Practice Passage Test - 5 | Practice Passages for MCAT PDF Download

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The earth's atmosphere has several layers of gases with different characteristics and temperatures. The atmosphere has a total mass of 5 × 10¹⁸ kg, and is composed mainly of nitrogen, oxygen, argon, and carbon dioxide, as well as water vapor to a variable degree. Seventy-five percent of this mass is within 11 km from the earth's surface. Although the limit between outer space and the atmosphere is not definite, the Kármán Line, approximately 100 km above sea level, is often taken as the boundary

The five main layers of atmosphere are the exosphere, thermosphere, mesosphere, stratosphere, and troposphere. The troposphere is the innermost layer, and extends to approximately 6 km above the earth's surface at the poles and approximately 20 km above sea level at the equator. The next layer is the stratosphere. The ozone layer, which is considered to be a layer of its own because of its unique composition, is within the stratosphere. Approximately 90 percent of the O₃ in the atmosphere is found in the ozone layer, although the actual concentrations are quite low (2-8 ppm). This layer is very important as it absorbs much of the ultraviolet (UV) light emitted from the sun. The mesosphere is the middle layer of the atmosphere with a temperature of approximately 100°C. The thermosphere and exosphere in that order are the outermost layers.

The pressure, density, and temperature of the atmosphere vary with altitude. Both pressure and density decrease with increasing altitude. At sea level, the density of air is 1.2 kg/m³ and drops by approximately 50 percent every 5.5 km.

The atmosphere contains greenhouse gases, which absorb and emit thermal infrared radiation, leading to the greenhouse effect. Greenhouse gases include water vapor, methane, nitrous oxide, carbon dioxide, and ozone. These gases are needed to maintain the temperature of the earth, which would otherwise be much colder. However, it is thought that an increase in the amount of greenhouse gases in the atmosphere has contributed to the increase in average temperature in the twentieth century. Methane is a very potent greenhouse gas. It can be oxidized in the atmosphere to produce carbon dioxide and water, with a half-life of 7 years.