Chapter 4. Reactions of air pollutants in the atmosphere

Table of Contents

4.1 General notes
4.2 Reactions of atmospheric oxygen
4.3 General reactions in the troposphere and stratosphere

4.1 General notes

Chemical composition of the troposphere has a relative constant composition of gases like oxygen, nitrogen, carbon dioxide and other compounds (Sharma, 2007). The air is utilized by organisms in the process of respiration to liberate chemical energy from organic substances during oxidation. The presence of nitrogen, oxygen and carbon dioxide in the air is of great importance, because these compounds keep cycling in nature between organisms and their environment through various cycles (e.g., carbon, nitrogen and water cycles).

Oxygen (O2) is available in sufficient amount near to the surface (Figure 4.1), it becomes scarce only at higher altitude, deep in ground and in water soaked soil. Oxygen dissolved in water is used by aquatic organisms. Some of the oxygen is absorbed from the air, while some is also released by plants growing in water. The amount of oxygen dissolved in water depends on the temperature and hydrostatic pressure (depth of the water). Warm water has a lower capacity to dissolved oxygen than the cold one.

Chemical structure of oxygen molecule.

Figure 4.1: Chemical structure of oxygen molecule.

Carbon dioxide (CO2) provides carbon supply for the plants for photosynthesis. Atmospheric nitrogen (N2) is directly used by some nitrogen fixing bacteria of some plants’ roots. Plants differ in their requirement of aeration. Reduced aeration of soil brings about a number of morphological and physiological effects on plants. Excess of carbon dioxide in soil air may produce some toxic substances like hydrogen sulphide (H2S), bicarbonates of iron and manganese, acetic acid, oxalic acid and other organic acids. Sometimes carbon dioxide itself has toxic and lethal effects on plants and some soil organisms.

In aquatic habitants the medium is generally deficient in oxygen content. This is due to the fact that much of dissolved oxygen in water defuses in the atmosphere. Carbon dioxide is about 200 times more soluble in water than oxygen. The solubility of these gases also depends on temperature and salinity of the water. The solubility of both these gases decreases in water as a result of increase in temperature and salinity. Thus the oxygen content of lakes is closely related to their thermal stratifications and salinity.