SGCh17 - Chapter Seventeen CHEMISTRY IN THE ATMOSPHERE...

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356 Chapter Seventeen CHEMISTRY IN THE ATMOSPHERE Earth's Atmosphere and Its Regions Depletion of the Ozone Layer The Greenhouse Effect and Acid Rain Photochemical Smog Indoor Air Pollution EARTH'S ATMOSPHERE AND ITS REGIONS STUDY OBJECTIVES 1. List the major components and several of the minor components of the atmosphere. 2. Describe the approximate altitude and characteristics of four regions of the Earth's atmosphere. 3. Describe the chemical processes responsible for light emission by the aurora borealis and aurora australis. Earth's Atmosphere. Earth's atmosphere is unique in the solar system due to its high oxygen (O 2 ) content. Molecular oxygen makes up 21 percent of the atmosphere by volume. Table 17.1 in the text shows that N 2 , O 2 , and Ar make up over 99.9% of the atmosphere by volume. The other noble gas elements (besides argon) are only present in trace amounts. Associated with the high oxygen content is the formation of ozone (O 3 ), a molecule that absorbs ultraviolet light and filters it from sunlight. This radiation is known to be harmful to many of the life forms that inhabit Earth's surface. Nitrogen is the major constituent of the atmosphere. Nitrogen molecules are held together by a triple bond and therefore are extremely stable. Biological and industrial nitrogen fixation converts the elemental form of nitrogen into nitrogen compounds. Lightning plays an important role by initiating the production of about 30 million tons of nitric acid each year. There are three steps in the mechanism of nitric acid production. Lightning provides the energy to break up the N 2 molecule and initiates the first step in the reaction sequence. N 2 (g) + O 2 (g) 2NO(g) 2NO(g) + O 2 (g) 2NO 2 (g) 2NO 2 (g) + H 2 O(l) HNO 3 (g)+ HNO 2 (g) The nitrogen in nitric acid is an essential plant nutrient. Carbon dioxide, the fourth most abundant component, makes up only 0.033% of the air. Carbon dioxide is extremely important to Earth's heat balance even though its relative concentration is low. In this chapter we will review the effects of air pollutants on the ozone layer, on the Earth's temperature, and on other phenomena related to the atmosphere. Regions of the Atmosphere. For convenience the atmosphere is divided into four regions with respect to altitude. The lowest region is the troposphere. This region extends up to about 10 km. The temperature decreases with altitude in the troposphere from about 25°C at sea level to –55°C at 10 km. The troposphere contains about 80% of the mass of the atmosphere and virtually all the precipitation, clouds, and water vapor. All the weather occurs in the troposphere. Remember that the atmosphere gradually thins out as the altitude increases. The atmospheric pressure at 10 km is about 230 mm Hg.
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