0.11 Rice air curriculum - lesson 3 (teacher): atmospheric gases

Objective

Now that students have a basic knowledge of the physical structure of the atmosphere, this lesson goes into much further detail about the most common gases that make up the atmosphere. The significance of the water cycle, the oxygen cycle, and the carbon cycle will be covered. The gases discussed today compose the vast majority of air molecules in the atmosphere; in later lessons, students will learn about air pollutants that greatly affect our health and environment even in very small quantities.

Background information

The atmosphere is made up of five main gases – nitrogen (N2), oxygen (O2), argon (Ar), water vapor (H2O), and carbon dioxide (CO2). There are many other gases in very trace amounts, some of which are called air pollutants if they harm the health of humans, plants, or animals, and some of which are greenhouse gases that trap in the Earth’s warmth.

To visualize the composition of the atmosphere, suppose you gathered 100 random molecules of dry air. Most likely, 78 of them would be nitrogen molecules (N2), 21 of them would be oxygen molecules (O2), and 1 of them would be an argon molecule (Ar). Depending on how warm and humid the air is, zero to four of the 100 molecules would be replaced by molecules of water vapor (H2O).

Now suppose that you could randomly choose 1 million molecules of air. Almost all of them (about 999,610 molecules) would be the gases we listed above: nitrogen (N2), oxygen (O2), argon (Ar), and water vapor (H2O). About 385 of them would be carbon dioxide (CO2). This is less than one-tenth of 1 percent of the Earth’s atmosphere, but in Lesson 6 we’ll see it has a huge effect on Earth’s climate. Most of the other air pollutants that we’ll study in later lessons have concentrations of less than 1 part per million, so we might not even see one in our collection of 1 million air molecules. However, we’ll learn later that these air pollutants can have important effects on our health and environment.

The main gases of the atmosphere undergo important cycles that are vital to life on Earth. The most apparent of these cycles is the water cycle , which includes all the movements of water between Earth’s atmosphere, bodies of water, and underground. The total amount of water on Earth is constant, but water is continually moving to different locations and changing its form (solid, liquid, or gas).

The water cycle

The following steps make up the water cycle. Energy from the Sun evaporates liquid water from lakes and oceans to become water vapor gas in the air. Plants also release water vapor to the atmosphere by transpiration . The water vapor condenses back to liquid water higher in the troposphere, because the cooler air there cannot hold as much water vapor as warm air (recall condensation on a cold glass of water). This condensation occurs on small particles, forming clouds . Clouds can also form on the ground – this is called fog.