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5. The Atmosphere.pdf

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James Hamilton

Geography 101 The Atmosphere Lecture Outline 1) Composition 2) Vertical Structure 3) Function 4) Atmospheric Issues Lecture Objectives and Content In this lecture we: (i) described the composition of the atmosphere, (ii) examined the vertical structure of the atmosphere in terms of temperature, (iii) reviewed the function of some atmospheric components, and (iv) identified environmental atmospheric issues with a focus on pollution. These notes are not complete, your must supplement these notes with your readings. Use these notes as a guide to focus your study. Reading This lecture reviews materials covered in Chapter 3 of Geosystems. Read the section Atmospheric Composition, Temperature, and Function (2nd Edition pages 64 - 70, 3rd Edition pages 62 - 69) and the materials under Variable Atmospheric Components (2nd Edition pages 70 - 86, 3rd edition pages 69 - 80). In the notes below, the figure numbers in bold refer to the 3rd edition, if the figure number in the second edition is different, it is given in unbolded text). The Atmosphere is the gaseous envelope that surrounds the Earth, it is a mixture of gases and suspended liquids and solids. Aerosols are liquids and solids (but not water or ice), that are suspended in the atmosphere. 1) Composition The atmosphere is composed of a mixture of gases and suspended liquids and solids. Some of the gaseous components have accumulated in the atmosphere as a result of volcanic activity while oxygen has accumulated from biological processes. The character of the atmosphere has changed from Earth’s early history. The early atmosphere had constituents such as methane, carbon dioxide, sulphur dioxide, water vapour, and hydrogen sulphide. The composition of the modern atmosphere is much different. The main gases of the modern atmosphere and their concentrations are listed in Table 3.1 (Table 3.2 in 2nd ed). These gases may be divided into Nonvariable and Variable Gases. For the nonvariable gases (nitrogen, oxygen, argon, neon, krypton, helium) the concentrations are relative stable over time and space. The variable gases (water vapour, carbon dioxide, methane, CFCs, ozone) are those that can experience relatively large changes in their concentration either over time or across space. Nonvariable Gases The concentrations of these gases is relatively stable over long periods. Nitrogen • exists as primarily as N 2 • very stable gas • enters into reactions only when abundant energy is supplied: - by lightning, - within an internal combustion engine (produces oxides of nitrogen: smog component) - when nitrogen fixing bacteria in the soil zone produce ammonia. Oxygen • exists primarily as O 2 but also occurs as O3 (ozone) in very low concentrations • oxygen is used for respiration by organisms, oxygen is produced by photosynthesis • oxygen in the lower atmosphere has it origin in photosynthesis over billions of years • it is a highly reactive gas, will form oxides with other materials (e.g. iron oxide) Noble Gases The inert gases Argon, Neon, Helium, Krypton are very stable and do not enter into chemical reactions in the atmosphere. Variable Gases These gases constitute a small percentage of the total volume and mass of the atmosphere, but they can experience spatial and temporal variations in their concentrations. Carbon Dioxide (CO 2) • gas is essential to photosynthesis • it is the mobile carbon in the Earth-atmospheric system • concentrations fluctuate naturally (carbon dioxide is more soluble in cold water, atmospheric concentrations tend to be lower during periods of continental glaciation) • in recent decades concentrations have increased from 280 ppm (~1850) to current concentrations of 395 ppm. This increase is attributed to anthropogenic sources, largely fossil fuel combustion. • one of the key greenhouse gases. Water Vapour (H 2O) • concentrations in the atmosphere are highly variable (~0.1 to 4.0 %) and are higher in lower atmosphere • water vapour has a short residence time in the atmosphere, it is rapidly cycled from the oceans and land to the atmosphere • another important greenhouse gas Ozone (O 3) • present in the stratosphere where it is produced from a photochemical reaction with ultraviolet radiation • stratospheric ozone is broken down by synthetic refrigerants such as CFCs (chlorofluorocarbons) • also present near ground level where it is a component of photochemical smog Methane (CH ) • produced from anaerobic decomposition and human industrial activities (rice paddies, termites, cows, wetlands, natural gas production) • occurs in low concentrations (1500 ppbv) but has experienced a doubling in the last 150 years • an important greenhouse gas Aerosols • liquids and solids (excluding water and ice) that are temporarily suspended in the atmosphere • includes natural sources such as volcanic ash, dust, soot, sea salts, spores etc • industrial activities also contribute particulate matter • influence the energy balance of the Earth by increasing scatter back to space 2) Vertical Structure From the base of the troposphere to the top of the atmosphere there is a reduction in density (Figure 3.3a) and atmospheric pressure (Figure 3.3b). Density is mass per unit volume, and pressure is a force applied per unit area. Density = Mass/Volume Pressure = Force/Area Force = Mass*Acceleration (mg) = Weight Thus, the atmospheric pressure, at any point in the atmosphere, is si
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