Environmental Engineering Reference
In-Depth Information
Source: Environmental Protection Agency
Smog
The two common types of smog—photochemical smog and industrial smog—are formed from the interactions
of pollutants with the atmosphere, catalyzed by solar radiation. For example, to form photochemical smog, ni-
trogen dioxide, NO
2
, reacts with the heat of UV radiation from the sun. To form industrial smog, sulfur diox-
ide, SO
2
, absorbs ultraviolet radiation in the atmosphere. Sulfur dioxide can also produce aerosols, which are
solid particles and droplets suspended in the atmosphere.
Acid Deposition
Acid deposition, also known as acid precipitation or (most commonly) acid rain, occurs when chemical reac-
tions occur in the atmosphere between pollutant emissions and atmospheric components. Ultimately, acidic
(below 7 on the pH scale) particulate matter falls to Earth's surface as either as precipitation, particulate, or gas.
The main contributors to acid deposition are sulfur dioxide and nitrogen dioxide. In the atmosphere, NO
2
re-
acts to form nitrous and nitric acids, which can fall as acid rain. In addition, sulfur dioxide can react with water
vapor to form sulfuric acid (H
2
SO
4
) and sulfate salts, resulting in acid precipitation.
The following table shows basic chemical reactions of pollutants in the atmosphere and their production of
smog and acid precipitation.
