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____________________________________________________________________________________________________ Subject Chemistry Paper No and Title 4: Environmental Chemistry Module No and Title 1, Atmosphere Module Tag CHE_P4_M1 4: ENVIRONMENTAL CHEMISTRY CHEMISTRY 1: Atmosphere ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 2.1. What is ‘Atmosphere’? 2.2. Significance of the Atmosphere 2.3. Composition of the Atmosphere 3. Evolution of the Atmosphere 3.1. Primitive or First Atmosphere 3.2. Reducing or Second Atmosphere 3.3. Oxidizing or Present Day Atmosphere 4. Structure of the Atmosphere and vertical temperature 4.1. Troposphere 4.2. Stratosphere 4.3. Mesosphere 4.4. Thermosphere 4.5. Exosphere 5. Heat budget of the earth’s atmosphere 5.1 Energy budget 5.2 Earth’s energy imbalance 5.3 Natural greenhouse effect 5.4 Climate sensitivity and forcing 4: ENVIRONMENTAL CHEMISTRY CHEMISTRY 1: Atmosphere ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you shall be able to: • Differentiate between air and atmosphere • Know the functions and composition of atmosphere • Identify the layered structure of atmosphere • Learn about the importance of each layer for human activity • Evaluate the effect of human activities on the atmosphere 2. Introduction 2.1. What is ‘Atmosphere’? The thick blanket of air which envelopes our planet and helps sustain life is called ‘the atmosphere’. Atmosphere behaves as a protecting shield for all life on Earth, making it unique in the solar system. The atmospheric gases which are used in respiration and photosynthesis are called ‘air’. The atmosphere of earthcontains mixture of gases which are forced to be maintainednear the earth’s surface by gravity. An increase in altitude will decrease the density of atmosphere. Maximumatmosphereis located close to the surface, within 16 kilometres. Though the atmosphere extends into outer space and there is no distinct border between them, but an imaginary line called the Kármán Line which is approximately at the height of 100 km iscalled as the boundary of Earth’s atmosphere. 2.2. Significance of the Atmosphere The atmosphere protects life on earth by serving the following functions: • Absorbs the energetic ultraviolet radiation while allow thepassing of important visible radiations. • Keeps the earth’s surface warm through heat retention, making possible a comfortable mean temperature of ±15°C • Prevents excessive heating of surface of the earth at day and excessive cooling at night thereby reducing the temperature variations • Contains nitrogen, oxygen and carbon dioxide gases essential for plant growth and for respiration • It protects the surface of earth and all life existing on earth from small meteorites that heat up in the atmosphere due to friction • Serves an integral part in the bio-geo chemical cycles of C, N, O, P and S • Helps in flow of energy and water vapours through dynamic processes of air flow • Helps in radio communication • Helps in movement of air crafts • Aids in dissipation, dispersion and decomposition of pollutants 4: ENVIRONMENTAL CHEMISTRY CHEMISTRY 1: Atmosphere ____________________________________________________________________________________________________ 2.3. Composition of the Atmosphere The atmosphere is having 78.08% nitrogen (N2), 20.95% oxygen (O2) and about 1% other gases. 21% 78% 1% Fig 1 Composition of the Atmosphere Evolution of the 3. Evolution of the atmosphere In present day, the evolution of the atmosphere is a long process, spanning over millions of years and related with the earth origin and evolution. The evolution process can be divided into three eras or stages: 3.1. Primitive or First Atmosphere The formation of earth took place about 5 billion years ago from the solar nebula. The atmosphere at that time would have consisted of primarily hydrogen (H2),water vapor (H2O), nitrogen (N2), carbon dioxide (CO2)and carbon monoxide (CO). These gases deliver to the space because temperatures were too high and earth’s gravity was weak because earth did not have a differentiated core. The accretion of earth took place about 3.5 billion years ago. It was an exothermic process therefore, heat will be released and absorbed by noble gases most of which escaped out. Other constituents chemically combined to form gases which held on the earth under its gravitational pull. The atmosphere consisted of H2, N2, methane (CH4), ammonia (NH3), H2O and noble gases. 