What Do Concentrations Mean? Comparing Concentrations of Gases in Our Atmosphere

In this activity, students will use a dilution experiment to understand the concept of part-per-million (ppm) and part-per-billion (ppb) measurements. Through discussion, they will be able to relate these dilutions to concentrations of gases in our atmosphere.


Certain gases, such as the greenhouse gases (for example, carbon dioxide, water vapor, methane, and ozone), occur in the atmosphere in miniscule amounts. In a random air sample from the troposphere, for example, you would likely find only about 350 molecules of carbon dioxide for every one million molecules of air mixture. Scientists would express this amount as 350 parts per million (ppm).

Gas concentrations can also be expressed in mass units. With gases in the atmosphere, we usually think in terms of volume and may express this as parts per million by volume (ppmv). Some substances occur in such small amounts that scientists measure them in even smaller amounts such as parts per billion by volume (ppbv) or even parts per trillion by volume (pptv).

Because these measurements are very important to atmospheric scientists, it is useful for students to realize just how important even vanishingly small amounts of certain gases can be. This exercise is designed to give students an appreciation of how many dilutions it takes to achieve a part-per-million dilution of a common substance (food coloring).

You may want to share some of the following interesting comparisons with your students and encourage them to come up with their own:

Part per million:

Part per billion:

Part per trillion:

Learning Goals

  1. Students will understand the concept of part-per-million (ppm) and part-per-billion (ppb) measurements.

  2. Students will appreciate the small scale of ppm and ppb measurements, as compared to the concentration scales that they may be familiar with.

  3. Students will relate the concepts of ppm and ppb to the concentration of gases in our atmosphere.

Alignment to National Standards

National Science Education Standards

Benchmarks for Science Literacy, Project 2061, AAAS

Grade Level/Time

Materials Per Team


Instructions are provided in the student guide.

Have students work in pairs. They should fill out the data chart as they proceed with the activity.

Assessment Ideas

Ask students to creatively prepare a drawing that shows the abundances of five of the most important gases in the earth's atmosphere. They may use pictures, charts, graphs, or drawings.







Water vapor (variable, up to) *




Carbon dioxide *






Methane *


Low-level ozone * (variable, up to) (troposphere)

0.01 to 0.5

High-level ozone (stratosphere)

0.04 to 0.2

Chlorine from CFCs


* = Greenhouse gases

Modifications for Alternative Learners

When you're finished with the activity, click on To Student Guide or Back to Activities List at the top of the page.