Acidity of rain

Why Do We Need to Know This Material Chapter 9 developed the concepts of chemical equilibria in gaseous systems this chapter extends those ideas to aqueous systems, which are important throughout chemistry and biology. Equilibria between acids, bases, and water in plant and animal cells are vital for the survival of individual organisms. To sustain human societies and protect our ecosystems, we also need these ideas to understand the acidity of rain, natural waters such as lakes and rivers, and municipal water supplies. 

Research on air pollution is complex. Forests and prairies cover vast areas, and the interplay of regional air pollutants is so subtle that it may take years to sort out all the environmental stresses. However, controls being put in place are already beginning to reduce the acidity of rain in North America and F.urope. Such controls will help us maintain our quality of life without losing our precious heritage of native plants. We can also help by using automobiles less and bicycles more or by taking public transportation when we can. 

Johnson, C. A., and L. Sigg (1985), "Acidity of Rain and Fog Conceptual Definitions and Practical Measurements of Acidity", Chimia 39, 59-61. 

Acids and bases are two of the most common types of substances in the laboratory and the everyday world. We need to know how to recognize them, what their characteristic reactions are, and why they are such important chemicals. We shall see that keeping the concentrations of acids and bases in plant and animal cells within certain limits is necessary for the survival of individual organisms, and controlling the acidity of rain, of natural waters such as lakes and rivers, and of municipal water supplies is necessary to sustain human societies. 

All these conclusions have been made on the assumption that the sedimentary load of the rivers is constant through the whole period of accumulation of manganese in the deep sea. WedepohF believes that the weathering of the earth surface is stronger at present than it has been in the past, due to the pollution and the higher acidity of rain-water caused by pollution. 

The acidity of rains — due both to S04 and NO3 — formed in the atmosphere — which may bring the pH of lakes to figures of 5-6 and even 4.5 is not the sole factor responsible for the toxicity of water for fishes, phytoplankton and zooplankton. In effect there are several chemical processes due to the interaction of acid rains and the soil, as well as the water of lakes. First of all it may occur that in the water, owing to the presence of carbonic acid system HC03"/(Na) (C03 ) may act as buffer in many lakes. This is not the case of the lakes in Scandinavia because the soil is mainly constituted by granites. 

As human dependence on fossil fuels has increased, so has the acidity of rain. About 200 years ago, rain had a pH between 6 and 7.6, almost neutral. Today, it is common in many regions for rain to have a pH between 4 and 4.5, or even lower. In fact, the pH of rain in Wheeling, West Virginia, has been measured at 1.8, midway between the acidity of lemon juice and that of stomach acid. What do you think might account for this extreme acidity ... 

The arsenic penetrates deeply into the wood and remains there for a long time. However, some of the chemical may migrate from treated wood into surrounding soil over time and may also be dislodged from the wood surface upon contact with skin. The amount and rate at which arsenic leaches, however, varies considerably depending on numerous factors, such as local climate, acidity of rain and soil, age of the wood product, and how much CCA was applied. Interestingly, the leaching occurs more with newer structures and decreases with time. 

The mass balance for sulphur in Fig. 7.17b represents the various fluxes integrated over the whole globe. Because all the different sulphur compounds shown in Fig. 7.18 have atmospheric residence times (see Section 3.3) of only a few days and so are not well mixed, their distributions in the air are often inhomogeneous. Indeed, for any particular region of the atmosphere, it is likely that one of the major sulphur sources will dominate and thence determine the acidity of rain and aerosols. In general, for remote—particularly marine—areas, the DMS-S02-SOj route is likely to control, whereas close to urbanized/industrialized land,... 

Predict the effect of increasing acidity of rain on the rate of formation of limestone caves. 

We humans have added considerably to the levels of N02(j ) and S02(g) in our air, causing a steady increase in the acidity of rain. Coal, for example, contains a significant amount of sulfur when coal is burned, the sulfur is... 

JACOBSON The lettuce plants in the greenhouse were exposed to rain from a nozzle which simulated both the intensity and the droplet diameter of ambient rainfall. We found in previous experiments that it s not only the acidity of rain that s important in determining the development of foliar lesions or growth effects in addition, it s the duration and frequency of exposure, the intensity of rainfall, and, of course, the cultural conditions of the plant, which change their predisposition to injury. 