4: ENVIRONMENTAL CHEMISTRY CHEMISTRY 1: Atmosphere ____________________________________________________________________________________________________ 3.2. Reducing or Second Atmosphere This atmosphere existed from 3.5 to 2 billion years ago. The hydrosphere formed about 4 billion years ago resulting in huge oceans from condensed water vapor. During this period, the earth cooled down and 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 while passing through the mantle. Large amounts of N2 andCO2were released into the atmosphere. Most of the CO2 dissolved in water and carbonate sediments built up. Free oxygen (O2) was not present. The other gases present were H2, N2, CH4, NH3 and carbon monoxide (CO) in trace amounts, hydrogen sulphide (H2S), sulphur dioxide (SO2), chlorine (Cl2) etc. 3.3. Oxidizing or Present Day Atmosphere Around 2.4 billion years ago, free O2 starts forming in the atmosphere. Though photosynthesizing cyanobacteria were present around 2.7 billion years ago, but the O2 released during photosynthesis was used in oxidation of metals like iron. The atmosphere became oxidizing and O2 consuming life forms began appearing in the oceans on earth. O2 was being generated by photolysis of water from ultraviolet (UV) radiation and also from photosynthesis by plants and cyanobacteria. However, higher in the atmosphere, O2 molecules were absorbing the UV radiations and getting converted to ozone (O3). Soon, an ozone layer was formed, which protects surface of the earth from high energy UV radiations making it feasible also for development of life on land. As higher life forms emerged on land, plants added to the atmospheric O2 while animals used it up for breathing and the O2 level stabilized around the 21% we know today. 4. Structure of the Atmosphere There are five layers of the earth’s atmosphere (Fig 4.) based on the variation in vertical temperature in each layer: 4.1. Troposphere Troposphere is closest layer to the earth’s surface. So, the layer in which we live is troposphere. It is about 18 km above the sea level at the equator and 7 km at poles.75% of the atmospheric gases are present in this layer and almost all of the dust particles and water vapors. All weather phenomenon like cloud formation, winds, rainfall, snowfall takes place in this layer. The air below this layer is warmer than any other layer because it is heated from the earth’s surface below. The layer of air which is warm tends to rise up, and a pocket of cold air flows to cover the space, giving rise to wind movement. The temperature will decrease at a rate of about 6.5 °C per km as the altitude increases. The average temperature near the surface is 15°C while it is -57° C at the top. Pressure, moisture content and density of air also decrease with height thinning of air. That is why people experience breathlessness at high altitudes in mountains. 4: ENVIRONMENTAL CHEMISTRY CHEMISTRY 1: Atmosphere ____________________________________________________________________________________________________ The troposphere ends when there is variation in temperature with height. This area, which is the top of troposphere and bottom of stratosphere, is known as tropopause. The inversion of temperature beyond tropopause prevents further convection of air thereby confining most of the weather phenomenon in troposphere. 4.2. Stratosphere Stratosphere is the second layer of earth’s atmosphere. It starts at the top of tropopause and is 50 km above the sea level. The term stratosphere is derived from a Greek word “strata” which means arrangement in horizontal layers. Approximately 99% of the atmosphere is located up to this height which includes almost all the gases. Weather balloons and jet aircrafts fly in this region as the air present in this layer is very thin. This increases the fuel efficiency of the aircrafts. The stratosphere also contains the ozone layer; at a height of about 30 km. The density or thickness of ozone layer above the surface of earth is measured in Dobson Units (DU). The ozone layer is thinnest near the equator (~260 DU) and its thickness increases towards the poles, though there are seasonal fluctuations. Ozone – which is a mainly formed photochemical smog – is considered to be dangerous for health if reached the ground level or near the surface of earth. However, in the stratosphere, the high energy UV radiation is absorbed by the ozone layer and heats up the stratosphere. This also prevent skin cancer and damage to vegetation. The ozone layer is created in the stratosphere when the oxygen molecules (O2) absorb UV radiations (240 nm) and dissociate to give atomic oxygen (O) which immediately react with another O2 molecule and
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