit trees like apple, pear, etc., are severely affected by acid rain. Since the acid concentration increases near the base of clouds by density, high altitude trees and vegetation may be exposed to pH levels as low as 3. Unique areas such as the Black Forest in Germany and sugar maples in Vermont (USA) are particularly threatened.

Material damage

Metallic surfaces exposed to acid rain are easily corroded. Textile fabrics, paper and leather products lose their material strength or disintegrate by acid rain.

Building materials such as limestone, marble, dolomite, mortar and slate are weakened on reaction with acid rains because of the formation of soluble compounds.

reaction of acid rain on  limestone marble dolomite mortar slate
Thus, acid rain is dangerous for historical monuments.

Acid Rain & Heart Disease

Acid rain is the deposition of acidic components in rain, snow, fog, dew or dry particles that come from sulfur dioxide and nitrogen oxides. These dangerous chemical pollutants can have a severe impact on human health, including pulmonary disorders and heart disease. Much research is being done to combat acid rain for the sake of the environment, aquatic life and human health.

Process of Acid Rain

  • The process of depositing air pollutants on the Earth's surface is known as atmospheric deposition. These pollutants are derived from natural sources, such as forest fires and volcanoes, or from anthropogenic (man-made) sources, such as electric power plants and automobiles. Atmospheric deposition is both wet and dry. Wet deposition is precipitation (for example, rain and snow), and dry deposition is the settling, impaction, or adsorption of particles in dry weather. If wet deposition is acidic, it is known as acid rain.
  • Measuring Acid Rain

  • Because acid rain looks, feels and tastes like clean water, pH measurements are taken to determine its acidity. According to the U.S. Environmental Protection Agency, pure water has a pH of 7.0, and normal rain has a pH of about 5.6. Water is considered acid if the pH is less than 7.0 and alkaline if the pH is greater than 7.0. The lower the pH, the greater is the acidity of the water.
  • Causes of Acid Rain

    Air pollution is the principal cause of acid rain. The burning of fossil fuels, such as coal and oil, from electric power generators forms sulfur dioxide. Automobile fumes cause nitrogen oxides to form. These gases are released into the atmosphere and travel with the wind for hundreds of miles from the originating city to the countryside, harming not only Earth's forests and lakes, but also the health of humanity.

  • Link Between Acid Rain and Heart Disease

    When you breathe the air pollutants (sulfur dioxide and nitrogen oxides) that cause acid rain, symptoms including coughing, shortness of breath, chest tightness, and chest pain can occur. The formulated particles can penetrate deep into the lungs and aggravate heart disease. The high levels of sulfur dioxide and nitrogen oxides associated with acid rain are particularly harmful to senior citizens and people with existing heart disease. As a result of these airborne particles, hospital admissions for heart disease are on the rise as well as higher morbidity rates from this ailment.

    Acid Rain Phenomenon


    Acid rain is a very dangerous weather phenomenon that can harm an area to a great extent. Usually raindrops carry some amount of acid in it. When the amount of acid goes beyond the normal range, then the rainfall is called acid rain. Acid rain is a result of extreme pollution in certain places on the earth. Acid rain can destroy many things of nature such as various living objects as well as non-living objects. However, acid rain sometimes remains unrecognized and the particular place suffers from this dangerous natural outburst for years.

    • The Reason behind Acid Rain:

    Acid rain is a result of pollution that includes acidic gases emerged from the factories and vehicles of a place. The level of sulfur dioxide and nitrogen oxide increases in the atmosphere, thus increasing the acidic level in the raindrops.

    Acids rain makes the entire atmosphere poisoned in a certain place. It raises the acidic level of the soil. Thus the water bodies like lakes, ponds, streams, etc. are affected. The chemical balance of these water bodies gets disturbed and water becomes poisonous for living beings. Places like Europe, North America, Canada, etc. are mostly affected by the acid rain.

    From the above discussion it is clear that the level of sulfur dioxide and nitrogen oxide must be decreased in the atmosphere to stop the menacing effects of acid rain. For this people have to be careful while using their vehicles. Fossil fuels should not be used in cars. In factories also some pollution control equipments must be used to reduce the emission of poisonous gases.

