Overview - Acid deposition

Acid deposition represents the mix of air pollutants that together lead to the acidification of soils and freshwaters. The term encompasses the popular idea of "acid rain", but also includes the direct uptake of pollutants by the ground in the absence of rain. This direct uptake is referred to as "dry deposition", while precipitation inputs are referred to as "wet deposition". In addition, the direct impaction of cloudwater on hills is sometimes referred to as "occult deposition". Today the contribution of SO₂ has fallen dramatically though with some more linearity since levels have fallen more closer to source than over the mountains (Fowler et al).

A number of miscellaneous compounds can also contribute to acid deposition, for example hydrochloric acid (HCl) and organic acids such as acetic acid and methanoic acid.

It is worth noting that strictly speaking not all the components of acid of acid deposition are acidic, all though they have an acidifying effect in soils and freshwaters. Hence sulphate and nitrate are not in themselves acidifying, while NH₃ is actually an alkali. This may be explained by the fact that SO₂ reacts to produce sulphuric acid (H₂SO₄), while NO_X reacts to produce HNO₂. Hence nitrate and sulphate are each associated with H⁺. In contrast, NH₃ is acidifying since when deposited (either as NH₃ or NH₄⁺) it may be oxidized in the soil by bacteria to nitrate. This oxidation process liberates 1 H⁺ from NH₃ and 2 H⁺ from NH₄⁺ (Sutton et al. 1993, Sutton and Fowler 1993). Deposition to an ecosystem may occur in the form of ammonium sulphate, which is in itself not acidic. However, the acidity from SO₂ is "trapped" in the NH₄⁺, which is then liberated following oxidation in the soil. (See Figure 1).

Figure 1: Effects of atmospheric NH₃ or NH₄⁺ input on soil proton balances. The production of H⁺ depends on the form of NHx input and the fate of the NH₄⁺ ion. M+ represents a metal ion that might be leached from the soil. Diagram modified from Binkley and Richter (1997). [Source: Sutton M.A., Pitcairn C.E.R. and Fowler D. (1993): The exchange of ammonia between the atmosphere and plant communities. In: Begon M. and Fitter A.H. (Eds.): Advances in Ecological Research 24, p301-393. Academic Press, London.]

Acid rain has been blamed for large-scale damage to aquatic ecosystems and forests in Scandinavian countries. Southeastern Canada, North eastern United States and Galloway (SW Scotland). In the 80’s Germany was concerned about acid rain effects on its forests. The most important active ions are H⁺, SO₄^2-, SO₃^- and NH₄⁺, however rainfall pH is not a good indicator of potential phytotoxicity ion concentrations and ratios are more significant except possibly <pH 2.5 (Cape 1993), once the pH falls below 3.0 the likelihood of visible injury lesions, ‘burning’ and premature senescence and leaf all increases. Reproduction in both birds and plants appears to be sensitive to acidification e.g. egg shell condition and seed production, viability and germination. Birds may be adversely affected through negative impacts on their food down the trophic levels. Crustaceans do not survive in acidified waters.

In assessing the risks to vegetation from acid deposition, consideration should be given to deposition characteristics over the country. Some areas will be more at risk than others because deposition is higher. Similarly for plants rooted in soil responses will be strongly dependent on soil type and bed rock geology and weathering rate and its buffering capacity. Areas most sensitive to acid deposition are characterised by minerals such as granite, gneiss and quartz rich rocks which contain little (lime) CaCO₃ and do not weather easily. Deposition rates of >30 kg ha-1 yr-1 cause adverse effects on sensitive aquatic systems (alkalinity <200m eq l-1 ). Communities most at risk include bryophytes and lichens particularly some of the epiphytes and species growing on acid soils with high Al₃⁺ contents. Acidified rain will lower soil pH, increase Al solubility and base cation leaching. High Al₃⁺ concentrations are toxic to fine roots, unless protected by mycorrhiza, reduce P availability through precipitate and compete with Ca for uptake sites (Sheppard 1980).

Sutton M.A., Pitcairn C.E.R. and Fowler D. (1993): The exchange of ammonia between the atmosphere and plant communities. In: Begon M. and Fitter A.H. [Eds.]: Advances in Ecological Research 24, p 301-393. Academic Press, London.

Sutton M.A. and Fowler D. (1993): Estimating the relative contribution of SOx , NOx and NHx inputs to effects of atmospheric deposition. In: Critical loads: concept and applications. (Grange-over-Sands Workshop, 2/1992) [Eds: Hornung M. and Skeffington R.A.] 119-126, HMSO, London.

Brady G.L.and Selle J.C. (1985): Acid Rain: The International Response. Intern. J. Environmental Studies, 1985, 44, 217-230.

Cape J.N. Direct damage to vegetation caused by acid rain and polluted cloud: definition of critical levels for forest trees. Environmental Pollution 82, 167-180.



