CHAPTER 1 Focus Points

• Types of gases: Gases in the atmosphere can be grouped into permanent gases and variable gases (or trace gases).

• Greenhouse gases: Greenhouse gases, which absorb and emit thermal energy, play a crucial role in Earth’s climate system.

• Sources and sinks: Gases enter and exit the atmosphere through natural and anthropogenic sources and sinks.

• Air pressure: Because it has mass, the atmosphere is held down against the planet’s surface by gravity, which gives it weight and creates pressure.

• Air density: Air density and pressure are greatest near sea level because the weight of the atmosphere above compresses the air, increasing its density.

• Thermal divisions: The atmosphere can be divided into four layers based on changes in temperature with altitude: the troposphere, stratosphere, mesosphere, and thermosphere.

• Troposphere: The troposphere gets colder with increasing altitude, and there is strong vertical mixing there. Earth’s weather occurs in the troposphere.

• Stratosphere: Storms and most clouds do not enter the stratosphere, and there is little vertical mixing there. Ozone molecules in the stratosphere block ultraviolet radiation.

• Categories of pollution: Anthropogenic air pollutants are grouped into two categories: primary pollutants and secondary pollutants.

• Concentration of air pollution: Air pollution is concentrated in valleys, by temperature inversions, and by wind.

• The Clean Air Act: Emissions of air pollutants in the United States have been reduced since the passage of the Clean Air Act in 1963.

• The ozonosphere: Without the ozonosphere, life on land would be burned by ultraviolet radiation.

• CFCs: Chlorine atoms from CFCs break down ozone molecules in the stratosphere.

• Montreal Protocol: After the enactment of the Montreal Protocol, the manufacture of CFCs was mostly phased out.