A broad term referring to a mixture of wet and dry deposited material from the atmosphere, which contains higher than normal amounts of nitric and sulphuric acids.
Acid rain formation results from both natural sources, such as volcanoes and decaying vegetation, and man- made sources, primarily emissions of sulphur dioxide and nitrogen oxides resulting from fossil fuel combustion.
Acid rain occurs when these gases react in the atmosphere with water, oxygen, and other chemicals and form various acidic compounds. A mild solution of sulphuric acid and nitric acid is created as a result.
Wet deposition refers to acidic rain, fog, and snow. If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to the ground in the form of either rain, snow, fog, or mist. When this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depends on several factors, including how acidic the water is; the chemistry and buffering capacity of the soils involved; and the types of fish, trees, and other living things that rely on the water.
In areas where the weather is dry, the acid chemicals may become incorporated into dust or smoke and fall to the ground through dry deposition. They stick to the ground, buildings, homes, cars, and trees. Dry deposited gases and particles can be washed from these surfaces by rainstorms, which lead to increased runoff. This runoff water makes the resulting mixture more acidic. Around half of the acidity in the atmosphere falls back to earth through dry deposition.
In the forest, a spring shower falls through the trees to the forest floor below. Some trickles over the ground and runs into streams, rivers, or lakes, and some of the water soaks into the soil. That soil may neutralise some or all of the acidity in acid rainwater. This is known as buffering capacity.
Differences in soil buffering capacity are an important reason why some areas that receive acid rain show a lot of damage, while other areas that receive about the same amount of acid rain do not appear to be harmed at all. The ability of forest soils to resist, or buffer, acidity depends on the thickness and composition of the soil, as well as the type of bedrock beneath the forest floor.
Acid rain doesn’t kill trees directly. Instead it damages their leaves, limits the nutrients available to them, or exposes them to toxic substances slowly released from the soil. Quite often, injury or death of trees is a result of these effects of acid rain in combination with one or more additional threats.
Scientists know that acidic water dissolves nutrients and helpful minerals in the soil and washes them away before trees and plants can use them to grow. Acid rain causes the release of substances that are toxic to trees and plants, such as aluminium, into the soil.
Scientists believe that this combination of loss of soil nutrients and increase of toxic aluminium may be one way that acid rain harms trees. Such substances also wash away in the runoff and are also released from the soil when the rainfall is more acidic.
However, trees can be damaged by acid rain even if the soil is well buffered. Forests in high mountain regions are often exposed to great amounts of acid because they tend to be surrounded by acidic clouds and fog that are more acidic than rainfall.
Scientists believe that when leaves are frequently bathed in this acid fog, nutrients in their leaves and needles are stripped away. This loss makes trees more likely to be damaged by other environmental factors, particularly cold winter weather.
Acid rain can harm other plants in the same way it harms trees. Food crops are not usually affected by acid rain because farmers frequently add fertilisers to the soil and by doing this, they replace the nutrients that have washed away.
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