    What Are the Best Acid Rain Solutions?

    Acid rain solutions focus on both prevention, and on helping to deal with the environmental ramifications once the acid rain does take place. So little can be done after the fact to improve the environment that the best acid rain solutions often focus on making sure it never happens in the first place. To do this, it is first necessary to identify the causes of acid rain together with processes that can be changed to aid in prevention.

    Acid rain is caused by sulfur emissions, which are put into the air through the burning of coal, often used for energy. Other sources of sulfur in the atmosphere include volcanoes and decomposing vegetation. Despite the fact these natural sources do exist, it is generally agreed that anthropogenic causes are the main sources of acid rain. This may make acid rain solutions easier to identify simply because if humans have caused it, then logically they should have the ability to change it.

    The best acid rain solutions focus on power plants and coal used in the production of energy. Technologies exist that can remove sulfur from the emissions coming out of coal plant. Low-sulfur varieties of coal can also be used to reduce sulfur emissions. Often, a combination of these acid rain solutions provides the best way to minimize the problem.

    The installation of scrubbers can help prevent as much as 95 percent of sulfur emitted by a carbon-fired electric power plant. The scrubbers work by spraying water with lime through the gas emission produced from burning the coal. The lime interacts with the sulfur dioxide, creating a sludge that is then captured and stored. The problem with this method is that retrofitting existing power plants with the technology is usually very expensive. It is cheaper if the technology is installed as the power plant is being built, and the inclusion of the technology is now required in many countries when new power plants are erected.

    What Are the Different Acid Rain Effects?

    Acid rain is a term that encompasses several ways in which acids fall from the sky and cause environmental damage. These acids come from air pollution, primarily sulfur dioxide and nitrogen oxide gases. They can fall as acid precipitation, or fall back directly as acidic particles and gases. Acid rain damage can range from polluted waterways to corroded statues and buildings to direct effects on human health.

    The worst acid rain effects on ecology are seen in waterways, such as lakes, streams, and marshes. The most sensitive areas are those that are located in watersheds in which the soil is not very effective at neutralizing acidic compounds. When this happens, the water becomes more acidic. This means it has a lower pH. In addition, aluminum is released into the water from the soil, and is highly toxic to many forms of aquatic life.

    Some plants and animals can tolerate acidic waters, but others will die as the pH decreases. While some lakes are naturally acidic, the pH of most streams and lakes is between six and eight. When the pH goes down to five, most fish eggs will not hatch. Some adult fish will die at lower pH levels, causing some acid lakes to be entirely devoid of fish. Even if the fish survive, they may be physically stressed and unable to effectively compete for habitat and food.

    More recent research has identified acid rain effects in the shallow waters of the coastal ocean. The ocean overall is not greatly affected, but the affects of acid rain are magnified in waters near the coast. They result in lower pH and a reduction in carbon storage.

    The lowered amount of carbon means that organisms such as corals, sea urchins, and some types of plankton lose the ability to make their hard outer shells. These types of organisms are necessary to provide food and living conditions for other ocean creatures. Their death could have serious effects on ocean ecosystems. For instance, corals form reefs that provide habitat for a substantial number of marine organisms.

    The deposition of nitrogen from the atmosphere has effects on both freshwater and ocean ecology. It can cause massive overgrowth of algae. Some of these can be toxic, and directly affect humans by contaminating shellfish. A common effect of algal growth is to use up all of the oxygen in the water. This can cause the formation of dead zones.

    Forests are another ecosystem that manifest acid rain effects. This is a combination of the direct effect on the leaves and needles of the trees, and changes in soil chemistry and microbiology. This can happen especially in high mountain regions, where trees are surrounded by fog and clouds that have more acid than the local rainfall. This can cause the loss of essential nutrients in the leaves.

    In addition, acid rain causes nutrients in the soil to be washed away, so they are unavailable for plants. The subsequent release of aluminum is toxic to trees and plants. A lowered pH can also kill beneficial soil microorganisms.

    It is thought that acid rain alone does not cause the death of trees in forests. Scientists think it predisposes them to other stresses, such as insect damage, drought, disease, or cold weather. Acting in concert with these other factors, some forests in areas with heavy amounts of acid rain have died completely.

