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Acid Deposition
Acid deposition is
a
general
term that includes more than simply acid rain. Acid rain suggests liquid
precipitation that has a more acidic level than normal. However, dry acidic
particles also can settle out of the atmosphere, and acidic vapors can interact
directly with plants and structures at the earth's surface. So to
broaden the
concept beyond acid rain, we use the term acid deposition to include both wet and
dry deposition of particles and capture of acidic vapors at the earth's surface.
Anthropogenic air pollutants can be created as gases or vapors, solids, or liquids and may be transformed to other states while in the atmosphere. For instance, SO2 is a gas emitted as a byproduct from burning coal and leads to the formation of sulfate (SO4) particles in the atmosphere, which combine with H2O to produce H2SO4, which is sulfuric acid. Oxides of nitrogen (e.g., NO, NO2) are gases produced by high-temperature combustion from, for instance, automobiles and power plants follow a similar pathway to become nitric acid in the atmosphere. These substances may be removed from the atmosphere either as dry or moist particles or as vapors or they may remain in the atmosphere and attach to naturally occurring precipitation particles then be rained out or snowed out.
Figure 1 sketches the stationary (e.g., smokestacks) sources, but we also have mobile (e.g., trucks, cars) sources that produce substances leading to acid deposition and how they are transported and transformed to become part of dry deposition or wet deposition after combining with atmospheric H20 either in cloud processes or scavenged by precipitation.
When these acidic materials reach the earth's surface they may lodge on plants or in the soil. Many eventually enter waterways and lead to a rise in the acidic level of streams, rivers, marshes and lakes. In some cases, such as Iowa, soils are slightly deficient in nutrients such as sulfur (for optimum agricultural production) and so a little additional sulfur being deposited on Iowa soils is not a detriment to agriculture production, in fact it may even be considered to have a positive effect. However, it may have a negative effect if it leaves the atmosphere as rain on the plants or structures. The point to be made is that there may be both positive and negative effects of acid deposition, depending on the subsystem of atmosphere/biosphere/lithosphere being considered in isolation.
Lakes and ponds tend to become repositories for some of these acids that fall as rain and are transported by surface run-off to standing water bodies. Sometimes these acid surges enter ponds at vulnerable times such as in spawning periods in the spring when a snow melt releases acidic material from accumulated deposition on snowpack over the winter.
Natural sources of acidic material, such as volcanoes and sea spray, produce a natural acidity to normal precipitation.
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