There are two common uses for the term “acidity”, viz. (i) hydrogen ion concentration which is an intensity factor, and (ii) base neutralizing capacity (BNC), a capacity factor (Skeffington, 1987). The current practice is to measure BNC by titrating to an equivalence point, where the concentrations of acidic species are equal to those of basic species. Usually, the equivalence points of carbonic acid are used in natural waters. Such usage defines acidity with respect to carbonic acid. Equivalence points do not occur at fixed pH is a much more appropriate parameter to calculate. Whereas it is impossible to calculate a proton budget, as free protons are not conservative species.
It is quite possible to calculate a BNC budget. BNC budgets can yield valuable information about the sources and transport of acidity in ecosystems. Acid rain has been implicated as the principal cause of the acidification of soils and fresh-waters in several countries. It may be deemed as rain with pH of less than 5.6.
This has nothing to do with rain pH in the absence of man-made emissions, but follows from the definition of acidity in natural waters. (It may be interesting to note here that pure water in equilibrium with atmospheric CO2 has a pH of 5.6 at 100°C and zero alkalinity by definition: this means that rain with pH less than 5.6 is acid rain) Clearly, the input of acid from the atmosphere imposes an acid load on ecosystems which receive it. On the other hand, it has also been a long-accepted notion that internal production of acid in ecosystems is very often fairly significant. Little is known about the relative extents of internal and external acid inputs. Forest canopy modifies the chemical composition of the rain as it falls to the ground. During passage through the canopy, the rainwater is spatially redistributed over the forest floor as through fall which includes drips and non-intercepted water, and stemflow which reaches the forest floor by running down along trunks of trees.
The interception loss is that water which is retained by the canopy and then lost by evaporation. Redistribution is accompanied by chemical changes of the water, involving gains or losses of materials through interaction with the vegetation. Some likely mechanisms of change include the wash-off of surface material (previously deposited from the atmosphere) and exchange of material from within the plant. In the case of a net gain, this is called leaching. In cases where the deposition of sulphate is greater in through fall and stemflow than in rain, it has been suggested that air pollutants, SO2 and particulate sulphate, could be responsible for the observed increase in sulphur deposition.
Rainwater generally loses hydrogen ions at the leaf surface, especially at the more polluted sites. Trees greatly reduce the acidity of water passing through their crowns, the reduction occurring at the expense of a loss of potassium and other base cations from the tree. The ground vegetation beneath the forest canopy further reduces rainwater acidity, but much of this is regained on passage through the humus layer.
Movement of acidity through deeper layers of soil is partly related to the amount of sulphate introduced in the rain but again, effects of both species and tree age have been detected. Nitrate, however, is retained in most ecosystems dining the summer months but may pass through with the drainage water during winter. Soil acidification appears to be an unavoidable consequence of terrestrial life.
In fact, the evolution of terrestrial organisms was not possible without the acquisition of acid tolerance. It follows from the acid-buffering properties of soils that nutrient deficiency and acid stress are the two sides of the same coin. The major cations (Ca, K, Mg) exist in the soil as bases. If removed, they are replaced by acids. Nutrient stress (deficiency) and acid stress (surplus, toxicity) are therefore intimately connected.
The change in vegetation from rich to poor soils accordingly includes acid stress as a cause. Several buffering processes operate in soil, the most important being the weathering of various minerals from rock. As long as calcareous minerals abound in the soil, the pH value will stay fairly high.
But should the pH decline because of a deposition of acid, a major buffering process will be set in, causing aluminium compounds to dissolve. Free aluminium in the soil is suspected of damaging tree roots, but it can reach out to the groundwater and surface water, this being a major cause of harm to the fish populations in acidic lakes. Acid rain can cause fish kills, and deteriorate monuments and buildings. The pH of naturally distilled rain water is around 5.6 to 5.7, but in some cases rain water may register a pH as low as 4.0. The acidity of rain water is mainly caused by H2SO4 and HNO3 Precipitation contains traces of certain acids, heavy metals and organic pollutants.
Most of the gaseous components of the atmosphere participate in elemental cycles which are regulated by biological oxidation-reduction reactions such as photosynthesis and respiration. The atmosphere is quite susceptible to anthropogenic emissions which upset the natural levels of the various gases. Thus the concentration of CO2 has increased globally whereas those of SO2, H2SO4, NO, NO2, HNO2 and HNO, have increased in many regions where atmospheric pollution is high. Natural sources of SOx are currently estimated to constitute only about 10 per cent of the total SOx emissions (Kish, 1981). These natural sources include sea spray, sulphate reduction in soils and waters, decomposition of organic matter, volcanic discharges and forest fires. Lightning discharges and breakdown of nitrates in soils contribute to natural supply of NOx.
At pH 4.3, the rain water contains about 50 micro-equivalents per liter of acidity (free H+ ions), whereas at pH 5.6, there are about 20 times less H+ ions. When fossil fuels are burnt, S and N are oxidized and there is a build-up in the atmosphere of CO2 and the oxides of S and N, which leads to acid-base interaction. Rain also traps wind-blown dust which may be strongly acidic or alkaline. Volcanic eruptions, wildfire, lightning, emissions from living organisms also release carbon, nitrogen and sulphur compounds into the atmosphere.
The HCI in acid rain comes mostly from the combustion and decomposition of organochlorine compounds and polyvinyl chloride plastics. The sulphuric acid comes from oxidation of sulphur in fossil fuels, and nitric acid comes from nitrogen fixation. Bases originate as the carbonates of windblown dust and from ammonia. The latter is derived from decomposition of urea in the soil.
Acid precipitation markedly affects water bodies. Lakes can lose as much as 70 per cent of their bicarbonate alkalinity in a decade, mainly because of the input of acid precipitation. Such a loss of alkalinity makes it difficult for crustaceans to moult their exoskeletons; in some cases, many crustaceans may die. Windermere (1980) reported that an inverse relationship exists between water pH and the species diversity of crustaceans. Another indirect effect of the fall in water pH is the release of heavy metals such as mercury and aluminium in water; these metals in turn produce adverse effects on the indigenous biota. Nothing very precise or definite is known about the direct effects of acid precipitation on human health or on vegetation (Kish, 1981).