Acid deposition sources

Fig. 10-11. The pH scale is a measure of hydrogen ion concentration. The pH of common substances is shown with various values along the scale. The Adirondack Lakes are located in the state of New York and are considered to be receptors of acidic deposition. Source U.S. Environmental Protection Agency, Acid Rain—Research Summary," EPA-600/8-79-028, Cincinnati, 1979.

A version of this chapter was published as Driscoll, C.T., Lambert, K. F. and Chen, L. (2005). Acidic deposition Sources and effects. In Encyclopedia of Hydrological Sciences, Anderson, Malcolm G. (Ed.), John Wiley Sons Ltd. Chichester. Reproduced with permission. 

Innes, J.L. (1989). Acid rain and trees. In, Acid deposition. Sources, effects and controls, ed. J.W.S. Longhurst, 229-242. British Library, London. 

On this topic, J.W.S. Longhurst also edited the volume Acid Deposition Sources, Effects and Controls , published jointly by the British Library and Technical Communications in 1989. 

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. 

The formation of acidic deposition is largely from the combustion of fossil fuels and the smelting of sulfide ores. Minor natural sources exist such as the formation of hydrochloric and sulfuric acid from gaseous volcanic eruptions. 

The documentation of regional level terrestrial consequences of acid deposition is complicated. For example, forested ecosystems m eastern North America can he influenced by other factors such as high atmospheric ozone concentrations, drought, insect outbreaks and disease, sometimes from non-native sources. However there is a general consensus on some impacts of acidic depositon on both soils and forests m sensitive regions. 

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). 

Demonstrations (see below) of the acidic properties of the oxides of nitrogen and sulfur show students some of the inorganic reactions that will be background for class discussions and homework assignments. The students are also expected to know the natural and industrial sources of each gas. They must also be reminded of chemistry topics they have already studied, such as gas behavior and equilibrium, that will provide relevant background for understanding acid deposition. 

Driscoll CT, Lawrence GB, Bulger AJ, Butler TJ, Cronan CS, Eagar C, Lambert KF, Likens GE, Stoddard JL, Weathers KC. 2001. Acidic deposition in the northeastern U.S. sources and inputs, ecosystems effects, and management strategies. BioScience 51 180-198. 

The inventory made by Buijsman in the Netherlands should be caracterised as a quite rough estimate. This is demonstrated by the assumptions on which he based his calculations. For instance he neglected the emission from stables for laek of reliable data. If the Government wants to reduce the acid precipitation, there will be paid attention to ammonia. As it is released from ground level sources reduction of ammonia emission has more effect on acid deposition in the region than in the case with S02 and NOx. 

It has been observed in this laboratory, as in many others, that compounds, which volatilize from the mass spectrometer direct insertion probe at a certain temperature under electron impact conditions, will often volatilize at a lower temperature (50°C or more) if the analysis is carried out under chemical ionization conditions with reagent gas sweeping around the probe tip and then into the source. For sulfuric acid deposited on a Fluoropore filter, a decrease in volatilization temperature of approximately 70°C has been observed for sulfuric acid when the analysis is carried out in the chemical ionization mode. The elution profile is sharper and better defined than that obtained under electron impact conditions. 

Elsewhere acid deposition tends to be more of a local problem. For example, in the western states, oxides of nitrogen produced by motor vehicles are a more important source of acid rain than is sulfur dioxide. Acid deposition in California, then, is more a consequence of extensive automobile traffic than of power generation. 

In short, while the focus has been primarily on sulfuric and nitric acids as a source of acid deposition, it is clear that organic acids can also contribute significantly. The gas-phase concentrations of the simplest carboxylic acids, formic acid and acetic acid, are relatively high even in remote regions, of the order of a ppb. Both natural and anthropogenic sources have been... 

Given that the source of oxidants for S02 in both the gas and liquid phases is the VOC-NO chemistiy discussed earlier and that a major contributor to acid deposition is nitric acid, it is clear that one cannot treat acid deposition and photochemical oxidant formation as separate phenomena. Rather, they are very closely intertwined and should be considered as a whole in developing cost-effective control strategies for both. For a representative description of this interaction, see the modeling study of Gao et al. (1996). 

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