This article throws light upon the top five cyclic processes of the atmosphere. The cyclic processes are: 1. The Hydrological Cycle 2. The Oxygen Cycle 3. The Nitrogen Cycle 4. The Carbon Cycle and 5. The Sulphur Cycle.
Cyclic Process # 1. The Hydrological Cycle:
Figure 2.4 shows a simplified version of the hydrological cycle. Water evaporates from land and water surfaces into the atmosphere.
Some water vapour is emitted because of transpiration. Combustion of fossil fuels, agricultural residues, forest residues and volcanic eruptions also cause emission of water vapour into the atmosphere.
The total amount of emission from the above mentioned sources is relatively small compared to that due to evaporation from land and water bodies.
In the atmosphere water vapour condenses to form cloud and finally precipitates as rain and snow. Some of the precipitates get stored in ponds, lakes, etc. and some run off to rivers and finally to the oceans. The remaining portion percolates the ground.
A part of this water is utilized by vegetation for their metabolic processes and the remaining portion finally ends up as ground water. Human beings utilize surface as well as ground water for domestic, agricultural and industrial purposes. Most of it is returned back to nature with impurity level higher than what was present initially at the source.
Cyclic Process # 2. The Oxygen Cycle:
The two essential life-sustaining materials are oxygen and water. Most of the water taken in by the living beings is returned back to nature as water. But the oxygen taken in by them is converted to CO2 and exhaled, Moreover, oxygen used for industrial processes is converted to oxides, such as H2O, CO2, CO, NOx, SO2 and other compounds containing oxygen.
Natural processes, like crustal oxidation and bacterial action also produce oxides. The only process by which molecular oxygen is regenerated is the Photosynthesis process. During photosynthesis vegetation’s take up CO2 and water and release oxygen according to the reaction:
A simplified version of the oxygen cycle is shown in Fig. 2.5.
Cyclic Process # 3. The Nitrogen Cycle:
Nitrogen is one of the basic ingredients (amino-acids) of all living beings. The only form in which nitrogen can be assimilated by living beings is ammonia and its compounds. Atmospheric molecular nitrogen is converted to ammonia and other nitrogenous compounds by various agencies.
These are then assimilated by vegetation and microorganisms and converted to proteins. Herbivores and carnivores cannot synthesize proteins. Herbivores get proteins from plants and carnivores get them from herbivores.
Bacterial decomposition of dead vegetation and animals converts proteins to ammonia, a part of which is taken up by vegetation for protein synthesis. Of the remaining portion a part escapes to the atmosphere and the rest is leached away. Some bacteria convert nitrates to nitrites to nitrogen.
Industrial operations, particularly combustion processes produce nitrogen oxides (NOx) which are finally converted to nitric acid (HNO3) as a result of tropospheric reactions. Nitric acid thus produced is precipitated as nitrates or acid rain.
Atmospheric nitrogen through industrial processes is converted to ammonia, which is then converted to nitrogenous fertilizers.
In a simplified way the nitrogen cycle is depicted in Fig. 2.6
Cyclic Process # 4. The Carbon Cycle:
Various compounds of carbon are emitted into the atmosphere. The major portion is emitted as CO2 and CO. The other carbon compounds emitted are hydrocarbons and various natural and synthetic organic compounds including halogenated compounds.
CO is oxidized to CO2 in the atmosphere. A large fraction of the emitted CO2 as well as that produced as a result of atmospheric oxidation of the emitted carbon compounds take part in the photosynthesis reaction. Another part is precipitated as CO2 dissolved in rain water. The remaining portion accumulates in the atmosphere.
The rate of accumulation of CO2 is increasing since the beginning of the ‘Industrial Revolution’. The concentration of CO2 in the atmosphere has increased during the last one hundred years from about 290 ppm to about 320 ppm. The recent rate of increase is approximately 0.7 ppm/year. It is apprehended that, the ‘greenhouse effect’ resulting from progressive build-up of CO2 in the atmosphere would cause global warming.
Hydrocarbons and other organic compounds emitted during industrial operations and natural processes are oxidized to aldehydes, acids and various other oxygenated compounds. These are subsequently wet precipitated. However, the rate of transformation of the organic compounds in the troposphere being slow, their tropospheric concentrations are on the rise. Like CO2 these compounds also cause ‘greenhouse effect’. In Fig. 2.7 a simplified version of the carbon cycle is depicted.
Cyclic Process # 5. The Sulphur Cycle:
The sulphur compounds released to the atmosphere are SO2, SO3, mercaptans, sulphuric acid and sulphates. Sulphur in h2s and that in mercaptans is oxidized to SO2. SO2 is further oxidized to so3, which is finally converted to h2so4.
Some of the h2so4 combine with air-borne alkaline particles and form sulphates. Free sulphuric acid and sulphates are precipitated on land and water bodies. Such deposits lower the pH of the receiving bodies. Acid deposits on land may interfere with vegetation growth. Aquatic life may also get affected at low pH.
H2SO4 and sulphates, which infiltrate the soil, are partly converted to SO2 and H2S due to bacterial action. These gases find their way to the atmosphere. The remaining sulfates are leached away and finally end up in surface and ground water. The sulphur cycle is shown in Fig. 2.8.
