Acid Rain Chemistry

As described at the opening of this chapter, rainfall acidity arises from oxidation products of nitrogen oxides (NO and NO2, called NO ) and sulfur dioxide (SO2). This demonstration illustrates SO2 chemistry with a glass pH electrode suspended in the air space of a flask. 

Prepare three stoppered S(X)-mL flasks containing SO2, H2O2, or NH3. 

Prepare SO2 by winding a conical coil on the end of a copper wire and filling the coil with 20 mg of sulfur 

Monitor the glass electrode suspended in the humid flask with a pH meter, preferably interfaced to a computer to record pH versus time. After 1 min, place the electrode in the airspace of the SO2 flask for 3 s. Observe a sudden drop in pH. Remove the electrode so that SO2 can diffuse from the liquid film back into the air. The pH changes are shown in the grtqih. 

When faced with a weak acid, you should immediately realize that [H ] [A ] = x and proceed to set up and solve the equation 

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Consequently, an intensive uptake of sulfur occurs in hydrosphere, lithosphere, and biosphere. This is the main reason that the content of gaseous sulfur species in the atmosphere is rather small and even in polluted air does not exceed 2-3 ppmv. In unpolluted atmosphere the concentration of most S compounds is at ppbv levels, despite the intense sulfuroutgassing from the Earth s interior. The atmospheric content of the major gaseous S species, either SO2 or H2S, is highly variable and is influenced by both natural and anthropogenic factors. The role of anthropogenic sulfur emission in acid rain chemistry will be discussed in Chapter 10. The influence of natural parameters, microbiological activity in particular, is described in Box 7. 

Irwin JG, Williams NL. 1988. Acid rain Chemistry and transport. Environ Pollut 50 29-59. 

Clarke, A. G., and Radojevic, M., Oxidation of S02 in rainwater and its role in acid rain chemistry. Atmos. Environ. 21, 1115 (1987). 

Irwin, J.G. and Williams, M.L. 1988. Acid rain chemistry and transport. Environmental Pollution,... 

M = Al, Ga, In, Tl). The solution chemistry of Al in particular has been extensively investigated because of its industrial importance in water treatment plants, its use in many toiletry formulations, its possible implication in both Altzheimer s disease and the deleterious effects of acid rain, and the ubiquity of Al cooking utensils.For example, hydrated aluminium sulphate (10-30 gm ) can be added to turbid water supplies at pH 6.5-7.5 to flocculate the colloids, some 3 million tonnes per annum being used worldwide for this application alone. Likewise kilotonne amounts of A1(OH)2.5C1o.5 in concentrated (6m) aqueous solution are used in the manufacture of deodorants and antiperspirants. 

Atmospheric emissions of sulphur dioxide are either measured or estimated at their source and are thus calculated on a provincial or state basis for both Canada and the United States (Figure 2). While much research and debate continues, computer-based simulation models can use this emission information to provide reasonable estimates of how sulphur dioxide and sulphate (the final oxidized form of sulphur dioxide) are transported, transformed, and deposited via atmospheric air masses to selected regions. Such "source-receptor" models are of varying complexity but all are evaluated on their ability to reproduce the measured pattern of sulphate deposition over a network of acid rain monitoring stations across United States and Canada. In a joint effort of the U.S. Environmental Protection Agency and the Canadian Atmospheric Environment Service, eleven linear-chemistry atmospheric models of sulphur deposition were evaluated using data from 1980. It was found that on an annual basis, all but three models were able to simulate the observed deposition patterns within the uncertainty limits of the observations (22). 

FIGURE 7.19 Mobility of acid rain—a multimedia problem. Reprinted from Opportunities in Chemistry, National Aeademy Press, 1985. 

In the 20 years since the Brunddand report, great developments have taken place in industries toward sustainable practices. As a case in point, the problem of acid rain, an issue of concern in 1987, has improved to a large extent, thanks to catalytic pollution abatement both in stationary and automotive emissions. Catalysis for Green Chemistry and Engineering will continue to have a cracial role in improving the environmental performance of industry [25-27]. Nowadays, catalytic procedures are often implemented according to the green chemistry... 

Acid rain monitoring data in North America have been gathered by Environment Canada and stored in the National Atmospheric Chemistry (NatChem) Database, details of which can be found at www.airquality.tor.ec.gc.ca/natchem. Analysis of the deposition chemistry data has confirmed that wet sulfate deposition did indeed decline in concert with the decline in SO2 emissions in both eastern Canada and the... 

The literature on acid rain is simply vast. One of the better introductory texts is Acid Rain Its Causes and its Effects on Inland Waters, by B. J. Mason, Clarendon Press, Oxford, 1992. Dozens of Websites supplement and update Mason s book. For a general but more widely ranging survey of pollution and its legacy, try Pollution Causes, Effects and Control (fourth edition), edited by Roy M. Harrison, Royal Society of Chemistry, Cambridge, 2001. 

Sulfur has four unique characteristics related to its occurrence and chemistry in soil. As sulfate, it is one of the principle counterions that keep the soil electrically neutral. Soil receives constant additions of sulfur through volcanic activity around the world and industrial pollution, usually in the form of acid rain. This means that soils usually have sufficient sulfur for plant growth. Lastly, plants can take and use sulfur dioxide from the air as a source of sulfur for growth [22,38],... 

The extraction of soil with water at pH 7 would seem to be a good way to study the soil inorganic chemistry. The most common solvent that soils are in contact with is water in the form of rain. However, rain is not neutral but acidic. Rainwater pH ranges from 3.8 to 5.6, depending on the air in which it forms. Acid rain that contains H2S04 and HN03 created by the reaction of gases in the atmosphere with water can have a pH as low as 2.0 [2],... 

Johnson, R. W., and Gordon, G. E. (Eds.). 1987. The Chemistry of Acid Rain Sources and Atmospheric Processes, Symposium Series No. 349. American Chemical Society, Washington, DC. 

Many aspects of sulfur chemistry are of concern if we are to reverse the apparent trend to increasingly acid rain while continuing to use all... 

In an ideal troposphere, O3 would react with NO yielding NO2 and ultimately regenerating O3 [Equations (19) and (20)], thus no over-production of ozone could occur. However, in the presence of VOCs, the resultant peroxy radicals formed can compete with O3 by also reacting with NO to form excess NO2, thus resulting in the formation of excess ozone. This is just one example of the complexity of atmospheric chemistry peroxy radicals and NO , have substantial implications for reaction with climate gases, acid rain fonnation, and other aspects of air quality. Saunders et al. and Jenkin et al. have provided a wealth of information on the tropospheric degradation of aliphatic and aromatic VOCs. Additionally, the interested reader may wish to consult References 10-17 for further discussion of these important topics. 

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