Gaseous composition of dry air.
| Constituent | Chemical symbol | % by volume |
|---|---|---|
| Nitrogen | N2 | 78.084 |
| Oxygen | O2 | 20.947 |
| Argon | Ar | 0.934 |
| Carbon dioxide | CO2 | 0.0350 |
• Also present are smaller amounts of Ne, He, CH4, Kr, H2 and in addition variable trace quantities of other pollutants such as NH3, SO2, CO, NO2, O3 and H2S. Water vapour present to about 4% in the atmosphere may not be uniformly distributed (may vary from time to time) but concentrated close to the oceans and large water bodies.
• The amount of N2 in the atmosphere is high because of its inertness due to the presence of strong N ≡N bond. Reactivity of O2 is greater than N2 and the amount of O2 is smaller than N2i n the atmosphere. Presence of O2 makes the atmosphere to be active and sustains life on the earth.
• CO2 and H2O are the main ingredients for photosynthesis.
Variation of atmospheric temperature, molar mass and pressure according to the altitude
• With the altitude mass is gradually decreasing and accordingly pressure is also coming down.But the temperature undergoes several inversions. Based on these thermal inversions atmosphere is divided into different regions. The region closer to the ground is the troposphere and the region above the troposphere is stratosphere. Ozone layer is located in the stratosphere.
• Water covers 70% of the earth’s surface. Very little of the world’s water is fresh water (2.6%).Most of the water (97.4%) is in the oceans. Most of the fresh water (76%) is frozen inglaciers and in the polar ice caps. Only a tiny fraction (0.01%) is available for human use.
Major cycles
• Knowledge of chemical cycles is very important to understand the fate of the chemicals, their abundance in different spheres, their possible environmental impacts and controlling the pollution problems.
Carbon cycle
• The only way that carbon gets into ecosystem is through photosynthesis.
• Animal gain carbon through their food.
• Decomposers get their carbon by digesting dead organisms.
• All living organisms return carbon to the air in the form of carbon dioxide through
respiration.
• If plants or animals die in situations where there are no decomposers (E.g. : deep
oceans) the carbon in them can get turned into fossil fuels over millions of years.
• The carbon in fossil fuels is released during the combustion.
• Microorganisms are important in the carbon cycle because they can quickly get the
carbon in dead material back into the atmosphere
Oxygen cycle
• Atmospheric oxygen is removed through combustion (chemical / biological) and respiration and replenished through photosynthesis.
• Most oxygen is stored in the oxide minerals of the earth crust and mantle but is bound to rocks and unavailable for use.
• Most available oxygen comes from photosynthesis and some is made in the atmosphere when sunlight breaks down (photolysis) water.
Nitrogen cycle
• Atmospheric nitrogen is fixed by bacteria. Some live free in the soil (Eg:- Azotobacter).Others like Rhizobium are found inside root nodules of leguminous plants (hat’s peas,beans and cloves). Atmospheric nitrogen is changed into ammonia, nitrites and then nitrates which all plants can absorb and use to make protein.
• Nitrogen in the plant proteins is passed onto animals through food chains.
• When living organisms die their nitrogen is returned to the soil in the form of ammonium compounds by micro organisms. Animals get rid of excess aminoacids via deamination in their liver. The nitrogen gets back into the soil via their urine.
• Ammonium compounds are changed into nitrates by nitrifying bacteria. Firstly nitrosomonas changes ammonium compounds into nitrites, then Nitrobacter changes the compounds into nitrates.
• Nitrates are converted back into atmospheric nitrogen by denitrifying bacteria like
Pseudomonas and Thiobacillus.
Hydrological cycle
• Optimum compositions of atmosphere, hydrosphere and earth’s surface are important on environmental equilibrium for sustainability of earth. If this equilibrium is disturbed the following problems could occur.
• Damaging effects on human health
• Damage on plants and hindrance of their growth
• Damage on marble buildings and metallic structures
• Increase in salinity / alkalinity
• Weathering of rocks
• Climatic changes (may cause drought / flood)
