Acid deposition

Acid rain is not a new phenomenon either in Asia or in other parts of the world. Acid rain was first reported in Europe in the 19th century (Ducros, 1854). Acid rain and its harmful effects have been extensively studied in the industrialized countries of Europe and North America over the last thirty years (Radojevic and Harrison, 1992). 

Normal, unpolluted rainwater has a pH close to 5.6, in consequence of the raindrops being in equilibrium with atmospheric concentration ofC02 (reaction (1) and Section 3) 

We should stress here that even completely unpolluted rainwater does not have a pH of 7.0, because it is not pure distilled deionizing water and it contains equilibrium amounts of atmospheric gases, namely, carbon dioxide at an average present concentration of 365 ppbv. Acid rain is generally defined as having pH lower than 5.6. Low acidic rainwater has a pH between 5.6-5.0, acidic rainwater, 5.0-4.5, strong acidic rainwater, 4.5-3.5, and extremely acidic, 3.5. 

Over the last ten years, the presence of acid rain has been identified at nnmerous sites in Asia. Recent industrial development in Asia has raised concerns about actual and potential acidification in the region (Rodhe et al, 1992 ESCAP, 1995). Current and projected manifestation of acid forming compound emissions and deposition with indication of main sensitive ecosystems is shown in Table 1. 

Accordingly, in this chapter, we consider acid rain monitoring patterns and relative chemistry of acid rain formation in the Asian region. 

Acid deposition, the deposition of acidic substances from the atmosphere onto Earth s surface, is another phenomenon associated with atmospheric pollution. Acid rain refers to the low pH of precipitation that has been observed in 

FIGURE 4-38 Mean pH of precipitation of the United States in 1990, based on weekly measurements at 170 sites (adapted from National Atmospheric Deposition Program, 1990). 

Although some sulfuric acid is emitted directly by fuel-burning equipment, most of the sulfur in fuel is oxidized to and emitted as sulfur dioxide (S02). Sulfur dioxide contains sulfur in the ( + IV) oxidation state and dissolves in water to form sulfurous acid (H2S03), a relatively weak acid. In the presence of hydroxyl radicals in the atmosphere, sulfur dioxide is oxidized to sulfur trioxide (S03), which contains sulfur in the (+VI) oxidation state. Sulfur trioxide reacts with water to form H2S04, a strong acid. 

Typically, the bulk of sulfur oxidation in the atmosphere occurs by heterogeneous processes in water droplets. The homogeneous oxidation of S02 to 

FINE PARTICULATE SULFATE AND NITRATE 0.1-2.0/jm DIAMETER NH4NO3 (NH4)2 SO4 

Pollution refers to changes in the equilibrium (or balance) of biological and non-biological systems, as a result of human activity. Note that many so-called pollutants are substances that occur naturally, such as ozone and oxides of sulfur and nitrogen. However, human activity has led to an increase in the concentrations of such substances, which upsets the delicate balance of natural cycles. 

Primary pollutants are emitted directly from the sources and remain unchanged once they enter the environment. Examples include particulate matter and inorganic gases, such as sulfur dioxide. Secondary pollutants, for example sulfurous acid, are formed in the atmosphere by chemical reactions involving primary pollutants and gases normally present in the air. 

Sulfur dioxide (SO2) is an important primary pollutant. It is pungent smelling, toxic gas, which damages the respiratory system and may lead to asthma attacks. It is highly soluble in water, and contributes to the formation of acid rain. 

Natural rain water is acidic, with a pH of around 5.6. This acidity arises from the reaction of carbon dioxide and water in the atmosphere. Carbon dioxide dissolved in water is sometimes referred to as carbonic acid, but only a very small percentage of molecular carbonic acid, H2CO3, exists in solution  

The carbonic acid molecules immediately dissociate in water to form hydrogencarbonate ions, HCO3, and oxonium ions, H30  

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Acid chlorides Acid coupling Acid deposition... 

Receptors. The receptor can be a person, animal, plant, material, or ecosystem. The criteria and hazardous air pollutants were so designated because, at sufficient concentrations, they can cause adverse health effects to human receptors. Some of the criteria pollutants also cause damage to plant receptors. An Air QuaUty Criteria Document (12) exists for each criteria pollutant and these documents summarize the most current Hterature concerning the effects of criteria pollutants on human health, animals, vegetation, and materials. The receptors which have generated much concern regarding acid deposition are certain aquatic and forest ecosystems, and there is also some concern that acid deposition adversely affects some materials. 

Acid Deposition. Acid deposition, the deposition of acids from the atmosphere to the surface of the earth, can be dry or wet. Dry deposition involves acid gases or their precursors or acid particles coming in contact with the earth s surface and thence being retained. The principal species associated with dry acid deposition are S02(g), acid sulfate particles, ie, H2SO4 and NH HSO, and HN02(g). Measurements of dry deposition are quite sparse, however, and usually only speciated as total and total NO3. In general, dry acid deposition is estimated to be a small fraction of the total... 

Since SO2 and NO2 are criteria pollutants, their emissions are regulated. In addition, for the purposes of abating acid deposition in the United States, the 1990 Clean Air Act Amendments require that nationwide SO2 and NO emissions be reduced by approximately 10 million and 2 million t/yr, respectively, by the year 2000. Reasons for these reductions are based on concerns which include acidification of lakes and streams, acidification of poorly buffered soils, and acid damage to materials. An additional major concern is that acid deposition is contributing to the die-back of forests at high elevations in the eastern United States and in Europe. 

