The Nature And Development Of Acid Rain

6 June 2016

Acid rain is usually measured to be a by-product of modern atmospheric pollution. Even in a pure, uncontaminated world, though, it is probable that the rainfall would be acidic. The absorption of carbon dioxide by atmospheric water produces weak carbonic acid, and nitric acid may be formed during thunderstorms, which provide enough energy for the synthesis of oxides of nitrogen (NOX ) from atmospheric oxygen and nitrogen.

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During volcanic outbreaks or forest fires, sulphur dioxide (SO2 ) is released into the atmosphere to give the necessary component for the formation of sulphuric acid. Phytoplankton in the oceans as well produces sulphur during their seasonal bloom period. The sulphur takes the shape of dimethyl sulphide (DMS) which is oxidized into SO2 and methane sulphonic acid (MSA). The MSA is eventually transformed into sulphate.

Acids formed in this manner fall out of the atmosphere in rain to turn out to be involved in various physical and biological processes once they reach the earth’s surface. The return of nitrogen and sulphur to the soil in naturally acid rain assists to uphold nutrient levels, for instance. The peculiar landscapes of limestone areas—typified by highly weathered bedrock, rivers flowing in steep-sided gorges or through inter-connected systems of under-ground stream channels and caves offer outstanding instances of what even moderately acid rain can do. (Stephen L. Baird, 2005)

In fact, since ‘acid rain’ comprises snow, hail and fog and rain, it would be more suitable to illustrate it as ‘acid precipitation’. The word ‘acid rain’ is most normally used for all kinds of ‘wet deposition’, though. A related process is ‘dry deposition’, which engrosses the fallout of the oxides of sulphur and nitrogen from the atmosphere, either as dry gases or adsorbed on other aerosols for example soot or fly ash.

To the extent that two-thirds of the acid precipitation over Britain falls as dry deposition in the form of gases and small particles. On contact with moisture in the form of fog, dew or surface water they make the same effects as the constituents of wet deposition. Presently, both wet and dry depositions are usually included in the term ‘acid rain’ and, to maintain continuity, that convention will be followed here.

Current concern over acid rain is not with the naturally produced variety, however quite with that which consequences from modern industrial activity. Technological advancement in a society frequently depends upon the availability of metallic ores, which can be smelted to make the great volume and variety of metals required for industrial and socio-economic development. Substantial amounts of SO2 are released into the atmosphere as a by-product of the smelting process, mainly when non-ferrous ores are involved. The burning of coal and oil, to provide energy for space heating or to fuel thermal electric power stations, as well produces SO2. The ongoing growth of transportation systems using the internal combustion engine—one more trait of a modern technological society—contributes to acid rain through the release of NOX into the atmosphere. (Conrad G. Schneider, 2001)

The table above shows that the biggest air pollutant that mobile sources contribute to acid rain is carbon monoxide. Of all of the carbon monoxide releases that contribute to acid rain, 81% of them come from mobile sources. The biggest other source is particulate matter, little particles of pollution that are released into the air by cars, trucks, and buses that are burning diesel fuel, fertilizers, pesticides, road construction, steel making, mining, and turning on fire places and wood stoves. 73% of the non-mobile sources that contribute to acid rain are caused by the release of particulate matter.

The table above shows how much mobile and other sources of pollution can make acid rain more of a problem. Seeing that carbon monoxide and particulate matter are the leading sources of pollution, by cutting down on these, acid rain will not be as much of a problem.

At first, the consequences of these pollutants were limited to the local areas in which they originated, and where their impact was often noticeable. As emissions increased, and the gases were steadily incorporated into the larger scale atmospheric circulation, the stage was set for an escalation of the problem. Sulphur compounds of anthropogenic origin are now held responsible for as much as sixty-five per cent of the acid rain in eastern North America, with nitrogen compounds accounting for the remainder.

In Europe, emission totals for SO2 and NOX are usually measured to split closer to seventy-five per cent and twenty-five per cent. Since the early 1970s, though, declining SO2 emissions and a growing output of NOX have combined to bring the relative proportions of Acid precipitation formed by human activities differs from natural acid precipitation not merely in its origins, however as well in its quality. (Hope Cristol, 2002)

Acid Rain and the pH Scale

The pH scale measures how acidic an object is. Objects that are not very acidic are called basic. The scale has values ranging from zero (the most acidic) to 14 (the most basic). As you can see from the pH scale above, pure water has a pH value of 7. This value is considered neutral—neither acidic or basic. Normal, clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides—produced from power plants and automobiles—the rain becomes much more acidic. Typical acid rain has a pH value of 4.0. A decrease in pH values from 5.0 to 4.0 means that the acidity is 10 times greater

The quality of the rain is determined by a series of chemical processes set in motion when acidic materials are released into the atmosphere. Some of the SO2 and NOX emitted will come back to the surface quite rapidly, and close to their source, as dry deposition. The rest will be carried up into the atmosphere, to be transformed into sulphuric and nitric acid, which will ultimately return to earth as acid rain. The processes involved are basically simple. Oxidation converts the gases into acids, in either a gas or liquid phase reaction. The latter is more effective.

The conversion of SO2 into sulphuric acid in the gas phase is sixteen per cent per hour in summer and three per cent per hour in winter. Equivalent conversion rates in the liquid phase are hundred per cent per hour in summer and twenty per cent per hour in winter. Regardless of the relatively slow conversion to acid in the gas phase, it is the main source of acid rain when clouds and rain are absent, or when humidity is low. (James Salzman, J.B. Ruhl, 2000)

The rate at which the chemical reactions occur will as well depend upon such variables as the concentration of heavy metals in the airborne particulate matter, the presence of ammonia and the intensity of sunlight. Airborne particles of manganese and iron, for instance, act as catalysts to accelerate the conversion of SO2 to sulphuric acid and sulphates. Natural ammonia may have similar effects. Sunshine gives the energy for the production of photo-oxidants—for instance ozone (O3 ), hydrogen peroxide (H2 O2 ) plus the hydroxyl radical (OH)—from other pollutants in the atmosphere, and these oxygen-rich compounds make possible the oxidation of SO2 and the NOX to sulphuric and nitric acid correspondingly.

The role of the photochemical component in the conversion process may account for the better acidity of summer rainfall in several areas. In the presence of water, these acids, and the other chemicals in the atmosphere, will dissociate into positively or negatively charged particles called ions. For instance, sulphuric acid in solution is a mixture of positively charged hydrogen ions (cations) and negatively charged sulphate ions (anions). It is these solutions, or ‘cocktails of ions’ that constitute acid rain. (Jonathan Watts, 2005)

Whatever the complexities involved in the creation of acid rain, the time scale is vital. The longer the original emissions remain in the atmosphere, the more probable it is that the reactions will be completed, and the sulphuric and nitric acids formed. Long Range Transportation of Atmospheric Pollution (LRTAP)—transportation in excess of 500 km—is one of the mechanisms by which this is accomplished.

The introduction of the taller smokestacks on smelters and thermal electric power stations, together with the higher exit velocities of the emissions, permitted the pollutants to be pushed higher into the atmosphere. This efficiently reduced local pollution concentrations, however caused the pollutants to remain in the atmosphere for longer periods of time, therefore escalating the likelihood that the acid conversion processes would be completed.

The release of pollutants at greater altitudes as well placed them outside the boundary layer circulation and into the larger scale atmospheric circulation system with its potential for much greater dispersion through the mechanisms of LRTAP. The net consequence was a noteworthy increase in the geographical extent of the problem of acid rain. (Krajick, K, 2001).

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