    Acid rain effects on human materials, such as statues, have been substantial. In addition to the damage caused by acid rain, the dry deposition of acidic particles has been a significant factor in the degradation of these items. Buildings and statues that are particularly vulnerable are those made of limestone and marble. In many areas, structures have been significantly corroded, and gravestone markings have flaked away. Metal items, such as bronze and copper, can also be corroded by acid rain.

    Human health can be directly impacted by acidic particles. Particulate matter can lodge in the lungs. The exposure to particulate matter in the air has been correlated with an increased mortality from heart and lung diseases. Also, such compounds increase the tendency towards bronchitis and asthma in exposed individuals.

    Sources of Acid Rain

    Natural Acid Rain

    The phenomenon of acid rain was discovered in Great Britain in the late 1800's, but was then essentially forgotten until the 1960's. It refers to precipitation that is significantly more acidic than natural, unpolluted rain which itself is mildly acidic due to the presence of carbon dioxide dissolved in atmospheric moisture forming carbonic acid:

    CO2(gas) + H2O(liquid) H2CO3(aq)

    The H2CO3(aq) is a weak acid, meaning it only partially ionizes to release H+(aq), with a resultant reduction in the pH of the system. pH is the - log [H+(aq)] therefore as the concentration of the H+(aq) increases the ph decreases. This partial ionization occurs in equilibrium as carbonic acid only ionizes 1.7%:

    H2CO3(aq)Equilibrium H+(aq) + HCO3-(aq)

    Due to this source of acidity, the pH of unpolluted, natural rain is about 5.6. Only rain that is appreciably more acidic than this, with a pH of less than 5.0 is considered to be truly acid rain.

    Acid Rain

    The two predominant acids in acid rain are sulfuric acid (H2SO4) and nitric acid (HNO3). These acids are created during the transprt of the air mass that contains the primary pollutants. Generally speaking, acid rain is precipitated far downwind from the source of the primary pollutants, namely gaseous sulfur dioxide (SO2) and nitrogen oxides such as NO2. Thus acid rain is a pollution problem that does respect provincial or national borders.

    On a global scale, most SO2 is produced by volcanoes and by the oxidation of sulfar gases produced by the decomposititon of plants. Because this natural sulfur dioxide is emitted high into the atmosphere or far from populated centers, the concentration of the gas in unpolluted air is quite small. However, considerable amounts of SO2 is presently being emitted into ground - level air, particulary over land masses in the Northern Hemisphere. The main human source of SO2 is the combustion, especially in electric power plants, of mined coal that tends to contain 1 to 5% sulfur. About half of the sulfur is trapped as inclusions in the mineral content of coal. If the coal is pulverized before combustion, this type of sulfur can be removed mechanically. The other half of the sulfur however, is bonded in the complex carbon structure and cannot be removed without expensive processing.

    Both nitrogen and sulfur oxides are known to react with water in the atmosphere to form acids. These reactions are the source of polluted acid rain. The formation of acid solutions by SO2 is explained as a two - step process. assume that sulfur dioxide molecules first react with water molecules, forming molecules of sulfurous acid:

    SO2(gas) + H2O(liquid) H2SO3(aq)

    Sulfurous acid molecules then react with water producing an equilibrium with H+(aq) and hydrogen sulfite. Sulfurous acid is considered a weak acid as it only partially ionizes into H+(aq):

    H2SO3(aq) + H2O(liquid) H+(aq) + HSO3-(aq)

    Sulfur dioxide can also react with oygen or ozone to form sulfur trioxide:

    2 SO2(gas) + O2(gas)2SO3(gas)

    SO2(gas) + O3(gas) SO3(gas) + O2(gas)
    The sulfur trioxide then reacts with atmospheric moisture to form sulfuric acid:
    SO3(gas) + H2O(liquid) H2SO4(aq)

    This sulfuric acid is a strong acid that ionizes 100% in atmospheric precipitaion to produce H+(aq) ions:

    H2SO4(aq)H+(aq) + HSO4-(aq)
    These H+(aq) are responsible for the acidic effects of acid rain.

