Acid Rain:Definition and Causes

Subject: Environment
Pages: 9
Words: 2409
Reading time:
9 min
Study level: College

Acid rain is precipitation with a pH below 5.6, attributed largely to impurities dissolved from emitted automobile exhausts as well as pollutants from combust hydrocarbons. Acid rain includes precipitation of acid fog, acid sleet and acid snow. The impacts of acid precipitation are acidic threats of rains; however, this may occur in sunny days. The problem of acid rain probably started in northeastern United States in between the years of 1950 and 1955.

Sulfur based acid depositions are higher than nitrogen ones especially in Europe and North America [1]. United States (US) and Canada have had a strained relation due to trans-boundary implications of industrial pollution. Canada has experienced negative impacts of acid rain as result industrial emissions in the US. RAINS (Regional Acidification and Information System) model was as early as 1960s to monitor emission of acid deposition in the atmosphere in Europe [2*]

Normal rain pH is about 5.6, which result from mixing of water vapor in the air with carbon to form weak carbonic acid [3]. Sulfur dioxide and nitrogen oxides form a significant portion of atmospheric pollutants [4]. As they accumulate in the air and interact with water vapor, sulfuric and nitric acids form. The dissolving affinities of sulfur dioxide and nitrogen oxides are high and at times are swept off to remote places by prevailing wind.

Hence, acid rain phenomena can have trans-boundary implications. Acid rains are associated with anthropogenic activities. Industrial operations with heavy combustion of sulfur coal have been associated with massive emission of the sulfur dioxide pollutant into the atmosphere. Automobile exhausts from fossil fuel combustion are among the lead causes of nitrogen oxide accumulation in the atmosphere. In China, coal combustion is primary cause of acid rain [5]. There are natural activities that release nitrogen oxide and sulfur dioxide in to the atmosphere. These natural activities include volcanic events, decomposing vegetation matter, fires and lightening.

The resulting acids condense and precipitate as rain. This rain has harmful impacts on natural ecosystems, humans and manmade forms. The massive use of fossil fuels in North America and Europe is globally a primary contributor to the atmospheric emission leading to the accumulation of Sulfur dioxide and nitrogen oxide [6]. These emit up to 70% of the global release of the gases [6]. In the United States and Europe, probes on acid deposition has mainly centered on Sulfur dioxide and less of nitrogen oxide [7].

Acid deposition is a synonymous term for acid rain. The term is widely applied to distinguish between the wet and dry acid depositions. Wet and dry deposition present the two forms with which the acid pollutants leave the atmosphere through precipitation to the earth surface [8]. Wet deposition refers to acid in the acidic rainwater, fog, dew, sleet, snow and hail while dry deposition refers to acidic gases and particles [8]. China acid deposition is subject to dry than wet deposition [5].

Towards the end of twentieth century, there was increased interest to understand the cause and effect associated with acid deposition [7]. Research work focused on damages caused on forests, water bodies and consequent threat on fisheries stocks. Among the earliest published works were on southern Norway. Trends have shown an upward of anthropogenic release of acid precursors since in the 1970s [7]. These were the industrial revolution times. Acid rains lead ultimately to acidification of surface waters.

This kind acidification of surface waters may be gradual ranging to decades before negative implications of this are noted [6]. The contamination of surface water has been associated with significant extermination of fish population in Scandinavian countries, the US, United Kingdom as well as Canada [5]. Fish and other aquatic species have narrow tolerance to highly acidic water. Accumulated acidification in the terrestrial ecosystems may lead to delayed aquatic acidification [6].

The difference in intensity of acidification of the water bodies is depended on their Sulfate Adsorption Capacities [6]. The scales for gauging the difference in intensity of acidification of water bodies are on spatial and temporal bases. Methyl-mercury impairs the human nervous system; its concentration levels correlate with acidification of water bodies [9]. Fish in the water bodies assimilate the methyl-mercury [9].

There is global evidence of forest damage caused by acid deposition. Acid deposition has adverse impacts on the plant kingdom [10]. There have been direct implications of sulfur dioxide, acidic mist or rain on forest areas in China [5]. Sulfur dioxide mainly combined with frost has led to forest damage in the Central Europe. An example is the forest dieback in the mid-1980s [5]. A survey done in 1983 in West Germany estimated almost a third of forests as damaged by the acid rain phenomenon.

In this part of Germany, acid depositions were by ozone fusion with acid mist than soil-mediated effects. This acid deposition affected the photosynthetic apparatus of the needles of the conifers trees in affected areas. Forest soil accumulation of acid depositions has led to delayed acidification of freshwaters. Acid rain falling on leaves or needles of trees causes a leaching effect.

