acid rain

acid rain Acid rain became one of the most emotive environmental issues of the 1970s and 1980s, yet the presence of high concentrations of acids in the urban rainfall of Manchester, in comparison with surrounding rural areas, was identified in a systematic way over a hundred years ago by Angus Smith, an industrial chemist who was employed as the first Inspector of Factories. As a contemporary environmental issue, the acid rain problem came to the forefront of scientific and public awareness in Europe after the first United Nations Environment Conference in Stockholm in 1972 when ‘evidence’ for a number of detrimental environmental impacts of acid rain was presented. The impacts identified were a rapid increase in the acidity of European rainfall, a parallel increase in the acidity of Swedish rivers and lakes, a decline in fish populations in these rivers and lakes, and a decline in forest growth. In the decades since this conference, the scientific community has endeavoured to evaluate in a systematic way whether there is a sound body of evidence to substantiate these claims. Providing substantive evidence for the effects listed above has proved to be a challenging task for environmental scientists, not least because of the complexity in linking essentially gaseous sources to the environmental effects of acid rain. In consequence, an understanding of the acid rain problem needs to focus upon some of the fundamental principles and concepts involved, of which the first is ‘What do we mean by the term “acid rain”?’

Natural rainfall contains many impurities, including dissolved gases, dusts, and salts which it picks up during its passage through the atmosphere to the ground. These constituents can come from a variety of natural sources, which include volcanic and biologically produced gases, sea spray and wind-blown dusts. Impurities in rainfall also come from a variety of polluting sources, such as fossil-fuel combustion and motor vehicles; these impurities include both gaseous and particulate emissions. In order to understand the effect that both natural and polluting substances have on the acidity of rainfall it is essential to start with an examination of the way in which acidity is measured.

Acidity is measured on what is called the pH scale. The scale was developed by chemists at the beginning of the twentieth century to provide a simple way of expressing the concentration of free hydrogen ions (H⁺) in a solution. (Free hydrogen is the hydrogen in a solution which is not part of the water molecule, H₂O.) The pH scale is potentially confusing to the non-chemist, since it is negative and is based upon logarithmic units. This means that the concentration of H⁺ increases as the pH value falls. Starting at a neutral pH of 7, where there are no free hydrogen ions, each single unit decrease in pH means that there is ten times more hydrogen in the solution. Between pH 6 and pH 2, therefore, there is a 10 000-fold increase in concentration. The relationship between hydrogen ion concentration and pH is shown in Fig. 1.

One of the most common natural atmospheric gases is carbon dioxide, CO₂, which dissolves in rainwater to form carbonic acid, giving it a pH of around 5.6. The presence of other natural acids can reduce the pH of unpolluted rain to around 5. Dust in rain can change the pH by making it either more or less acidic according to the chemical properties of the dust. While natural rainfall is acidic, the presence of pollutant gases, particularly those containing oxides of nitrogen, sulphur, and chlorine, serves to increase the acidity. In the industrial regions of North America and Europe the presence of these dissolved gases generally increases the acidity by a factor of 10 (i.e. it produces rainfall with a pH of around 4).

A second fundamental issue in understanding the acid rain problem is to identify the types of pollutant gaseous emissions which produce acid rain and to identify how, and at what rate, acid rain is subsequently produced. As indicated above, the principal pollutant emissions leading to the formation of acid rain are oxides of nitrogen and sulphur. Sulphur is present in fossil fuels and is liberated by their combustion, especially in power generating stations and by the smelting of ores. Nitrogen oxides are generated during the combustion process in the internal combustion engine because of the presence of nitrogen in the air. The degree to which this process occurs depends on the temperature of the reaction. Some nitrogen gases, especially ammonia, are alkaline when dissolved in rain. As ammonia is oxidized, however, it will eventually contribute to acidity. Some 80 per cent of ammonia in the atmosphere is estimated to have come from livestock wastes. Chlorine is a third acid-forming gas, of which 75 per cent in Europe is estimated to come from the burning of fossil fuels. The rate at which sulphur, nitrogen, and chlorine are oxidized in the atmosphere to produce acid rain depends in part on the presence of volatile organic carbons that are also liberated by industrial processes. Furthermore, industry produces considerable quantities of carbon dioxide which influence the amount of carbonic acid present in rainfall. Emissions from point sources such as chimney stacks can be dispersed downwind for hundreds or even thousands of kilometres and are usually retained within an atmospheric layer less than a kilometre thick. The rate at which acids are produced from gases through the oxidation process is fairly slow, and it has been estimated that the conversion rate is only between 1 and 3 per cent per hour. These gases and oxides are also diluted by as much as 10 000 times in the atmosphere as they disperse.

