ACID DEPOSITION, ITS EFFECTS, AND CRITICAL LOADS
Acid deposition is the deposition of air pollutants to ecosystems leading to acidification and eutrophication, which cause negative effects in these ecosystems. The components contributing to acid deposition are sulfur compounds (SO2 and SO4), oxidized nitrogen compounds (NO, NO2, HNO2, HNO3, and NO3), and reduced nitrogen compounds (NH3 and NH4). Some rain is naturally acidic because of the carbon dioxide (CO2) in air that dissolves with rain water and forms a weak acid. This kind of acid is actually beneficial because it helps dissolve minerals in the soil that both plants and animals need. Ammonia acts as a base in the atmosphere, neutralizing nitric and sulfuric acid, but in soil ammonia it can be converted by microorganisms to nitric acid, producing additional acid in the process.
The first sign of the effects of acid deposition was the loss of fish populations in Canadian, Scandinavian, and U. S. lakes in the early 1960s. It was found that the water in the lakes was acidified to a point at which fish eggs no longer produced young specimens. This was caused by acid deposition; precipitation had introduced so much acid in the lakes that pH levels had declined below the critical limit for egg hatching. A few years later, reports from Canada, the United States, and Germany indicated unusual damage in forests. Trees showed decreased foliage or needles, and this damage even progressed to the point that trees would die, a phenomenon that happened in the border area of former East Germany, Poland, and former Czechoslovakia. This tree die - back was also attributed to acidification of the soil. However, exposure to increased oxidant concentrations was determined to be another cause. In the mid - 1980s, nitrogen deposition to terrestrial and aquatic ecosystems was found to cause negative effects through eutrophication. Acidification and eutrophication are both caused by atmospheric deposition of pollutants, and the combination increases the effects, causing too high nutrient concentrations in soil and groundwater. Nitrate and ammonium are beneficial, even essential, for vegetation growth, but in too high concentration they lead to the loss of diversity, especially in oligotrophic ecosystems (i. e., ecosystems adapted to low nutrient availability). This problem has mainly been encountered in central and northwestern Europe.
The effects of acid deposition include changes in terrestrial and aquatic ecosystems showing acidification of soils, shifts in plant community composition, loss of species diversity, forest damage, water acidification and loss of fish populations, reduction in growth yield of sensitive agricultural crops, and damage to cultural heritage and to building materials. The common factor for all these effects is that pollutants, or their precursors, are emitted, transported in the atmosphere, and deposited by way of precipitation (wet deposition) or as gases or particles (dry deposition). In some cases, acid deposition occurs 1000 km or more from where the responsible emissions were generated.
If deposition exceeds sustainable levels, effects can occur depending on the duration of the exposure and the level of exceedance. Sustainable levels can be represented by critical loads and critical levels. The critical load is formally defined by the United Nations Economic Commission for Europe (UNECE) as ‘‘a quantitative estimate of exposure to one or more pollutants below which significant harmful effects on sensitive elements of the environment do not occur according to present knowledge.’’ This definition can be applied to a wide range of phenomena and not merely acid rain. Any process in which damage will occur if depositions exceed natural sink processes has a critical load set by those sink processes. The concept of critical loads or levels has been used to provide a scientific basis for the incorporation of environmental impacts in the development of national and international policies to control transboundary air pollutants, within the framework of the UNECE Convention on the Control of Transboundary Air Pollution. The concept is based on the precautionary principle: Critical loads and levels are set to prevent any long-term damage to the most sensitive known elements of any ecosystem. The critical loads approach is an example of an effects-based approach to pollution control, which considers that the impact of deposited material depends on where it lands. Similar industrial plants may have different pollution impacts depending on the different capacities of the receiving environments to absorb, buffer, or tolerate pollutant loads. The policy goal with effects-based strategies is ultimately to ensure that the abatement on emissions from all plants is adequate so that no absorptive capacities, nor air quality standards, are exceeded.