In China’s Liu Chong Guan, the tree species Pinus massonica significantly lost their needle up to 50% due to acid deposition [5]. In addition, this resulted to high tree cover loss. This Liu Chong Guan forest damage happened in year 2000. Sulfur based acid deposition is considered as principal cause of leaching of hydrogen ions and other cation elements on land. Forest soils that are sensitive to acid rain are high in humic acid [11]. Hence, pH has regulated the solubility capacities of these soils. This explains the reason why cation leaching is not excessive in very acidic soils at rates similar to those of deposition.

Watersheds for countries like United States, Canada and Norway that are prone to acid deposition have very low rates. Another perspective held by scientist is that such soils experience leaching when the acid rains are strongly acidified. However, many scientists do not hold this as a fact. Soil characteristics studies in North America and northern Europe show that disturbance under conifers affects acidity levels as well as depth of humus more than under hardwood cover [11].

Soils under mixed forests response resemble that of conifers [11]. Research done on sunny months of 1988 showed watercourses in the eastern and northeastern areas of the United States had lower pH than previously inferred. Factors that outline landscape danger of acidification by rainfall are the very factors, which are known to enhance the acid concentration in the soils through the natural way of soil formation. Quantitative analysis of impacts of acid deposition on land and water masses should focus on system models of upstream soils and water [12].

Some vehicle surfaces corrode on coming into contact with acid deposition. Mast corrosion impacts have resulted due to surface evaporation on vehicle exteriors. The affected surfaces have irregular shaped spots, which are noticeable on dark colored vehicles. This corroded spots are permanent; such spots are repairable through repainting.

Sulfur-based acid deposition is common on exposed limestone objects; it corrodes in a neutralization reaction. This deposition causes corrosion the surfaces, this leads to loss of aesthetic value [3].In the town of Malta, the impacts of sulfate corrosion on external walls of churches correlated with distance from power stations [13]. The power stations were the local predominant sources of sulfur-based acid deposition.

The sulfur-based depositions were contained in soot particles released during combustion processes. Calcite the component of limestone reacts with sulfur acid deposition. The resulting compound is calcium sulfite. Further reaction result in a stable gypsum compound [13]. This is readily soluble in rainwater and washed away. However, the pH of the rainwater determines this. Exposed carbonate rocks undergo similar reactions in areas with high sulfur dioxide release. Areas prone to sulfur dioxide pollution, the degree of sulphation on objects made of limestone can be an indicator of air quality [13].

The level of damage by the sulfur dioxide pollution is indicative of the economic loss resulting thereof. For instance in the case of Malta the heavy pollution was as a result of hard coal and fuel oil combustion. [13], reckons 260 000 tons of coal and 230 000 tons of fuel oil annually. The consistency of prevailing winds blowing in a particular direction may result in varying intensities of acid deposition. Winds act as a dispersal agent for the precursor element of acid deposition. Strength of wind also determines the amount of the particles dispersed from the point of source.

Complex compounds formed in the atmosphere from sulfur dioxide and nitrogen oxides release can impair visibility. In the eastern parts of the US, almost 70% of poor visibility in the air is as a result sulfur based acid deposition. This problem has affected tourism activity in the national parks, especially in Shenandoah and Great Smoky Mountains. The United States Environment Protection Agency (USEPA) launched the Acid Rain Program to counter the problem of acid rain by improving recreational and residential visibility as well as lowering the emission levels. The program has budgeted for an annual expenditure of US$1 billion annually. The Chinese State Environmental Protection Administration projects about U.S. $13 billion budget to address the damages of the phenomenon of acid rain in China [5].

The phenomenon of acid rain affects human health in two stages. These are the pre-deposition and post-deposition. The pre-deposition stage involves contact with the acid particles in the ambient air [9]. In the post-deposition stage, contact occurs after the deposition of acid particles in water or soil medium [9]. Threats by acid deposition aggravate when the mobility of particles is high. These particles are in the form of aerosols. Complications of bronchitis and asthma become severe with exposure to acid deposition. Prolonged or high concentration exposure aggravates the human health effects. In London in 1952, more than 4000 people died of complication ill health [3].

The Acid Rain Program by USEPA targets to reduce the health implications of acid rain in the US. Building knowledge on the impacts of Sulfur dioxide and nitrogen oxide on health is enshrouded in three main branches of study: epidemiology and diseases evidence, human clinical studies and animal toxicology [9]. Diseases evidence gives insight information on the reflex indicators after contact with emitted pollutants [9]. These studies provide information on both short-term and long-term trends.

