BACKGROUND:
Atmosphere which comprises nitrogen, oxygen, carbon dioxide, argon etc. and a layer that surrounds a planet has different layers – troposphere, mesosphere, thermosphere and exosphere. This atmosphere of 78% nitrogen, 21% oxygen, 0.93% argon and 0.04% carbon dioxide protects us from harmful UV radiation emitted by the Sun. Atmosphere that scatters the sunlight helps in regulating various processes – water cycle, temperature and many other natural processes.
IMPORTANCE:
Atmosphere which has many salient features is important for human survival. Essential gases, trapping temperature, protecting from ultraviolet rays are a few of the advantages of Atmosphere. The Troposphere extending to about 12 kilometer high and the lowermost layer contains 75% of the entire atmosphere. Tropopause, the upper part acts as a boundary between troposphere and stratosphere. The decreasing temperature with increase in height in the troposphere is of the order of 6.50C per kilometer. Stratosphere extending up to 50 kilometers comprises the ozone layer which protects us from UV radiation – Aero planes and birds fly in the lower level of ozone. Mesosphere extends from 50 to 85 kilometers just above the stratosphere and is the coldest layer (temperature decreases with increase in altitude – average temperature being about -850C). Thermosphere extending from 85 to about 700 kilometers and is very hot with temperature ranging between 5000C to 20000C. The layer absorbs energetic ultraviolet rays of the sun. Exosphere which has no boundary merges out in the space and extends to about 10,000 km and is the first layer to shield the Earth from meteors, asteroids and cosmic rays. Air density is low because of hydrogen which is the element on earth – Satellites orbiting within this or below. Ionosphere is characterized by a distinct layer of atmosphere and extends from 80 km to 1000 km above the earth’s surface (includes thermosphere and parts of mesosphere). This layer uses solar radiations to ionize atoms and molecules resulting in a layer of electrons which reflect and modifies radio waves used for communication and navigation.
Source: internet
WATER VAPOR:
The cycling among ice, liquid and vapour shapes the climate by delivering rain and snow. The distribution and cycling of Earth’s water changes due to climate change. The warmer temperatures induce increased evaporation from oceans, lakes, streams and soil leading to higher water vapor in the atmosphere. Depending upon the regional temperatures, it may either rain or snow. Water vapor which traps heat is also a greenhouse gas but, unlike other gases that linger in the atmosphere for years, water vapor usually stays for a few days before falling back to Earth as precipitation. Warm temperatures lead to more water vapor leading to warmer temperatures which creates a positive feedback loop – it is a good thing because, the effects of one process enhance the effect of another.
EARLY ATMOSPHERE AND NATURAL REACTIONS:
The atmospheric reactions of various classes are implicated in the destruction of Earth’s protective Ozone layer. It has been established that the Ozone layer above the Antarctica has been disappearing since 1970 resulting in commonly known as ‘Ozone hole’ – 2009 the hole reached 24.1 million km2 and the largest ever recorded was in 2000 when it measured 29.9 million km2.
Satellite Photos of Earth Reveal the Sizes of the Antarctic Ozone Hole over Time
Source: Internet
Temperature in the troposphere decreases with increasing altitude and this gradient leads to continuous mixing of the upper and lower regions within the layer. The thermally induced turbulence results in temperature and pressure fluctuations which is termed ‘weather’ – percentage composition except for water vapor and ozone is the same up to about 100 km altitude. However, water vapor concentration falls off quickly with altitude. Stratosphere contains highest amount of ozone – ultraviolent light causes a wide variety of chemical reactions that are harmful to organisms. Ozone responsible for pungent smell associates with lightning discharges and electric motors which is toxic and a significant air pollutant in cities specially. The source of ozone in troposphere is emission of NO2 from cars which can absorb 420 – 300 nm to yield oxygen atoms Ozone undergoing cleavage reaction to give O2 and O.
Ozone, the protective layer that absorbs ultraviolet light is being damaged by chlorofluorocarbons (CFCs) used in refrigerants and propellants in aerosol cans. Chlorofluorocarbons being volatile do not undergo chemical reactions until they are carried to the top of the atmosphere eventually entering stratosphere where ultraviolet light splits it into chlorine which reacts with one molecule of ozone to result in two O2 molecules. Chlorine atoms are a catalyst the increases the rate at which ozone is converted to oxygen. Chlorine forms hydrochloric acid with water molecule and carried back into the troposphere to finally get washed out in rainfall.