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Acid deposition Acid deposition represents the mix of air pollutants that together lead to the acidification of soils and freshwaters. The term encompasses the popular idea of "acid rain", but also includes the direct uptake of pollutants by the ground in the absence of rain. This direct uptake is referred to as "dry deposition", while precipitation inputs are referred to as "wet deposition". In addition, the direct impaction of cloudwater on hills is sometimes referred to as "occult deposition". Today the contribution of SO₂ has fallen dramatically though with some more linearity since levels have fallen more closer to source than over the mountains (Fowler et al). The pollutants that contribute to acid deposition include: sulphur dioxide (SO₂) and its reaction product sulphate (SO₄^2-) nitrogen oxides (NO_X) and the various reaction products which include nitric acid (HNO₃ ), nitrate (NO₃^-) and peroxyacetyle nitrate (PAN). ammonia (NH₃) and its reaction product ammonium (NH₄⁺) Other overviews are provided on the different primary pollutants. A number of miscellaneous compounds can also contribute to acid deposition, for example hydrochloric acid (HCl) and organic acids such as acetic acid and methanoic acid. It is worth noting that strictly speaking not all the components of acid of acid deposition are acidic, all though they have an acidifying effect in soils and freshwaters. Hence sulphate and nitrate are not in themselves acidifying, while NH₃ is actually an alkali. This may be explained by the fact that SO₂ reacts to produce sulphuric acid (H₂SO₄), while NO_X reacts to produce HNO₂. Hence nitrate and sulphate are each associated with H⁺. In contrast, NH₃ is acidifying since when deposited (either as NH₃ or NH₄⁺) it may be oxidized in the soil by bacteria to nitrate. This oxidation process liberates 1 H⁺ from NH₃ and 2 H⁺ from NH₄⁺ (Sutton et al. 1993, Sutton and Fowler 1993). Deposition to an ecosystem may occur in the form of ammonium sulphate, which is in itself not acidic. However, the acidity from SO₂ is "trapped" in the NH₄⁺, which is then liberated following oxidation in the soil. (See Figure 1). Figure 1: Effects of atmospheric NH₃ or NH₄⁺ input on soil proton balances. The production of H⁺ depends on the form of NHx input and the fate of the NH₄⁺ ion. M+ represents a metal ion that might be leached from the soil. Diagram modified from Binkley and Richter (1997). [Source: Sutton M.A., Pitcairn C.E.R. and Fowler D. (1993): The exchange of ammonia between the atmosphere and plant communities. In: Begon M. and Fitter A.H. (Eds.): Advances in Ecological Research 24, p301-393. Academic Press, London.] Acid rain has been blamed for large-scale damage to aquatic ecosystems and forests in Scandinavian countries. Southeastern Canada, North eastern United States and Galloway (SW Scotland). In the 80’s Germany was concerned about acid rain effects on its forests. The most important active ions are H⁺, SO₄^2-, SO₃^- and NH₄⁺, however rainfall pH is not a good indicator of potential phytotoxicity ion concentrations and ratios are more significant except possibly <pH 2.5 (Cape 1993), once the pH falls below 3.0 the likelihood of visible injury lesions, ‘burning’ and premature senescence and leaf all increases. Reproduction in both birds and plants appears to be sensitive to acidification e.g. egg shell condition and seed production, viability and germination. Birds may be adversely affected through negative impacts on their food down the trophic levels. Crustaceans do not survive in acidified waters. In assessing the risks to vegetation from acid deposition, consideration should be given to deposition characteristics over the country. Some areas will be more at risk than others because deposition is higher. Similarly for plants rooted in soil responses will be strongly dependent on soil type and bed rock geology and weathering rate and its buffering capacity. Areas most sensitive to acid deposition are characterised by minerals such as granite, gneiss and quartz rich rocks which contain little (lime) CaCO₃ and do not weather easily. Deposition rates of >30 kg ha-1 yr-1 cause adverse effects on sensitive aquatic systems (alkalinity <200m eq l-1 ). Communities most at risk include bryophytes and lichens particularly some of the epiphytes and species growing on acid soils with high Al₃⁺ contents. Acidified rain will lower soil pH, increase Al solubility and base cation leaching. High Al₃⁺ concentrations are toxic to fine roots, unless protected by mycorrhiza, reduce P availability through precipitate and compete with Ca for uptake sites (Sheppard 1980). References: Sutton M.A., Pitcairn C.E.R. and Fowler D. (1993): The exchange of ammonia between the atmosphere and plant communities. In: Begon M. and Fitter A.H. [Eds.]: Advances in Ecological Research 24, p 301-393. Academic Press, London. Sutton M.A. and Fowler D. (1993): Estimating the relative contribution of SOx , NOx and NHx inputs to effects of atmospheric deposition. In: Critical loads: concept and applications. (Grange-over-Sands Workshop, 2/1992) [Eds: Hornung M. and Skeffington R.A.] 119-126, HMSO, London. Brady G.L.and Selle J.C. (1985): Acid Rain: The International Response. Intern. J. Environmental Studies, 1985, 44, 217-230. Cape J.N. Direct damage to vegetation caused by acid rain and polluted cloud: definition of critical levels for forest trees. Environmental Pollution 82, 167-180.

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