NOTES ON SEDIMENTARY FACIES

   Composition of atmosphere

*Composition of the atmosphere*

The earth's atmosphere is a multilayered gaseous envelope which surrounds the earth for all sides and is attached to the earth's surface by it.(earth's) gravitational force. The air is a mechanic mixture of several gases. Basically, the atmosph is composed of three major constituents,

 (1) gases

 (2) water vapour, and

 (3) aerosols

*Gases:-    The gaseous composition of the atmosphere consists of two categories of gases e.g. (i) constant or permanent gases, and (ii) variable gases. It may be mentioned that the study of atmospheric gases started in the 18th century when carbon dioxide was discovered and studied in 1752 followed by the discovery of gaseous nitrogen (N J by Rutherford in 1772 and was named as mephitic air. The oxygen atmosphere in parts per million (ppm), parts per billion (ppb) and percent by volume. The following gas laws have been formulated from time to time

(a) Boyle's gas law (pertaining to relation-

ships among air pressure, volume of gas and density of gas),

 (b) Lussac's law showing relationship among pressure, volume of gas and temperature,

 (c) Charle's law (showing relationship between pressure and temperature with constant gas volume), (d) combined gas law (to derive any one variable with two known variables) etc.

(i) The major constant gases are nitrogen, oxygen and argon which constitute 78, 20.9 and 0.9 percent by volume of the gaseous composition was discovered and studied first by Joseph Priestly and the last gas to be discovered in 1874 was argon.

The nitrogen and molecular oxygen constitute 7 8 .1 an(j 20.9 per cent of gaseous composition of the present day atmosphere respectively, while argon represents 0.9 per cent. Thus, it is obvious that gaseous nitrogen, molecular oxygen and argon constitute about 99.9 per cent of the gaseous composition of the atmosphere. 

Nitrogen accounts for the largest percentage of atmospheric gases but it is not as  much active as other constant gases. Nitrogen is  very important for all life-forms in the biosphere  because it is an essential part of amino acids which  make up proteins. Nitrogen generally exists in  seven forms in the atmosphere e.g. molecular  nitrogen (N2), Oxides of nitrogen (e.g. N^O =  nitrous oxides, NO = nitric oxide, and NO =  nitrogen peroxide) and hydrogen - nitrogen com- pounds (e.g. NH = amino, NH3= ammonia, and  H N 02= nitrous acids). Though nitrogen constitutes the largest proportion of atmospheric gases by volume, but living organisms cannot use nitrogen  directly rather they obtain nitrogen in the form of  ammonium salts and nitrate through their roots  from the soils while animals get nitrogen from the  plants and their products by eating them. The  nitrogen cycle involves

 (i) transfer of atmospheric  nitrogen into soils (known as nitrogen fixation), 

(ii) mineralization, nitrification and transfer ol  nitrogen from soils to plants and animals, and 

(iii) denitrification and return of nitrogen to the  atmosphere. 

*OXYGEN:-   Oxygen is chemically very active because it combines with majority of elements in the  biosphere. It generally forms about 70 per cent  atoms in living matter and plays a very important  role in the formation of carbohydrates, fats and  proteins. The molecular oxygen (0 2) mostly  occurs upto the height of 60 km in the lower  atmosphere while dissociated oxygen (0) is  present above the molecular oxygen layer. Oxy- gen is produced through the process of photosyn- thesis by autotrophic green plants of terrestrial  ecosystems and by phytoplanktons of marine  ecosystems and to a lesser extent by the reduction  of various mineral oxides. The oxygen is con- sumed in respiration by animals including man  and in industrial combustion (burning of fossil  fuels) and burning of wood. The proportion of  molecular oxygen in the atmosphere is likely to  decrease due to anthropogenic factors and gradual  increase in the consumption of oxygen effected by  increased burning of fossil fuels and wood and  deforestation resulting into increase in the atmos- pheric carbon dioxide due to less consumption of  C 0 2 and decrease in the production of 0 2 may  cause imbalance in the gaseous composition of the  atmosphere.

 (1) Variable or minor gases include water
vapour, carbon dioxide, ozone, hydrogen, helium, neon, xenon, krypton, methane etc. Water vapour is very important because it is responsible for all
sorts of precipitation whether in liquid or solid form. Ozone (0 3) is life saving gas because it filters the incoming shortwave solar radiation and absorbs ultraviolet rays and thus protects the earth
from becoming too hot. Ozone defined as 'a three- atom isotope of oxygen (0 3)' or ’merely a triatomic form of oxygen ( 0 3), is a faintly blue irritating gas with a characteristic pungent odour'. Ozone is 'a strong oxidizing agent which can at high concen-tration decompose with an explosion’. It may be pointed out that ozone is present almost at all altitudes in the atmosphere but the bulk of its concentration is present in a layer from 10 km to 50 km up in the atmosphere and within this zone the highest concentration of ozone is between the altitudes of 12 km and 35 km in the stratosphere. Ozone gas is unstable because it is created as well as destroyed or disintegrated. The oxygen mol-ecules are broken up or separated in the atmos-pheric layer between the altitudes of 80 to 100 km by ultraviolet solar radiation or by an electric discharge in oxygen or air during a thunderstorm in the troposphere in the following manner :

0 2 -> O + O

(Oxygen breaks up in two separate oxygen molecules)

These separated oxygen atoms (O) are then combined with oxygen molecules ( 0 2) and thus ozone (0 3) is formed.

