Acidity of precipitation

Nearly 80% of the atmosphere consists of elemental nitrogen. This nitrogen, collected from different altitudes, exhibits a constant isotopic composition (Dole et al. 1954 Sweeney et al. 1978) and represents the zero-point of the naturally occurring isotope variations. Besides the overwhelming predominance of elemental nitrogen, there are various other nitrogen compounds in the atmosphere, which play a key role in atmospheric pollution and determining the acidity of precipitation. 

DMS is photochemically oxidized in the atmosphere to methanesulfonic and sulfuric acids. These strong acids contribute, along with nitric and organic acids, to the natural acidity of precipitation. Recent problems with acid rain have aroused interest in the anthropogenic and natural sources of volatile sulfur compounds (2). 

CCN). Changes in the concentrations of CCN may alter the cloud droplet concentration, the droplet surface reflectivity, the radiative properties of clouds (cloud albedo) (2), and hence, the earth s climate (8-101. This mechanism has been proposed for the remote atmosphere, where the radiative properties of clouds are theoretically predicted to be extremely sensitive to the number of CCN present (ID). Additionally, these sulfate particles enhance the acidity of precipitation due to the formation of sulfuric acid after cloud water dissolution (11). The importance of sulfate aerosol particles to both radiative climate and rainwater acidity illustrates the need to document the sources of sulfur to the remote atmosphere. 

The mean acidity of precipitation in North America for the period 1976-79 ( ), is shown in Figure 9. The maximum values (i.e., minimum pH) are associated with the eastern part of the continent, and are closely related to the major emissions in the Ohio valley. In northeastern USA and Canada, both deposition and sulphate concentrations are at a maximum in summer. 

The acidity of precipitation is mainly governed by its content of sulphate, nitrate and ammonium ions. It is closely related to the chmical composition of the aerosols, and may to a large extent depend on the pathway of the polluted air masses. The effects of the acid precipitation are not simply related to the acidity of the precipitation, but are the result of complex interactions in which all the major ions in precipitation are of significance. 

The acidity of precipitation is commonly presented as pH. However, it is important to also examine all of the other ions in a sample to understand the effects on the environment. There has been a suggestion put forth to limit the sulfate deposition without regard to the precipitation acidity. It is presumed that a sulfate reduction will automatically result in higher pH values, and, thus, a double environment benefit will result. But without better quantification of all sources of sulfate and their relation to acidity, any reduction strategy may not bring about the desired result. 

The retrospective study has been formulated with a clear paradigm of the parameters of the corrosion system. It is assumed that the significant contributors to the corrosion of metals exposed to the atmosphere are water, sea salts, sulfur oxides, nitrogen oxides, and the acidity of precipitated water. 

An accelerated MIST test procedure has been developed which duplicates the mechanisms of corrosion of anodized aluminum trim in automotive environments. Blush and bloom of anodized aluminum automotive trim is more severe in environments with acid precipitation and this effect can be duplicated in an acidified MIST test procedure. Pitting of anodized aluminum is more prevelant in automotive environments with high chloride concentrations and this effect can be duplicated in a neutral chloride MIST test procedure. A change in the mechanism of corrosion of anodized aluminum trim from pitting to blush and bloom in chloride containing environments occurs in the pH range of 2 to 4. These results indicate that blush and bloom of anodized aluminum will become more severe as the acidity of precipitation increases. Thus more expensive trim materials such as bimetal are being used by the automotive industry. 

Galloway (J ) reports that the relative contribution of H2SO4, HNO3, and HCl to the acidity of precipitation is difficult to determine because the acids are not present as such in solution but rather as dissociated ions. However, using the absolute concentration of SO4, NO3 and Cl it is possible to determine their relative contribution. Likens and Bormann ( ) and Glass et al ( ) report that precipitation data for the northeastern United States indicate that 60 to 70% of the acidity in acid precipitation is due to sulfuric acid, 30 to 40% to nitric acid, and 5% to hydrochloric acid. For this reason, the following disucssion of the effects of acids on concrete will focus on effects of sulfuric acid. 

English scientists C. Crowther and H.G. Ruston demonstrate that acidity of precipitation decreases the further one moves from the center of Leeds, England. They associate these levels of acidity with coal combustion at factories in Leeds. 

Swedish scientist Hans Egner, working in the same vein of agricultural science as MacIntyre and Young, set up the first large-scale precipitation chemistry network in Europe. Acidity of precipitation is one of the parameters tested. 

The haze phenomenon in Southeast Asia is a major air pollution problem and its impacton the chemistry of precipitation is still unknown. It has been speculated that forest fires could raise the acidity of precipitation. While it is known that forest fires produce acid precursor gases such as SO2, NO and NO2, the non-carbonaceous matter of particles produced by these fires contains Ca and K, which give rise to alkalinity in rainwater. There is a need for a research project to study both the emissions from the forest fires in Southeast Asia and their impact on precipitation chemistry and rainfall acidity. 

Thus, the acidity of precipitation has increased during the latest decades in many developed and developing countries of Asia. There exist clearly transboundary air pollution problems. The harmful single and synergetic effects of acidity, SO2, NO ... 

