Friday, 3 July 2015

Environment and Ecology: Part 3

Biogeochemical Cycles Contd.

Oxygen cycle
It describes the movement of oxygen within and between its three main reservoirs: The atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis.

Reservoirs  

Vast amount of molecular oxygen is contained in rocks and minerals within the Earth (99.5%) Only a small fraction has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.49%). The main source of oxygen within the biosphere and atmosphere is photosynthesis, which breaks down carbon dioxide and water to create sugars and oxygen:

6CO2 + 6H2O+ energy → C6H12O6 + 6O2



Atmospheric oxygen also comes from photolysis, where high energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component molecules. The free H and N atoms escape into space leaving O2 in the atmosphere:

2H2O + energy → 4H + O2
2N2O + energy → 4N + O2



The main way oxygen is lost from the atmosphere is via respiration and decay mechanisms in which animals consume oxygen and releases carbon dioxide.
The lithosphere also consumes free oxygen by chemical weathering and surface reactions. Eg)  formation of iron-oxides (rust)
4FeO + 3O2 → 2Fe2O3

Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create carbonate shell material (CaCO3) rich in molecular oxygen. When the organism dies, its shell is deposited on the shallow sea floor and buried over time to create limestone rock.

Weathering processes initiated by organisms can also free oxygen from the land mass. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.

NOTE : Factual Data

Storage Capacity of O: Lithosphere > Atmosphere > Biosphere
Residence time of O2      : Lithosphere > Atmosphere > Biosphere

Gain of atmospheric O2 (in descending order of contribution)
  1. Photosynthesis (land)
  2. Photosynthesis (ocean)
  3. Photolysis of N2O    
  4. Photolysis of H2O

Loss of atmospheric O2 (in descending order of contribution)
  1. Aerobic Respiration
  2. Microbial Oxidation
  3. Combustion of Fossil Fuel  
  4. Photochemical Oxidation
  5. Chemical Weathering
  6. Fixation of N2 by Lightning , Surface Reaction of O3
  7. Fixation of N2 by Industry
  8. Oxidation of Volcanic Gases


Ozone

Ozone or trioxygen(O3), an inorganic molecule, is an allotrope* of oxygen that is much less stable than the diatomic allotrope O2, breaking down in the lower atmosphere to normal dioxygen.

*Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner.

The highest levels of ozone in the atmosphere are in the stratosphere - ozone layer between about 10 and 50 km above the surface. It is present in low concentrations, nevertheless, the ozone layer is extremely important to modern life, as it absorbs harmful ultraviolet radiation.


1. Creation: Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges.

O2 + uv energy → 2O

Each oxygen atom then quickly combines with an oxygen molecule to form an ozone molecule:
O + O2 + uv energy → O3






2. Ozone-Oxygen Cycle / Chapman cycle: Ozone molecules formed, absorb radiation having an appropriate wavelength and triatomic ozone molecule becomes diatomic molecular oxygen plus a free oxygen atom
O3 + uv energy → O2 + O

The atomic oxygen produced quickly reacts with another oxygen molecule to reform ozone:
O + O2 → O3 + Heat energy

Thus, the absorbed solar energy also raises the temperature of the atmosphere within the ozone layer, creating a thermal barrier that helps trap the atmosphere below, as opposed to bleeding out into space.

3. Removal: if an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:
3 + O· → 2O2
And if two oxygen atoms meet, they react to form one oxygen molecule:
2O· → O2

The overall amount of ozone in the stratosphere is determined by a balance between production by solar radiation and removal. The removal rate is slow, since the concentration of O atoms is very low.

Ozone Depletion

Ozone depletion is largely a result of man-made substances. Certain free radicals like hydroxyl (OH), nitric oxide (NO) and chlorine (Cl) and bromine (Br), catalyze the recombination reaction, leading to an ozone layer that is thinner than it would be if the catalysts were not present. Most of the OH and NO are naturally present in the stratosphere, but human activities like emissions of chlorofluorocarbons (CFCs) and halons, has greatly increased the Cl and Br concentrations, leading to ozone depletion.





Chlorine, fluorine and carbon atoms are extremely stable. This extreme stability allows CFCs to slowly make their way into the stratosphere (most molecules decompose before they can cross into the stratosphere from the troposphere). This prolonged life in the atmosphere allows them to reach great altitudes where photons are more energetic. When the CFC's come into contact with these high energy photons, their individual components are freed from the whole.


Cl + O3 → ClO + O2
ClO + O. → Cl + O2
Overall reaction : O3 + O. → 2O2

Chlorine initiates the breakdown of ozone and combines with a freed oxygen to create two oxygen molecules. After each reaction, chlorine begins the destructive cycle again with another ozone molecule. One chlorine atom can thereby destroy thousands of ozone molecules. Because ozone molecules are being broken down they are unable to absorb any ultraviolet light so we experience more intense UV radiation at the earths surface. 

Ozone depleting substances (ODSs)

Substances which deplete the ozone layer and are widely used in refrigerators, airconditioners, fire extinguishers, in dry cleaning, as solvents for cleaning, electronic equipment and as agricultural fumigants.


The Montreal Protocol on Substances that Deplete the Ozone Layer - is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. Ozone depleting substances controlled by Montreal Protocol include:

Chlorofluorocarbons (CFCs) - used in refrigeration, air conditioning and heat pump systems
Halon - used historically as fire suppression agents and fire fighting
Carbon tetrachloride/ Tetrachloromethane (CCl4) - limited solvent use in laboratories and chemical and pharmaceutical industry
Methyl chloroform (CH3CCl3) - limited solvent use in laboratories and chemical and pharmaceutical industry
Hydrobromofluorocarbons (HBFCs) - historically used in fire suppression systems and fire fighting
Hydrochlorofluorocarbons (HCFCs) - used in refrigeration, air conditioning and heat pump systems
Methyl bromide (CH3Br) - historically used in fumigation, soil treatment, pest control, quarantine, market gardening
Bromochloromethane (CH2BrCl) - historically used in the manufacture of biocides.

The Ozone Hole

Refers to the reduced concentrations of ozone directly over the continent of Antarctica, that an enormous hole in the ozone layer had developed over the region (it had begun to develop in the mid 1970s)

Reasons :
As Antarctica is surrounded by water, winds over the continent blow in a unique clockwise direction creating a so called "polar vortex" that effectively contains a single static air mass over the continent. So, air over Antarctica does not mix with air in the rest of the earth's atmosphere. 

Antarctica has the coldest winter temperatures on earth. These chilling temperatures result in the formation of polar stratospheric clouds (PSCs) which are a conglomeration of frozen H2O and HNO3. PSCs form an electrostatic attraction with CFC molecules and other halogenated compounds

During spring, the PSCs melt in the stratosphere and release all of the halogenated compounds that were previously absorbed. In the antarctic summer, high energy photons are able to photolyze the halogenated compounds, freeing halogen radicals that then catalytically destroy O3. As Antarctica is constantly surrounded by a polar vortex, radical halogens are not able to be diluted over the entire globe. The ozone hole develops as result of this process.

In years with a strong polar vortex, the ozone hole is seen to expand in diameter, whereas in years with a weaker polar vortex, the ozone hole is noted to shrink.

Low level ozone / tropospheric ozone

It is an atmospheric pollutant, formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides. Ozone reacts directly with some hydrocarbons and thus begins their removal from the air, but the products are key components of smog.

Ozone photolysis by UV light leads to production of the hydroxyl radical HO•, which plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such asperoxyacyl nitrates, which can be powerful eye irritants.

Ozone acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.

The atmospheric lifetime of tropospheric ozone is about 22 days.










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