Source area acidic deposition

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. 

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 ... 

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... 

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. 

---