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2. Mechanism of acid deposition

We should not think that acid deposition is simply rain that has pH value below 5.6. That reflects only a part of the acid deposition phenomena.
Pollutants such as sulfur dioxide and nitrogen oxides are emitted to the atmosphere when we burn fossil fuels such as oil and coal from boilers in factories and power plants as well as from automobile emissions. These pollutants are converted to sulfuric and nitric acids and come back to the ground (acid deposition). Acid deposition takes place in two processes.
One process is wet deposition. Sulfuric and nitric acids are incorporated into clouds, and fall onto the ground in the form of rain, snow and mists. When a large amount of acid are dissolved in, rain shows strong acidity. This phenomenon is known as acid rain. The other process, called dry deposition, occurs on fine and cloudy days. Airborne acids come directly through winds, and deposit themselves on trees, buildings, and even human respiratory systems.
These deposited acids increase acidity in soil, lake water etc., then affect living creatures such as trees and fish. Their impacts depend on the amount of acid deposited more so than the degree of acidity. In other words, a large amount of relatively weak (high pH) precipitation would cause more significant impact than a small amount of relatively strong acidic(low pH) rain. It is, therefore, quite important to consider not only pH but also the total amount of acid deposition.

Mechanism of Acid Deposition

Mechanism of Acid Deposition (Source:Hiroshi HARA)

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