Rainwater, acid rain

The HNO3 may be washed out of the air by rainwater (acid rain), or it may react with traces of NH3 in the air to form solid NH4NO3, 2, particulate pollutant. 

Rainwater is naturally slightly acidic due to the dissolved carbon dioxide. Acid rain results when acidic sulfur and nitrogen oxides produced during the combustion of coal and oil react with rainwater (see Box 10.1). 

Anthropogenic Modifications of the Acid-Base Balance of Rainwater Alkalinity in Cloud Water "Acid Rain"... 

Sulfur Dioxide. Sulfur dioxide and nitrogen oxides, which are produced in modern society when coal, gas, and oil are burned in cars, power plants, and factories, react with water vapor in the air to form acids that negatively affect organic materials and even metals and stone when dissolved in airborne rainwater, the oxides of sulfur and nitrogen are the main cause of the formation of acid rain (see below). 

As regards the pollutants monitoring, from the measurements available so far it could be concluded that acid rain is coming to be a major problem in Asia. In many industrially developed and new developed countries such as Japan, China, Taiwan, South Korea, Thailand etc., values of pH <5 are encountered at many sites, and they represent more than 50% of monitored rain events on a regional scale. In some developing countries of South-East Asia (Myanmar, Laos, Cambodia) most rainwater pH measurements tend to be around 5.6, the pH of natural rainwater, and the acid rain precipitation is mainly due to localized industrial pollution. There is some evidence that pH values below 5 at unpolluted sites may be due to the contribution of weak organic acids, such as formic and acetic acids (Radojevic, 1998). 

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],... 

Sulfuric acid is a stronger acid than sulfurous [pAa(l) < 0, p7fa(2) = 1.99 at 25 °C and infinite dilution] rain as acidic as pH 2.1 has been recorded at Hubbard Brook, New Hampshire, and the pH of water droplets in clouds can be as low as 1.5 (for comparison, the pH of rainwater saturated with atmospheric CO2 is about 5.6 at 15 °C). Acid rain destroys building materials (especially marble), kills fish and vegetation, accelerates metallic corrosion (Sections 16.5 and 16.7), and can be directly harmful to humans (e.g., it causes the alligator skin condition reported in Cubatao, Brazil). Sulfate rain is not completely without redeeming features, as many soils (e.g., in southern Alberta, Canada) are sulfur-deficient. On balance, however, its acidity is unacceptable, and sulfur oxide emissions must be controlled at the source. Several control measures are possible ... 

Acid rain threatens lakes and forests throughout the world. Monitoring the pH of rainwater is a critical component of programs to measure and reduce the production of acid rain. 

Acid Rain Acid rain, a second major environmental problem, results primarily from the production of sulfur dioxide, SC 2, that accompanies the burning of sulfur-containing coal in power-generating plants. Sulfur dioxide is slowly converted to SO3 by reaction with oxygen in air, and SO3 dissolves in rainwater to yield dilute sulfuric acid, H2SO4. 

Sulfur dioxide is slowly oxidized in the atmosphere to S03, which dissolves in rainwater to give sulfuric acid. The burning of sulfur-containing fuels is thus a major cause of acid rain (Section 9.9). In the laboratory, S02 is conveniently prepared by treating sodium sulfite with dilute acid ... 

Nitric acid (HNOs) in the atmosphere, formed from N02 gas dissolving in the rainwater, also contributes to the production of acid rain. 

Fossil fuels, such as oil, coal and natural gas, all contain some sulfur. When these fuels are burned they produce many different gases. Concern has grown in recent years about the effects of one of these gases, sulfur dioxide. When sulfur dioxide dissolves in rainwater it forms an acidic solution which has become known as acid rain. 

Rainwater is essentially free of mineral solutes. It is usually slightly acidic due to the presence of dissolved carbon dioxide, or more highly acidic because of acid rain-forming constituents. As a result of its slight acidity and lack of alkalinity and dissolved calcium salts, rainwater is chemically aggressive toward some kinds of mineral matter, which it breaks down by a process called chemical weathering. 

Manmade pollutants reach rainwater through the air. The best known is acid rain, which contains sulfur compounds. A large variety of other pollutants of urban life and industry are lifted into the atmosphere and washed down by rain. As clouds move inland or rise up mountains, their water condenses and gradually rains out. Isotopic fractionation increases the deuterium and lsO content of the condensed rain, depleting the concentration of these isotopes in the vapor remaining in the cloud. As the cloud moves on into the continent or up a mountain, the rain produced becomes progressively depleted in the heavy isotopes. Surveys have been developed into a source of information on distance of recharge from the... 

The actual measured pH of rainwater in equilibrium with atmospheric pC02 is somewhere around 5.6. Values of pH below 5.6 can either be due to an increased pC02 or to industrial emissions causing what is known as acid rain. As shown, the dissolution of C02 in water is an cause of acid water (Table 2.10). However, considering that soils and/or geologic systems are sources of HCOJ and COj", when water contacts soil or... 

Acid rain was at one time an important point of contention between the United States and Canada. Much of this acid was the result of the emission of sulfur oxides by coal-fired electricity generating plants in southern Indiana and Ohio. These sulfur oxides, when dissolved in rainwater, formed sulfuric acid and hence acid rain. How many metric tonnes of Indiana coal, which averages 3.5% sulfur by weight, would yield the H2S04 required to produce a 0.9 in. rainfall of pH 3.90 precipitation over a 104 mile2 area ... 

By the way, these dissolution and acid forming reactions of SO2 in rainwater are only the first step in forming acid rain. The last step is the oxidation of these sulfur species to form sulfate ions (SO4-). Thus, the form of the acid in the rain water is sulfuric acid (H2SO4), which is a stronger acid than sulfurous acid (H2SO3), and which tends to reduce the pH of the rain even more. 

The atmosphere is a major source of soil acidity. Even in unpolluted environments rainwater is slightly acidic, having a pH of about 5.7 due to the dissolution of atmospheric CO2 to form the weak carbonic acid (see Worked example 5.4). The CO2 concentration in the partially enclosed soil pore system can be significantly higher (typically up to about 10 times) than in the free atmosphere due to respiration of soil microorganisms and plant roots. This results in a lower pH. In areas affected by industrial pollution, sulfur dioxide and nitrogen oxides dissolve in rainwater to produce sulfuric and nitric acids (acid rain), which are both strong acids and cause even more acidity. 

In recent years large areas of the statue s left side have been observed to darken as if the patina were being removed. Scientists speculated that this darkening may be the result of acid rain, which converts brochantite to the more soluble antlerite, which is then washed off by rainwater. 

Sulfuric acid is the most wideiy produced industriai chemicai. it is aiso formed when suifur oxides, emitted into the atmosphere by burning fossii fueis high in suifur content, dissoive in water. This makes rainwater acidic, forming acid rain. 

Note also that because fossil fuels also contain some sulfur, burning them usually results in the formation of sulfur dioxide gas. This, when dissolved in falling rainwater, is one cause of acid rain . Some modem power plants try to desulfurise the gases before releasing them into the air. 

Acid rain Rainwater that has been rendered acidic from absorption of airborne nitrogen and sulfur oxides produced mainly by mankind. 

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