Fossil fuels, acidic atmospheric deposition

In spite of the occurrence of natural events such as the eruption of Krakatoa, scientists are now well aware that human activities can have serious long-term effects on the Earth s atmosphere. The hrst such effect to be noticed historically was the increase in acid precipitation resulting from the combustion of fossil fuels. Acid precipitation is also known as acid rain or acid deposition. The second, discovered in the mid-20th century, was the depletion of stratospheric ozone. More recently, atmospheric scientists established a link between so-called greenhouse gases and global climate change. 

Selection of pollution control methods is generally based on the need to control ambient air quaUty in order to achieve compliance with standards for critetia pollutants, or, in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. For instance, tall stacks which disperse effluent modify the transport of pollutants and can thus reduce nearby SO2 deposition from sulfur-containing fossil fuel combustion. Although better dispersion aloft can solve a local problem, if done from numerous sources it can unfortunately cause a regional one, such as the acid rain now evident in the northeastern United States and Canada (see Atmospheric models). References 3—15 discuss atmospheric dilution as a control measure. The better approach, however, is to control emissions at the source. 

Acid deposition occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to the earth in rain, fog or snow. Approximately 20 million tons of SOj are emitted annually in the United States, mostly from the burning of fossil fuels by electric utilities. Acid rain damages lakes, harms forests and buildings, contributes to reduced visibility, and is suspected of damaging health. 

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. 

Acid deposition acid rain a form of pollution depletion in which pollutants, such as nitrogen oxides and sulfur oxides, are transferred from the atmosphere to soil or water often referred to as atmospheric self-cleaning. The pollutants usually arise from the use of fossil fuels. 

Sulfuric acid plays a major role in air quality and is a primary contributor to acid deposition. The combustion of fossil fuels, which contain sulfur as an impurity, results in the production of sulfur oxides. Sulfur oxides react with water in the atmosphere to produce sulfuric acid, but they may also undergo other reactions leading to dry deposition. Clean air is slightly acidic, with a pH of approximately 5.6. The acidic conditions are primarily due to the presence of carbonic acid produced from the carbon dioxide present in the atmosphere. 

Scientists have discovered that air pollution from the burning of fossil fuels is the major cause of acid rain. Acidic deposition, or acid ram, as it is commonly known, occurs when emissions of sulfur dioxide (SO ) and oxides of nitrogen (NOx) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds. This mixture forms a mild solution of sulfuric acid and nitric acid. Sunlight increases the rate of most of these reactions. 

The OECD project showed that the acidification of precipitation was due to an increased content of sulphuric and nitric acid. The main cause was identified as the increased use of fossil fuels, including motor vehicle traffic. In the atmosphere, sulphur dioxide and nitrogen oxides from combustion processes are oxidized to sulphuric acid and nitric acid, which are taken up and deposited by the precipitation. 

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