Environmental scientists have identified a number of ways by which the oxides produced by atmospheric conversion can reach the ground surface without necessarily involving rainfall. The oxides can exist in dusts which reach vegetation surfaces or the ground surface as dry particulate fallout, or as dissolved constituents which may reach the ground surface as wet fallout, which includes not only rain, but snow, mist, and low cloud. In the case of low cloud, acid droplets are deposited on vegetated surfaces by a process called ‘occult deposition’. A generalized scheme showing the production, dispersal, chemical transformation, and deposition of acids is shown in Fig. 2.

It has been estimated that the emission of sulphur through the combustion of fossil fuels increased from 1850, when emissions were around 0.5 Mtonne (0.5 × 10⁶ metric tonnes), to 1965, when emissions reached a peak of some 3.5 Mtonne. After 1965, global emissions declined sharply as a result of changing practices in industry and the use of cleaner fuels such as gas and nuclear power. While sulphur emissions have declined, nitrogen emissions have increased, particularly as a result of the increase in road traffic. Although there have been substantial reductions in sulphur emissions, there is little evidence to suggest that this is having a dramatic impact upon the acidity of rainfall.

In trying to link the acidity of rainfall to the acidity of soils and water it is essential to understand the role that soils play in releasing chemicals into solution as a result of natural weathering. Soils are made up of two major components: organic matter, which results from the decomposition of vegetation, and the physically and chemically altered parent material. In soils with a high content of organic matter, and in areas of high rainfall, water draining through the soil becomes more acidic with the release of organic acids. Some parent materials, especially those containing carbonate rocks, can buffer this acidity and raise the pH of drainage waters. The pathways that water takes through the soil to the rivers are also important because they will determine whether or not the drainage water comes into contact with that part of the soil which can buffer the acidity. The sensitivity of different areas of the UK to acid inputs is highly variable. In regions with high rainfall, acid soils, and hard crystalline rocks, there is a potentially high susceptibility to the input of acid rain with little opportunity to buffer the input (Fig. 3).

The presence of hydrogen ions in rainfall, soil solutions, rivers, and lakes has a secondary impact on the environment through the release of potentially toxic metals, particularly aluminium, which is present in trace amounts in almost all soils. Aluminium is most toxic in the pH range 5–6. It has a number of known effects, such as impairing respiration in fish and possibly reducing the growth rate of phytoplankton communities in fresh water.

Vegetation is important for two reasons. First, it produces leaf litter, which decomposes in the soil to produce organic matter which in turn liberates organic acids. Some tree species especially conifers, are known to produce more organic acids than others. Scientific evidence obtained from studies of lake sediments in North Wales has, however, indicated that acidification began well before the planting of upland forests. It seems, nevertheless, that the rate of acidification increased after forest plantation. Vegetation also plays an important role in trapping dusts and acid aerosols by occult deposition. The combination of these two processes, in addition to the release of organic acids from leaf surfaces, also serves to increase the acidity of water dripping through the forest canopy (as throughfall: see hydrological cycle). Some studies have shown that the pH of throughfall can be at least one unit lower than the acidity of rain as it reaches the canopy surface, that is, the concentration of hydrogen ions is ten times greater. The effect of acidity upon forest growth rates is unproved. Forest decline is not a specific disease with a single cause; many factors, such as acidity, climate change, and the preferential removal of essential elements like magnesium from soils, may all play a part in the decline in forest growth.

The body of scientific evidence accumulated since the 1972 United Nations Stockholm conference has improved our understanding of the acid rain problem. There are, however, many unresolved issues and only recently have scientists begun to turn their attention to the next major phase in acid rain research, which is to find out whether or not the observed chemical and biological changes to the environment are reversible and sustainable.

Ian D. L. Foster

Bibliography

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