Download The Process of Stratospheric Ozone Depletion and more Slides Environmental Science in PDF only on Docsity! Q.14 Emission, accumulation, and transport. The principal steps in stratospheric ozone depletion caused by human activities are shown in Figure Q6-1. The process begins with the emission, at Earth’s surface, of source gases containing the halogens chlorine and bromine (see Q7). The halogen source gases, often referred to as ozone-depleting substances (ODSs), include manufactured chemi- cals released to the atmosphere in a variety of applications, such as refrigeration, air conditioning, and foam blowing. Chlorofluoro- carbons (CFCs) are an important example of chlorine-containing gases. Emitted source gases accumulate in the lower atmosphere (troposphere) and are transported to the stratosphere by natural air motions. The accumulation occurs because most source gases are highly unreactive in the lower atmosphere. Small amounts of these gases dissolve in ocean waters. The low reactivity of these manufac- tured halogenated gases is one property that makes them well suited for specialized applications such as refrigeration. Some halogen gases are emitted in substantial quantities from natural sources (see Q7). These emissions also accumulate in the troposphere, are transported to the stratosphere, and participate in ozone destruction reactions. These naturally emitted gases are part of the natural balance of ozone production and destruction that predates the large release of manufactured halogenated gases. The initial step in the depletion of stratospheric ozone by human activities is the emission, at Earth’s surface, of gases con- taining chlorine and bromine. Most of these gases accumulate in the lower atmosphere because they are unreactive and do not dissolve readily in rain or snow. Natural air motions transport these accumulated gases to the stratosphere, where they are converted to more reactive gases. Some of these gases then participate in reactions that destroy ozone. Finally, when air returns to the lower atmosphere, these reactive chlorine and bromine gases are removed from Earth’s atmosphere by rain and snow. What are the principal steps in stratospheric ozone depletion caused by human activities? Q6 Section II: THE OZONE DEPLETION PROCESS Principal Steps in the Depletion of Stratospheric Ozone Halogen source gases are emitted at Earth's surface by human activities and natural processes. Halogen source gases accumulate in the atmosphere and are globally distributed throughout the lower atmosphere by winds and other air motions. Halogen source gases are transported to the stratosphere by air motions. Most halogen source gases are converted in the stratosphere to reactive halogen gases in chemical reactions involving ultraviolet radiation from the Sun. Air containing reactive halogen gases returns to the troposphere where the gases are removed by moisture in clouds and rain. Reactive halogen gases cause chemical depletion of stratospheric ozone over the globe. Emissions Accumulation Transport Chemical reaction Conversion Removal Low-temperature surface reactions on polar stratospheric clouds (PSCs) significantly increase reactive halogen gases and thereby cause severe ozone loss in polar regions in late winter and early spring. Figure Q6-1. Principal steps in stratospheric ozone deple- tion. The stratospheric ozone depletion process begins with the emission of halogen source gases by human activities and natural processes. Those emitted by human activities are also called ozone-depleting substances (ODSs). Subsequent steps are accumulation, transport, conversion, chemical reaction, and removal. Ozone depletion by halogen source gases occurs glob- ally. Large seasonal ozone losses occur in polar regions as a result of reactions involving polar stratospheric clouds (PSCs). Ozone depletion ends when reactive halogen gases are removed by rain and snow in the troposphere and deposited on Earth’s surface.
