Acid deposition source area

Acid deposition has been known to exist since early in the industrial age. The principle pollutants responsible for the elevated levels of acidity are the oxidized forms of sulphur and nitrogen that have been emitted as by-products from non-ferrous smelters, fossil-fueled power generating stations, and motor vehicles. The pollutants are transported substantial distances from the source areas by the atmosphere. They are deposited on receptor regions remote from the sources as acidic rain, snow, and fog or as gasses and dry particulates. 

As treated in other chapters of this book, air masses often transport acidic pollutants thousands of kilometres from their original source prior to deposition. Because air mass and storm movements tend to follow regular patterns, there is a strong linkage between the sources of pollutants and the areas that receive the acidic deposition. In eastern North America, the air mass movements and storm tracks are, on the average, from southwest towards the northeast. This serves to carry the emitted pollutants from the industrial "heartland over the more rural and comparatively pristine area of the northeast United States and southeastern Qmada (14). The spatial distribution of sulphate deposition over the eastern United States and Canada in 1980 is shown in Figure 4 (17). 

NO in combination with SOp has a synergistic corrosion effect especially indoors on electrical contact materials, copper and steel. The influence of acid precipitation may differ for different metals and depends also on the pollution level. The atmospheric corrosion of metals due to acid deposition is in most regions mainly a local problem restricted to areas close to the pollution source. 

The atmospheric corrosion of metals caused by acid deposition is mainly a local problem restricted to areas close to the pollution source. 

If the receptor region is, for example, the Adirondack Mountains region of New York State, a possible specific question of the overall source-receptor problem would be which states contribute to acid deposition in the area. One could make the question even more specific, by asking what fraction of the sulfate deposition in the Adirondacks is emitted as SO2 in the state of Ohio. The development of source-receptor relationships is a key policy question associated with acid deposition. 

The source-receptor relationships we just discussed, if available, tell us the fraction of acid deposition at a receptor that results from emissions of a particular source over a given averaging time. While this information is valuable, we would like to know something more in order to design emission control strategies. What we need to calculate is how much deposition of, say, sulfate at a receptor site will be reduced if S02 emissions by a certain source are reduced by a certain amount. Let us use as an example the estimate presented in the previous section. Assume that the utility S02 emissions in the Lower Ohio Valley are reduced by 50% (cut in half). This area as of about two decades ago (according to the RADM results) appeared to contribute on average 1.8kg(S)ha 1 yr 1 to the sulfur deposition on the Adirondacks. What would be the contribution after the emission reductions ... 

Unfortunately, development of reliable source-receptor relationships remains a challenging task. The magnitude of the task may be appreciated by envisioning North America, an area of about 2500 by 2500 km gridded in cells of 250 by 250 km. This results in some 100 cells, each of which is in principle both a source and a receptor of acidity. The source-to-receptor relation is the contribution of each source to acid deposition at each receptor. Therefore one needs to quantify 10,000 elements of the source-receptor matrix. 

Trends in acidic deposition mirror emission trends in the source area. For example, over the past 30 years in eastern North America, sulfate deposition has declined but nitrogen and ammonium deposition have remained relatively stable (Figure 3.3). Decreases in precipitation sulfate have coincided... 

Atypically, there are a few materials which occur in an elemental form. Perhaps the most notable example is sulphur, which occurs in underground deposits in areas such as Louisiana, Southern Italy and Poland. It can be brought to the surface using the Frasch process in which it is first melted by superheated steam and then forced to the surface by compressed air. This produces sulphur of high purity. Substantial quantities of sulphur are also removed and recovered from natural gas and crude oil (petroleum). This amounted to 24 million tonnes out of a total world sulphur production of 37 million tonnes in 1991, and clearly demonstrates the vast scale on which the oil and petrochemical industries operate since crude oil normally contains between 0.1 and 2.5% of sulphur, depending on its source. Desulphurization of flue gases from some U.K. power stations will be another source of sulphur in the future. Over 80% of all sulphur is converted into sulphuric acid, and approximately half of this is then used in fertilizer manufacture. 

Acid deposition is a global phenomenon with its effects most clearly experienced in the industrial nations of the northern hemisphere. However as papers in this volume illustrate, the effects are now occuring in developing nations as well. The precursors to acid deposition are emitted to the atmosphere from natural and anthropogenic sources, of the latter transportation and power generation are the most important sources. Transport and transformation reactions in the atmosphere distribute the emissions over large areas. 

The ores of most importance are fluorspar, CaF2 fluorapatite, Ca (P0 2Fj cryoHte [15096-52-3], Na AlF. Fluorspar is the primary commercial source of fluoiine. Twenty-six percent of the world s high quaHty deposits of fluorspar are ia North America. Most of that is ia Mexico. United States production ia 1987—1991 was 314,500 metric tons, most of which occurred ia the Illinois-Kentucky area. Imported fluorspar ia 1990—1991 represented about 82% of U.S. consumption 31% of U.S. fluorspar imports were from Mexico and 29% from China compared to 66% from Mexico ia the 1973—1978 period. The majority of the fluorine ia the earth s cmst is ia phosphate rock ia the form of fluorapatite which has an average fluorine concentration of 3.5%. Recovery of these fluorine values as by-product fluorosiHcic acid from phosphate production has grown steadily, partially because of environmental requirements (see Phosphoric acid and THE phosphates). 

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