Human clinical studies involve pre-designed short-term exposures of individuals [9]. The information generated from this kind of study helps infer on the level of human risk. Predesigned tests are mild but do not have adverse impacts on volunteers. Animal studies involve both short-term and chronic impacts [9]. Tissue analysis on concentration of pollutants forms main part of the studies. Test on lung irritancy provide robust procedures of assessing the amount of inhaled sulfur oxide aerosols [9]. In depth, lung functions tests under sulfur dioxide inhalation have explored on bronchial provocation testing, lung clearance and defense, cilia beat frequency and tracheal clearance rate.

There is no single solution to the problem of acid rain. Solutions provided need individual responsibility and corporate or community action. Foremost there is need for a global wide sensitization on the negative effects of acid rain on the environment. This creates a suitable platform for implementing preventive and control measures. Proper management of the rising global human population is a compounding factor in the acid rain phenomenon just as in other environmental concerns.

Arguably, as the population grows so do the demands for resources and economic activities. There is nexus between population density and pollution; for instance, the urban places. Urban places experience heavy vehicular traffic causing massive release of exhaust gasses. Decongestion may ameliorate this. Substituting hydrocarbons in combustion processes can help lower emissions. However, where necessary to combust these, environmentally sound technologies should be applied. For instance, the adoption of cleaner production concept by industrial plants prevents the emission of greenhouse gasses rather than manage them.

There are two ways of lowering sulfur dioxide emission from combustion processes: the use of low sulfur coal for combustion or installing chemical filters in exhaust chimneys [14]. Vehicle exhaust systems can be fitted with filters that trap nitrogen oxide before it escapes. Use of cleaner energy may result to switching to renewable energy sources [14]. Environments that have suffered severe damage from acid deposition, restoration works may help regain environmental health. For instance, reforestation programs for areas damaged by acid deposition. Individual responsibility should focus on safeguarding the environment by voluntary actions.

This may include switching energy-consuming appliances when not in use, opting for public than private commuting services and using fuels that are either free or low in nitrogen oxides. Lessons learnt from acid deposition should be used in researching for practical solutions. These may form the bases for launching action programs such as USEPA’s Acid Rain Program.

Acid deposition has damaging effect on the environment. The degree, dimension and type of damage vary from proximate to remote locations from the source. There is absence of a single clear-cut solution to this problem; hence lack of harmony among implementers to reverse, manage and prevent negative impacts. Strained relationship between neighboring states on impacts of acid deposition is clear indication for need for collaborative action.

Rarely would one state be willing to bear with the adverse impacts of acid deposition by another. There is a great deal of economic cost involved in taking action on acid deposition. Solution should not narrow on acid deposition but on the wider environment protection spectrum. It is evident that moderation of human activities including human development provides the general platform for solutions, but this is not practical in all cases.

It appears that there is no international watchdog credible enough to ensure compliance to multilateral treaties (“soft laws”) by all stakeholders. Thus, this begs the morals and consciousness of each individual to uphold their integrity. Healthy environment will not only serve present interest but also for the future. The improvement of health and sanitation has led to rapid population growth; ironically, this is straining the limited environment resources. This scenario presents a gloomy precedence to the future generation. There is need for better stewardship so that the present generation leaves behind a legacy.

Collective action should focus on protective actions on the lithosphere, hydrosphere, atmosphere and biosphere. Protective action needs no international policing or coercion but voluntary. The USEPA is implementing its initiative-Acid Rain Program to check the underlying issues and provide alternative solutions. This is an initiative worth aping by other countries facing domestic challenges of acid deposition.


  1. Likens, G. E., Driscoll, C.T. and Buso, D.C. 272, 1996, 244-246, American Association for the Advancement of Science. Web.
  2. Schopp, W., Posch, M., Mylona, S., and Johanson, M. Hydrology and Earth System Sciences. 7, 2003, 436-446. Web.
  3. USEPA. Web.
  4. IUPAC. Web.
  5. American Chemical Society. Web.
  6. Galloway, J. N.Water, Air and Soil Pollution 85, 1995, 15-24. Web.
  7. NASA. Web.
  8. Durst, R.A., Davison, W. & Koch, W. F. Pure & Applied Chernistry. 66, 1994, 649-658. Web.
  9. Goyer A. R.,, Bachmann, J., Clarkson, T. W., Ferris, Jr., B. G., Paul, J. G., Perl, D. P., Rall, D. P., Schlesinger, M. and Wood, J. M. Environmental Health Perspectives. 60, 1985, 355-368. Web.
  10. suite101.. Web.
  11. Krug, E. C. and Frink, C. R. Science, 221, 1983, 520-525. Web.
  12. Cosby, B. J., Hornberger, G. M., Galloway, J. N. and Wright, R. F. Water Resources Research, 21, 1985, 51-63. Web.
  13. Vella, J. A., Camilleri, A. and Adami, J. P. T. Environmental Geochemistry and Health, 18, 1996, 165-170. Web.
  14. USEPA. Web.