0 2 + O + M —> 0 3 +M 

or 0 2 + O -> 0 3 (Ozone)

where M denotes energy and momentum balance produced by the collision of oxygen molecules (0 2) with another individual atom or molecule. It is important to note that most of the atmospheric ozone is formed in the atmosphere over tropical areas from where some ozone is transported by atmospheric circulation to the polar areas up in the atmosphere. Ozone ( 0 3) is also transformed back into oxygen by further collision of ozone with monoatomic oxygen (individual molecule of oxygen, O) in the following manner:

Ozone (0 3) + O -» Oxygen ( 0 2) + Oxygen (0)

The presence of ozone layer in the atmos-phere is very crucial and significant for plants and animals in general and human being in particular because it provides a protective cover, known as earth's umbrella, to all organisms (including plants, animals, micro-organisms and man) in the biospheric ecosystem against their exposure to ultraviolet solar radiation. In fact, the ozone layer filters the solar radiation by absorbing ultraviolet  rays and allowing only those radiation waves to  reach the earth's surface which are essential for the  maintenance of life on the planet earth.Any  change in the equilibrium level of ozone in the  atmosphere will adversely affect the life in the  biosphere. There has been much hue and cry about  the depletion of ozone since 1987. The ozone  depletion refers to breaking of 0 3 into 0 2 + O. The  main culprits of ozone depletion are halogenated  gases (e.g. chlorofluorocarbons), halons and  nitrogen oxides.

*Carbon dioxide - (an example of variable gas)represents 0.03 per cent of the total atmospheric gases. The gaseous carbon (C 02) plays two  significant roles e.g. (i) carbon dioxide helps inthe process of photosynthesis wherin carbon,hydrogen, and oxygen are combined by theautotrophic green plants of terrestrial and marineecosystems with the help of sunlight and thus theorganic compounds are formed; (ii) carbon dioxideis transparent to incoming shortwave solar radia-tion but is opaque for outgoing longwaveterrestrial radiation. Thus, carbon dioxide in-creases greenhouse effect and keeps the earthsurface warm by re-radiating the terrestrialradiation back to the earth's surface. It is evidentthat carbon dioxide is most significant greenhousegas. It may be mentioned that the concentration ofatmospheric carbon dioxide is gradually increas-ing due to anthropogenic activities, namely burn-ing of fossil fuels and wood and deforestationleading to probable climate change through globalwarming. It is estimated that at the beginning ofindustrial revolution (1860 A.D.) the concentra- tion of atmospheric carbon dioxide was 290 ppmbut it is supposed to have crossed the 350 ppm level. 

* (2) Aerosols - Suspended particulate matter (SPM) in the atmosphere including solid particles of varying  sizes and liquid droplets are collectively called  aerosols which include dust particles from vol- canic eruptions, exposed (ploughed) soil cover,  deserts, rocks, etc.. salt particles from seas and  oceans; meteoric particles, organic matter (bacteria, seeds, spores, pollen etc.); smoke and soot.  The aerosols are also variable elements of the  atmosphere. The concentration of particulate mat- ter decreases with increasing altitude of the  atmosphere. The overall amount of particulate  matter in the atmosphere varies from as little as 100  parts per cubic centimeter to several million parts  per cubic centimeter' (Oliver and Hidore, 2003).  

Aerosols are concentrated mostly in the lower  Atmosphere whi le upper atmosphere receives particulate  matter from disintegration of meteors resulting  into meteoric dust, violent volcanic eruptions,  nuclear explosions, strong duststorms travelling  over warm deserts and ploughed fields. Most of the solid particles are kept in  suspension in the atmosphere. These particulates  help in selective scattering of shortwave electro- magnetic solar radiation which adds varied  charming colour of red and orange at sunrise and  sunset. Similarly, selective scattering of the  electromangnetic shortwave solarradiation causes  blue appearance of the sky. Some of the aerosols,  mainly water droplets, absorb certain amount of  solar radiation while some amount of radiant solar  energy is reflected back to the space. 

Solid  particulate matter mainly salt particles become  hygroscopic nuclei and thus help in the formation  of water drops, fogs, clouds and varied forms of  condensation and precipitation. The smoke over  the cities, when combined with sulphur dioxide,  makes poisonous urban smogs. It is evident that  the presence of particulate matter in the atmos- phere causes variations in weather conditions.

* (3) Water Vapour - Water vapour, though considered in the category of atmospheric gases, needs separate  consideration because it is a very important  constituent of the atmosphere and is responsible  for different types of condensation and precipitation. In fact, water vapour is a gaseous form of  water. The process of evaporation is responsible  for the transformation of water into vapour. There  is much spatial and temporal variation in vapour  content in the atmosphere. The content of water  vapour ranges horizontally from 0.02 per cent by  volume in the cold dry air over polar areas to 5 per  cent by volume over moist tropical areas. The content of vapour in the surface air in the moist  tropical areas, at 50°and 70° latitudes is 2.6%, 0.9%  and 0.2% (by volume) respectively. The content of  water vapour also decreases with increasing alti- tude in the atmosphere. Water vapour is also  considered as a primary greenhouse gas because it  absorbs some portion (though very little) of  incoming shortwave solar radiation and major  portion of outgoing longwave terrestrial radiation  and thus helps in keeping the earth's surface warm. More than 90 per cent of the total atmospheric vapour is found upto the height of 5 km. If there is condensation of all the atmospheric  vapour at a time, there would result a one-inch thick  layer of water around the earth. Even this meagre amount of water vapour in the atmosphere is  responsible for various types of weather phenom- ena. The moisture content in the atmosphere  creates several forms of condensation and precipi- tation e.g. clouds, fogs, dew, rainfall, frost,  hailstorm, ice, snowfall etc. Vapour is almost  transparent for incoming shortwave solar radiation so that the electromagnetic radiation waves reach  the earth's surface without much obstacles but  vapour is less transparent for outgoing shortwave  terrestrial radiation and therefore it helps in heating  the earth's surface and lower portion of  the atmosphere because it absorbs terrestrial  radiation.