4: Atmospheric Composition
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By the end of this chapter, you should be able to:
- explain the role that each atmospheric constituent plays in atmospheric structure and weather
- identify changes in minor and trace gas amounts and the impacts these changes have on the atmosphere
- explain how the atmosphere cleanses itself using methane as an example
- use chemical equations to show how ozone is formed in the stratosphere and the troposphere and how they differ
- diagram the lifecycle of aerosol particles with an emphasis on their role in weather
The atmosphere consists mostly of dry air - mostly molecular nitrogen (78%), molecular oxygen (21%), and Argon (0.9%) - and highly variable amounts of water vapor (from parts per million in air to a few percent). Now we will consider gases and particles in the atmosphere at trace levels. The most abundant of the trace gases in the global atmosphere is carbon dioxide (~400 parts per million, or 400 x 10-6), but there are thousands of trace gases with fractions much less than a few parts per million. Some, particularly the reactive hydroxyl (OH) radical, are important even though their abundance is less than 1 part per trillion (10-12). The atmosphere also contains small particles with sizes from nanometers (10-9 m) to microns (10-6 m) coming from many sources. These trace gases and particles are as important to atmospheric structure and weather as are nitrogen, oxygen, and water vapor and they also play a huge role in human and ecological health and global climate. In this chapter, we will examine the atmosphere’s composition and its changes over time. The atmosphere is continually inundated with surface emissions of gases and particles (and some from space) but it has chemical mechanisms to clean itself. We will see how two atmospheric pollutants - ozone and small particles - are produced. In later lessons, we will see that without these chemical processes and particles, there would be no clouds and, thus, no real weather.
- 4.1: Atmospheric Composition
- Key features of the gases include their compressibility, their transparency in the visible, their momentum, and their heat capacity. Water vapor has the additional important feature of existing in the vapor, liquid, and solid phases in the atmosphere and on Earth’s surface. The most important properties of small particles include their ability to dissolve in water in order to be Cloud Condensation Nuclei or to maintain a lattice structure similar to ice to be Ice Nuclei.
- 4.2: Changes in Atmospheric Composition
- Since the rise of oxygen, 2 billion years ago, the nitrogen and oxygen fractions in the atmosphere have been stable. Water vapor is highly variable but, on average, appears to also have been fairly stable. Recent data from satellites and sondes indicate that perceptible water (the total amount of water that is in a column from the surface to space) has increased 1.3 ± 0.3% per decade over the oceans in the past 25 years (Trenberth et al., Climate Dynamics, 2005).
- 4.3: Other Trace Gases
- Hundreds of different trace gases have been measured in the atmosphere and perhaps thousands more have yet to be measured. Many of these are volatile organic compounds (VOCs). Volatile means that the compound may exist in the liquid or solid phase but that it easily evaporates. Organic means that the compound contains carbon but is not carbon dioxide, carbon monoxide, or carbides and carbonates found in rocks.
- 4.4: Stratospheric Ozone Formation
- Ozone is ozone no matter where it is in the atmosphere. Good ozone is good only because it is in the stratosphere where we cannot breathe it. Bad ozone also absorbs solar ultraviolet light, but it is down near Earth's surface where we can breathe it. For UV protection, we are interested in the total number of ozone molecules between us and the Sun. 90% of ozone molecules are in the stratosphere and 10% are in the troposphere - some down near Earth's surface where we can breathe them.
- 4.5: The Story of the Atmosphere's PAC-MAN
- The atmosphere's oxidation capacity is its ability to clean itself of all of the gases that are emitted into it. What does stratospheric ozone have to do with the atmosphere’s oxidation capacity, which mostly occurs in the troposphere and mostly by the atmosphere's PAC-MAN, hydroxyl (OH)? It turns out that natural dynamic processes actually pull air down from the stratosphere and mix it into the troposphere, eventually mixing some of this ozone to Earth’s surface.
- 4.6: Where do Cloud Condensation Nuclei (CCN) come from?
- Atmospheric particles come from many different sources. Good cloud condensation nuclei (CCN) must be small particles, so that they do not settle too fast, and must be hydrophilic, which means that water can stick. They can be either soluble (i.e., dissolvable in water), or insoluble, but most are soluble.
Thumbnail: Image of the largest Antarctic ozone hole ever recorded over the South Pole in September 2006. (Public Domain; NASA via Wikipedia).