The number of molecules in a narrow atmospheric column over a square centimeter area. This provides information about the total amount of that substance in the column, but does not provide any information about how it is distributed: uniformly mixed in the atmosphere (i.e. CO2), vs concentrated near the surface (i.e. pollution), vs concentrated at some other level (i.e., the ozone layer). Because the number of atoms or molecules is usually large, scientific notation is used. To make plots and graphs easier to read, such measurements can be expressed in terms of powers of 10. For example: 1 x 10^13 NO2 molecules in a column over one square centimeter of the surface (10,000,000,000,000 NO2 molecules in an atmospheric column). Related MY NASA DATA Activity: Parts Per Billion Activity (PowerPoint)