In dry deposition, SO2 increasingly predominates over NO2 as the major dry acid pollutant away from the source, although ozone may be a potentially critical pollutant at long distances from the sources. In areas where acid rain is dominant, sulphuric and nitric acids contribute in about a 70/30 ratio to overall acidity of rain (Section 4.4.6). It is probable that there is not a linear relationship between SO2 emissions and 804 deposition as the supply of oxidants/catalysts may be a limiting factor. Opinions differ on the degree of this effect for wet deposition (acid rain)(Section 4.3.2(iii)). 

The level of acidity from precipitation in Europe has been discussed earlier in Section 4.4.4 dealing with the acidity of rain. In the following sections we examine how this may be modified in the process of run-off to surface waters by catchments, and also its importance in relation to other sources of acid input. Also, the section examines the extent and mechanisms for damage to fish from acidification of surface waters. 

First, there would appear to be an apparent discrepancy in the fact that since 1974 acidity of rain in these areas has not increased (indeed sulphur emissions in Western Europe have fallen), but the trend to loss in fish populations in Scandinavia and Scotland has on the whole continued. This, on the face of it, can be explained by a continued leaching out of base cations and loss of buffering capacity from the top soils through which most of the run-off occurs, and the slow response time of fish populations in lakes to loss of spawning trout in tributory streams. However, at the same time an explanation must also be found for the following ... 

A. Mason, Sulphur and Nitrogen Contributions to the Acidity of Rain Atmospheric Environment, 12, 1978. 

The acidity of rain is very significant in zinc corrosion and, since the acidity occurs mainly from sulfur dioxide, further details are in Section II.B. Essentially, if rain is below about pH 5 (Fig. 2.7) the corrosion rate will be increased (see Fig. 1.11). Sulfur emissions are discussed by Likens et al. (1979) zinc is attacked only slightly by pure air, and zinc oxide forms, which is converted to hydroxide when moisture is present. Even if the moisture content is considerable, attack remains slight, but the hydroxide films formed (Schikorr, 1964 a,b) have a relatively minor protective effect. The zinc hydroxide reacts further with carbon dioxide in the atmosphere, forming a basic zinc carbonate. This film is very protective and is mainly responsible for the excellent resistance of zinc to ordinary atmospheres. 

In aggressive atmospheres the corrosion rate is linear with time. When zinc forms a protective patina in milder atmospheres the corrosion rate decreases with time. The corrosion rate increases with increases in sulfur compound in the atmosphere. Corrosion also increases as the time of wetness increases. The acidity of rain affects the corrosion rate, increasing the rate if the pH is less than 5. Sheltering from direct rainfall reduces the corrosion rate. 

The Clean Air Act, and its 1990 amendments, have provisions that target acid rain. These provisions force electrical utilities— which are the most significant source of SO2—to lower their SO2 emissions gradually over time ( Figure 14.22). The decrease in SO2 emissions has been significant, and the acidity of rain in the Northeast has already stabilized and should decrease in the coming years. Scientists expect most lakes, streams, and forests to recover once the pH of the rain returns to normal levels. 

Sulfur dioxide from the combustion of coal and from other sources appears to be a major cause of the marked increase in acidity of rain in the eastern United States in the past few decades.This acid rain has been shown to contain sulfuric and nitric acids.The SO is oxidized in moist, polluted air to H2SO4. Acid rain is discussed in the essay on acid rain at the end of Section 17.2. 

Although it is true that some SO also dissolves in water vapor to form sulfurous acid (H SOj), sulfurous acid is a very weak acid, so its effect on pH is much less than the effect of HjSO, which is a strong acid. The average level of HjSOj might increase the acidity of rain by about half a pH unit, whereas commonly observed levels of H SO can lower pH by several units. 

As precipitation chemistry is intimately related to atmospheric composition, air quality becomes a starting point to understand the acidity of rain water. An evaluation of atmospheric SQi content in Brazilian natural, industrial and urban areas is presented in Table 1. 

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