Basic Chemical Processes

A basic chemical process in the atmosphere is the oxidation of substances by atmospheric oxygen. Thus, sulfur dioxide (SO2) is oxidized to sulfur trioxide (SO3), and nitric oxide to nitrogen dioxide. Similarly, many organic substances are oxidized, for example, aldehydes to organic acids and unsaturated hydrocarbons. While pollutant clouds are transported and dispersed to varying degrees, they also age. Pollutant cloud aging is a complex combination of homogeneous and heterogeneous reactions and physical processes (such as nucleation, coagulation, and the Brownian motion). Chemically unlike species can make contact and further branch the complex pattern (see Figure 5.4.1). Table 5.4.1 summarizes the major removal reactions and sinks. Most of these reactions are not understood in detail.

Sulfur oxides, in particular SO2, have been studied with respect to atmospheric chemistry. However, an understanding of the chemistry of SO2 in the atmosphere is still far from complete. Most evidence suggests that the eventual fate of atmospheric SO2 is oxidation to sulfate. One problem that complicates understanding atmospheric SO2 processes is that reaction paths can be homogeneous and heterogeneous. Two processes convert SO2 to sulfate: cat-alytical and photochemical.

CATALYTIC OXIDATION OF SO2

In clear air, SO2 is slowly oxidized to SO3 by homogeneous reactions. However, studies show that the rate of SO2 oxidation in a power plant plume can be 10 to 100 times the clear-air photooxidation rate (Gartrell, Thomas, and Carpenter 1963). Such a rapid rate of reaction is similar to that of oxidation in solution in the presence of a catalyst.

SO2 dissolves readily in water droplets and can be oxidized by dissolved oxygen in the presence of metal salts, such as iron and manganese. The overall reaction can be expressed as:

catalyst

Catalysts for the reaction include sulfates and chlorides of manganese and iron which usually exists in air as sus-

From the Source

From the Natural Atmosphere:

From the

From the Source

From the Natural Atmosphere:

From the

Source:

From the Source:

c) Photochemical Chain Reactions—Principle of Smog Formation FIG. 5.4.1 Examples of chemical reactions in the atmosphere.

Source:

b) Oxidation From the Natural Atmosphere

From the Source:

c) Photochemical Chain Reactions—Principle of Smog Formation FIG. 5.4.1 Examples of chemical reactions in the atmosphere.

0 0

Post a comment