‘Ozone hole’ is a misconception as it is not formed due to ozone depletion during winter – ozone is not formed over the pole or is destroyed. Ozone build up over the South Pole during winter is due to transport from mid-latitude stratosphere and the lack of destruction by sunlight. Interesting chemistry in the atmosphere over South Pole in winter sets the stage for rapid ozone depletion leading to ozone hole.
GLOBAL REACTION TO OZONE: A fivefold increase in CFCs resulted in 2.5% decrease in stratospheric ozone levels between 1978 and 1988. It would have been disastrous continuing the use of CFCs. This resulted in 43 countries signing Montreal Protocol committing to reduce CFC emissions by 50% around 2000. Subsequently, joined by many other countries, they agreed to phase out CFCs completely by 21st Century. However, it is estimated that chlorine levels will not fall to the level at which the Antarctic ozone hole was first observed until 2050.
ATMOSPHERIC CHEMISTRY
While we dealt a bit on the stratification of the atmosphere, let us look at the same in a different way – the layer in which gaseous composition is generally uniform (no change with altitude) is termed ‘homosphere’ which extends up to 80 km; beyond this altitude, chemical constituents change significantly with height and the layer is termed ‘heterosphere’.
While Nitrogen, Oxygen, Argon, Neon, Helium, Methane, Krypton and Hydrogen are gaseous composition of earth’s atmosphere, water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen dioxide and ozone for important variable gases. The earth’s radiation budget is regulated by solar energy – the average flux of solar radiation at outer limit of earth’s atmosphere is called ‘solar constant’. Electromagnetic waves travel at 300,000,000 meters per second. Visible light is in the range of 0.4-0.7 um and includes radiation that can be perceived by human eye.
While weather related phenomenon occurs in troposphere, stratosphere has the following:
The significant feature of atmospheric chemistry is the occurrence of photochemical reactions resulting from absorption by molecules of light photons designated as hv.
The composition of Earth’s atmosphere is vulnerable to degradation through reactions with common automobile and industrial chemicals. Smog for example, was originally described as the unpleasant combination of smoke and fog together with sulfur dioxide. Generally, the term denotes photo chemically oxidizing atmosphere – sulfur dioxide being a reducing compound, it is termed sulfurous smog. Automobiles contributing hydrocarbons and nitrogen oxides are principle components that facilitate smog formation.
Methane released from underground sources as natural gas and a result of fermentation of organic matter is the most abundant hydrocarbon in the atmosphere. Reactive hydrocarbons produced as automobile exhaust in the presence of NO under temperature inversion, low humidity and sunlight produce undesirable photochemical smog manifested by particulate matter, ozone and aldehydes. Methane reacts with oxygen atoms (produced by photochemical dissociation of NO2 to O and NO) to generate hydroxyl radical and an alkyl (methyl) radical. Since methane is abundant in the atmosphere, it accounts for a significant fraction of total hydroxyl reactions. The methyl radical reacts rapidly with molecular oxygen to form peroxyl radicals (H3C+O2+M — H3COO+M. This is an important reaction in smog formation as the oxidation of NO by peroxyl radicals is means of regenerating NO2 in the atmosphere after photochemical dissociation to NO. Hydrocarbons may undergo heterogeneous reactions on particles in the atmosphere – dust composed of metal oxides or charcoal has catalytic effect upon oxidation of organic compounds.
These fundamental reactions transform into quite complicated reactions such as photochemical reaction producing oxygen atoms; reactions involving oxygen species; organic free radicals from hydrocarbons and propagation, branching and termination by a variety of reactions.
Apart from these, there are a number of significant atmospheric reactions involving nitrogen oxides, water, nitrous acid and nitric acid which ultimately is responsible for disappearance of atmospheric CO. In conclusion, organic compounds form species which react with NO directly rather than with NO2.
Various organic compounds (organic acids, alcohols, aldehydes, ketones, esters and organic nitrates) and particulate matter (aerosols) are produced from smog. Aerosols impact visibility of the atmosphere.
IMPACTS:
Ozone, Peroxyacetyl nitrate (PAN) and nitrogen oxides are major oxidants involved in smog out of which PAN has highest toxicity to plants attacking younger leaves causing ‘bronzing’ and ‘glazing’. Alkyl hydro peroxides cause genetic effects – DNA damage. Low toxicity of nitrogen oxides, PAN, hydro peroxides and other oxidants render ozone as the greatest threat to plants in smoggy conditions.
CONCLUSION:
The several layers atmosphere with troposphere being closest and complicated natural chemical reactions taking place due to several factor including solar radiation, modern activities of different kind injecting several other pollutants into this atmosphere certainly make it much more complicated to comprehend individually. A comprehensive understanding perhaps may provide some clue to our understanding of the climate change and its recent impacts globally.
