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Lecture Note on ATMOSPHERE

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Lecture Note on ATMOSPHERE Lecture Note on ATMOSPHERE -Dr S P Singh Department of Chemistry, A N College, Patna ORIGIN The origin of the earth’s atmosphere is spanned over millions of years. It involves the following stages; Stage 1: Primitive Atmosphere ∑ The formation of earth comprising of primarily H2, water vapor (H2O), N2, CO2 and CO took place about 5 billion years ago from the solar nebula. These gases deliver to the space because of having very high temperatures. ∑ There was no differentiated core resulting thereby weak earth’s gravity. ∑ About 3.5 billion years ago, the accretion of earth took place leading to an exothermic process. Heat was released and absorbed by noble gases; most of which escaped out. ∑ Many constituents chemically combined to form gases which held on the earth under its gravitational pull. ∑ The atmosphere consisted of H2, N2, CH4, NH3, H2O, and noble gases. Stage 2: Reducing Atmosphere ∑ This atmosphere existed from 3.5 to 2 billion years ago. ∑ *The hydrosphere formed about 4 billion years ago resulting in formation of the huge oceans from condensed water vapour. ∑ The differentiation of layers took place into a solid core, liquid mantle and thin crust. The solid core resulted in a strong gravitational force. The energy released from accretion of particles and decay of radioactive isotopes caused massive volcanic eruptions. ∑ Gases dissolved in the molten magma were reduced. Large amounts of N2 and CO2 were released into the atmosphere. Most of the CO2 dissolved in water leading to the formation of carbonate sediments. The other gases present were H2, N2, CH4, NH3, H2S, SO2, Cl2 and CO in trace amounts, etc. but free oxygen (O2) was not present. Stage 3: Oxidizing Atmosphere ∑ Photosynthesizing Cyanobacteria were present around 2.7 billion years ago, but the O2 released during photosynthesis was used in oxidation of metals like iron but free O2 started forming in the atmosphere around 2.4 billion years ago. ∑ The atmosphere became oxidizing and O2 consuming life forms began appearing in the oceans on the earth. ∑ Photolysis of water from ultraviolet (UV) radiation resulted in generation of O2. ∑ O2 molecules were absorbing the UV radiations and getting converted to ozone (O3). Soon, an ozone layer was formed, which started protecting the surface of the earth from high energy UV radiations coming from the Sun. 1OBJECTIVES • To learn the importance of atmosphere. • To to explain the composition of atmosphere. • To differentiate Earth’s various atmospheric layers • To know the characteristics of different layers of the atmosphere. • To learn heat budget of the Earth Atmospheric system INTRODUCTION • Atmosphere envelopes the mother earth. It comprises of various gases (air) along with water vapour (moisture) and dust particles. • Gases create pressure, and allow water vapour to exist on Earth’s surface. It is the gravitational pull of the earth which keeps on maintaining gases and moisture near to the Earth. Both of them are the essential components for existence of life. • An imaginary line called the Karmin line demarcates the boarder of the atmosphere from the outer space roughly at the height of 100 km from the sea level. • The components of the atmosphere changes with the change in time and place. It warms the Earth’s surface. SIGNIFICANCE The atmosphere of earth possesses a lot of significance such as ∑ containing gases; O2 plays role in respiration, CO2 in photosynthesis, larger % of nitrogen to prevent excessive oxidation, moisture in creating comfortable environment, etc. ∑ having properties of heat retention to maintain warmth, energy absorption and radiation to maintain energy / heat balance to prevent excessive heating and cooling. ∑ protecting living beings from harmful UV rays from the Sun. ∑ allowing the bio-geo chemical cycles of C, N, O, P and S. ∑ helping in radio communications, air fly and dynamic processes of air flow, etc. COMPOSITION OF ATMOSPHERE The atmosphere is divided into two layers (i) the heterosphere and (ii) the homosphere. ∑ The outermost sphere of the atmosphere is known as the heterosphere, where the gases are distributed on distinct layers in accordance with their atomic weight. Gravitational force also plays a vital role. In general, the lighter elements like hydrogen and helium make up the outer layer and the heavier elements such as nitrogen and oxygen remain at the lower layer. ∑ The homosphere lies between the Earth’s surface and heterosphere. The gases are uniformly distributed in this layer. The envelop of gases that is what we call Earth’s Atmosphere is bound to remain with the planet more or less permanently due to gravity. Within 50miles above the surface, the air is so thoroughly mixed by turbulence that the variation of permanent constituent gases is minimal. Two gases nitrogen and oxygen comprise of 99% of dry gases by volume. Water vapour is a variable constituent which can increase upto 4%. There are many other gases which are anthropogenic leading to air pollution. TROPOSHERE (~10 °C to -60°C) • The troposphere is the lowest layer of Earth's atmosphere. • It ranges from Earth's surface to an average height of about 12 km (7.5 miles; 39,000 ft). Its altitude varies from about 9 km (5.6 miles; 30,000 ft) at the geographic poles to 17 km (11 miles; 56,000 ft) at the Equator. • The troposphere contains roughly 80% of the mass of Earth's atmosphere. 