Overview

Overview

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 lesson 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.

Learning Objectives

By the end of this lesson, you should be able to:

• explain the role that each of several atmospheric constituents 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

Questions?

If you have any questions, please post them to the Course Questions discussion forum. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.