The major foundations for our present understanding of acid rain and its effects were laid by Eville Gorham. On the basis of his research in England and Canada, Gorham showed as early as 1955 that much of the acidity of precipitation near industrial regions can be attributed to combustion emissions, that progressive acidification of surface waters can be traced to precipitation, and that the free acidity in soils receiving acid precipitation results primarily from sulfuric acid. 

Moller, D. and L. Horvath (1989) Estimation of natnral acidity of precipitation water on global scale. Iddjaras 93, 324-335... 

As the coming years will again be characterized by a sharp rise in SO2 emissions in The Netherlands, it is to be expected that the acidity of precipitation will Increase as well, possibly up to values exceeding the 1967 figure at De Blit when the pH was 3 78 on a yearly basis. In this paper an attempt will be made to indicate a relationship between the development of acid concentration of precipitation in The Netherlands and total interior emissions of SO2 and NO. ... 

The considerable increase in the acidity of precipitation in the years after World War II coincided with enormous industrial expansion and the nearly exponentially rising consumption of energy in Western Europe accompanied by the emissions of SO2 and NOx- During a period of 15 to 20 years, the pH of precipitation declined at many measuring locations of the IMI network, on a yearly basis, from 6 to 4, which means an increase in acidity by at least a factor of 100. At some locations the increase was even larger. 

To describe the acid precipitation phenomenon, a number of techniques have been used to quantify the acidity, identify the acidic and basic components and to identify the sources of acidity. The methodology used is often determined by the available data. Although all valid models should reach consistent conclusions, the sensitivity of each technique to random and systematic errors should be determined to provide confidence limits for the results. This paper examines the acidity of precipitation in southern California with the charge and conductivity balances, direct measurements, source strength calculations and scavenging models. Some of the limitations of each approach are also discussed. 

The accelerated decay is due to the increased acidity of precipitation in industrial environments. This precipitation, in equilibrium with atmospheric CO2J acquires a pH value of 5.6. But SO2 has caused the precipitation to locally achieve values as low as 2.1 in the northeastern (NE) United States (13, p. 1176). 

Nitrogen dioxide produced primarily during combustion processes by the oxidation of atmospheric nitrogen is the main cause of the acidity of precipitation in the Los Angeles Basin here, N0 is more than twice as concentrated as SOT (14, Table 1). 

Yet, controlled studies of weathering promise the generation of data usable,not only for the determination of weathering rates> but also for the reconstruction of the history of the acidity of precipitation. As shown earlier in the introduction, the absence of nitrates and sulfates in the weathered rock indicates that GO is the cause of decay this reveals that the pH of the precipitation has been nearly 5.6. Since the presence of NO2 and SO2 considerably reduce the pH of the precipitation, the occurrence, quantities, and the depth to which the weathered products have penetrated in dated stones (e.g. monuments in graveyards) may form bases for determinations ofactual acidity when the pH of the precipitation is below 5.6. It will, however, be necessary to determine the reaction rates in laboratory conditions at known acidity levels and correlate them with reaction rates in ambient conditions,both for stones which are directly exposed to precipitation and those which weather due to acid aerosols while protected under the dome. 

It cannot be concluded that direct atirospheric sources are responsible for elevated mercury levels in either lake water or fish flesh. However, it seems likely that the acidity of precipitation is sonehow modifying the cherioal nature of mercury present in Cranberry Lake water, availing the mercury to the food web. 

HICKS On the basis of the sulfur regional experiments, to what extent do you suspect that the acidity of precipitation is indeed regional ... 

There is an urgent need for more field experimentation work to assess the degree and location of possible damage and/or yield loss to susceptible crops. Better understanding is needed of the principal pollutant damage mechanisms for spruce and fir trees, and the responses of different catchment areas to changes in acidity of precipitation and to liming. 

Several analysts in this field take the view that the acidity of precipitation has increased over a substantial area of Western Europe. Both Grannat and Oden show that acidity has intensified and extended spatially over the period 1955-1970. Oden extended his analysis into the early 1970 s and concluded that between 1970 and 1973 pH values had stabilised at constant levels in many parts of Europe. 

The contributing role of nitrate is expected to grow as NO emissions continue to increase, whereas SO2 emissions are constant or falling. Also, as sulphates in the atmosphere occur mainly as ammonium sulphate, i.e. neutral, and nitrates occur as HNO3, i.e. acid, nitrogenous pollutants are likely to have a stronger effect on the acidity of precipitation than do sulphurous pollutants. 

In the preceding sections we have set out the strong circumstantial evidence supporting the relationship between acidification, of which acid precipitation is a major cause, and fish populations of lakes, principally in Scandinavia, Scotland and NE America, and also described likely mechanisms by which this process takes place. The question remains as to whether a strong correlation has actually been found between acidification levels and fish populations. This is clearly of importance in assessing the benefits likely to be realised in measures to reduce acidity of precipitation. 

B. Fisher, Deposition of Sulphur and the Acidity of precipitation over Ireland Atmospheric Environment, 16, 11, 1982. 

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