50% of the total mass of the atmosphere is located in the lower 5.6 km (3.5 mi; 18,000 ft) of the troposphere. • The troposphere is mostly heated through energy transfer from the surface. This results from the Sun's radiation striking the earth and the earth then warming the air above it. Thus the lower section is the warmest section of troposphere. • The temperature usually declines with increasing altitude in the troposphere. The rate of change of air temperature with height is called the "lapse rate". In the troposphere, the lapse rate is generally about 6.5 deg C per kilometer increase in altitude. • The troposphere is bounded above by the tropopause, a boundary marked in most places by a temperature inversion (i.e. a layer of relatively warm air above a colder one), and in others by a zone which is isothermal with height. • Nearly all atmospheric water vapour or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place. • Because warm air tends to rise and cool air tends to sink, the troposphere is a location of much movement of air, or "turbulence". Hence, the troposphere is described by meteorologists as being "well-mixed". • If pollutants are injected into the troposphere, they are mixed throughout its depth in a few days and, usually within a week or so, will fall back to the ground with the rain (e.g., acid rain). Thus, the troposphere has a self-cleaning mechanism. • Most conventional aviation activity takes place in the troposphere. STRATOSPHERE (-60°C to 0°C) • The stratosphere lies above the troposphere and is separated from it by the tropopause. • It ranges from roughly 12 km (7.5 miles; 39,000 ft) above Earth's surface (Sea level) to an altitude of about 50 to 55 km (31 to 34 miles; 164,000 to 180,000 ft). • It contains the ozone layer. Temperatures rises with increasing altitude due to absorption of ultraviolet radiation (UV) radiation from the Sun by the Ozone layer which, in turn, increases the motion of the ozone molecules. The ozone molecules then collide with other molecules in the air, increasing its temperature. • The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence. • Particles that travel from the troposphere into the stratosphere can stay aloft for many years without returning to the ground. For example, large volcanic eruptions force ash to be projected into the stratosphere, where it may remain for years and causing slight global cooling in the process. • The stratosphere is almost completely free from clouds and other forms of weather. However, polar stratospheric or nacreous clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest. • The importance of the ozone layer lies with the fact that (1) ozone helps the earth to maintain its heat balance, and (2) ozone reduces the amount of harmful UV radiation that reaches the earth's surface. Ozone is produced and destroyed as well in the stratosphere. Ozone destruction can be both natural (UV radiation or molecular collisions) or man- made (e.g., chlorofluorocarbons). MESOSPHERE (0°C to -90°C) • The mesosphere is the third highest layer of Earth's atmosphere that ranges from altitude of about 50 km (31 miles; 160,000 ft) to the mesopause at 80–85 km (50–53 miles; 260,000–280,000 ft) above sea level. • Temperatures decreases with increasing altitude to the mesopause (the point of minimum temperature at the boundary between the mesosphere and the thermosphere atmospheric regions). • Due to lack of solar heating and very strong radiative cooling from CO2, the mesosphere is the coldest zone on Earth. It has an average temperature around −85 °C(−120 °F; 190 K). • Below the mesopause, the air is so cold that even the very scarce water vapour at this altitude can be sublimated into polar-mesospheric noctilucent clouds. Noctilucent clouds, or night shining clouds, are tenuous cloud like phenomena in the upper atmosphere of Earth. • The mesosphere is also the layer where most metereors burn up upon atmospheric entrance. A meteoroid is a small rocky or metallic body in outer space. THERMOSPHERE (-90°C to 500-1500°C) • The thermosphere of Earth's atmosphere ranges from an altitude of about 80 km (50 miles; 260,000 ft) up to the thermopause at an altitude range of 500–1000 km (310– 620 mi; 1,600,000–3,300,000 ft).
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