Subgroup on Metals of the Tri-Academj Committee on Acid Deposition, Acid Deposition Effects on Geochemical Cjcling and Biological Availability of Trace Elements, National Academy Press, Washington, D.C., 1985. 

U.S. Environmental Protection Agency (EPA), The Acidic Deposition Phenomenon and Its Effect, vol. 2 Effects Sciences, Critical Assessment Review Papers, Report EPA 600/9-83-016AF, EPA, Washington, D.C., 1984, pp. 4—11. 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

A variety of models have been developed to study acid deposition. Sulfuric acid is formed relatively slowly in the atmosphere, so its concentrations are beUeved to be more uniform than o2one, especially in and around cities. Also, the impacts are viewed as more regional in nature. This allows an even coarser hori2ontal resolution, on the order of 80 to 100 km, to be used in acid deposition models. Atmospheric models of acid deposition have been used to determine where reductions in sulfur dioxide emissions would be most effective. Many of the ecosystems that are most sensitive to damage from acid deposition are located in the northeastern United States and southeastern Canada. Early acid deposition models helped to estabUsh that sulfuric acid and its precursors are transported over long distances, eg, from the Ohio River Valley to New England (86—88). Models have also been used to show that sulfuric acid deposition is nearly linear in response to changing levels of emissions of sulfur dioxide (89). 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

J. S. Chang and co-workers. The Regional Acid Deposition Model and Engineering Model, State-of-Science—Technology Report 4, National Acid Precipitation Assessment Program, Washington, D.C., 1989. 

Concentration of acidic deposits had occurred at crevices between tubes and baffles. Sufficient wastage eventually accumulated at the baffle crevice and caused catastrophic failure. 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

Clean systems containing acidic deposits frequently. 

RGAR, Acid Deposition in the United Kingdom 1986-1988, Third Report of the United Kingdom... 

A receptor is something which is adversely affected by polluted air. A receptor may be a person or animal that breathes the air and whose health may be adversely affected thereby, or whose eyes may be irritated or whose skin made dirty. It may be a tree or plant that dies, or the growth yield or appearance of which is adversely affected. It may be some material such as paper, leather, cloth, metal, stone, or paint that is affected. Some properties of the atmosphere itself, such as its ability to transmit radiant energy, may be affected. Aquatic life in lakes and some soils are adversely affected by acidification via acidic deposition. 

The harmful effects of air pollutants on human beings have been the major reason for efforts to understand and control their sources. During the past two decades, research on acidic deposition on water-based ecosystems has helped to reemphasize the importance of air pollutants in other receptors, such as soil-based ecosystems (1). When discussing the impact of air pollutants on ecosystems, the matter of scale becomes important. We will discuss three examples of elements which interact with air, water, and soil media on different geographic scales. These are the carbon cycle on a global scale, the sulfur cycle on a regional scale, and the fluoride cycle on a local scale. 

Acid deposition refers to the transport of acid constituents from the atmosphere to the earth s surface. This process includes dry deposition of SO2, NO2, HNO3, and particulate sulfate matter and wet deposition ("acid rain") to surfaces. This process is widespread and alters distribution of plant and aquatic species, soil composition, pH of water, and nutrient content, depending on the circumstances. 

The impact of acid deposition on forests depends on the quantity of acidic components received by the forest system, the species present, and the soil composition. Numerous studies have shown that widespread areas in the eastern portion of North America and parts of Europe are being... 

When a forest system is subjected to acid deposition, the foliar canopy can initially provide some neutralizing capacity. If the quantity of acid components is too high, this limited neutralizing capacity is overcome. As the acid components reach the forest floor, the soil composition determines their impact. The soil composition may have sufficient buffering capacity to neutralize the acid components. However, alteration of soil pH can result in mobilization or leaching of important minerals in the soil. In some instances, trace metals such as Ca or Mg may be removed from the soil, altering the A1 tolerance for trees. 

This interaction between airborne acid components and the tree-soil system may alter the ability of the trees to tolerate other environmental stressors such as drought, insects, and other air pollutants like ozone. In Germany, considerable attention is focused the role of ozone and acid deposition as a cause of forest damage. Forest damage is a complex problem involving the interaction of acid deposition, other air pollutants, forestry practices, and naturally occurring soil conditions. 

Acid deposition and the alteration of the pH of aquatic systems has led to the acidification of lakes and ponds in various locations in the world. Low-pH conditions result in lakes which contain no fish species. 

Acid rain is the popular term for a very complex environmental problem. Over the past 25 years, evidence has accumulated on changes in aquatic life and soil pH in Scandinavia, Canada, and the northeastern United States. Many believe that these changes are caused by acidic deposition traceable to pollutant acid precursors that result from the burning of fossil fuels. Acid rain is only one component of acidic deposition, a more appropriate description of this phenomenon. Acidic deposition is the combined total of wet and dry deposition, with wet acidic deposition being commonly referred to as acid rain. 

The United States has established a National Acid Deposition Program (NADP), and Canada has established the CANSAP program, which consists... 

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