    Car engines burn gasoline using air air as a source of oxygen. The primary component of air is nitrogen and its exposure to the high temperatures inside car engines forms the pollutant nitrogen monoxide (NO). Sunlight provides the energy that promotes a series of secondary reactions producing nitrogen dioxide and ground level ozone that both participate in acid rain reactions.

    Direct scavening of NO2 bt atmospheric water contributes little nitric acid (HNO3) on account of the low solubility of NO2 in water:

    2NO2(gas) + 2H2O(liquid) Equilibrium HNO2(aq) + HNO3(aq)

    Acid rain containing considerable amounts of nitric acid likely originated without water:

    NO2(gas) + O3(gas) NO3(gas) + O(2(gas)

    The gaseous NO3 then reacts with any reactive hydrogen donor ( X ) in the atmosphere:

    NO3(gas) + XH(gas) HNO3(aq)

    Nitric acid like sulfuric acid is strong an completely ionizes into aqueous hydrogen and nitrate ions:

    HNO3(aq) H+(aq) + NO3-(aq)

    Technologies are now developing for the recovery of oxides from exuast gases. For example, some industries have added sulfur oxide recovery units to smoke stacks at large smelters. In addition to reducing sulfur dioxide emissions by over 50%, the sulfuric acid produced is sold as a valuable bi-product.

    Common chemical formula Acid rain

    Raining vinegar

    In some places in world the rain water has become so polluted with chemicals that it is like vinegar. This type of rain is called acid rain.

    Pure water is neutral and has a pH of 7.

    Natural rain water is slightly acidic mainly because of dissolved CO2 which produces carbonic acid or H2CO3

    H2O(l) + CO2(g) <==> H2CO3(aq)

    The pH of unpolluted rainwater ranges from pH 5 to 6.

    Acid rain is rain water with a pH of less than 5.

    In some parts of the Northern Hemisphere the pH of the rain water has been as low as 2!

    Acid rain is caused by caused by industrial pollutants.

    The main industrial gases responsible are SO2 and NOx (a mixture of NO and NO2).

    Major sources of industrial sulfur dioxide.

    SO2(g) comes from mining smelters and the burning of coal.

    i) The roasting of minerals releases SO2(g) from
    Metal sulfide + oxygen ==> Metal oxide + SO2(g)

    ii) Electrical power stations that burn coal produce sulfur dioxide from the sulfur impurities in the coal.

    S(s) + O2(g) ==> SO2(g)

    The SO2(g) combines with water to produce sulfurous acid.

    H2O(l) + SO2(g) ==> H2SO3(g)

    Note: Sulfur dioxide is not readily oxidized to sulfur trioxide in dry clean air. Water droplets and dust particles however, catalyse the reaction between O2 and SO2 in the air producing sulfur trioixde, SO3.This dissolves in water and produces sulfuric acid which is a much stronger acid. This can cause considerable damage to buildings, vegetation and fish populations by destroying fish eggs.

    SO2(g) + ½O2(g) ==> SO3(g)
    H2O(l) + SO3(g) ==> H2SO4(aq)

    Source of nitrogen oxides

    Sources of NOx are more widespread. Nitrogen is a diatomic molecule and is fairly inert because its triple bond. However, at temperatures over 1300°C, nitrogen combines with oxygen to form nitrogen monoxide.

    N2(g) + O2(g) 2NO(g)

    These high temperatures can be achieved by
    i) the internal combustion engine (human activity)
    ii) lightning in the atmosphere (natural source)

    The nitrogen monoxide slowly combines with oxygen to form soluble nitrogen dioxide gas.

    2NO(g) + O2(g) => 2NO2(g)

    Nitrogen dioxide readily dissolves in water producing a mixture of nitric and nitrous acids.

    2NO2(g) + H2O(l) ==> HNO3(aq) + HNO2(g)

    Acidic rain is mainly caused by atmospheric pollutants of sulfur dioxide and nitrogen oxides.

    The chemical formula of acidic rain is dependent upon the type of acids present. Acidic rain is a complex mixture of nitrous, nitric, sulfurous and sulfuric acids which all combine to lower the pH.