Final answer:
The concentration of carbon dioxide in Earth's atmosphere is typically stated in parts per million (ppm), which shows the number of CO2 molecules per million molecules of air.
Step-by-step explanation:
The concentration of carbon dioxide in Earth's atmosphere is usually stated in parts per million (ppm). This unit represents the number of molecules of a substance, such as carbon dioxide, per million molecules of air. In more technical terms, one part per million by volume (ppmv) is equivalent to one molecule of CO2 in a million molecules of air, often used to describe the concentration of greenhouse gases.
When it comes to expressing very dilute concentrations, ppm and parts per billion (ppb) are the standard units used. Since the atmosphere is a mixture of gases, concentration by volume is a more relevant measure than concentration by mass, and therefore ppmv is the appropriate unit.