Earth's atmosphere does not exhibit notable stratification by molecular weight until about 100 km up. The lower well-mixed portion is called the homosphere, and

Earth's atmosphere does not exhibit notable stratification by molecular weight until about 100 km up. The lower well-mixed portion is called the homosphere, and it is where these standard atmospheric composition values are measured. This near-uniform composition is also why, for instance, chlorofluorocarbons emitted at the surface reach the ozone layer and decompose it catalytically, even though their molecular weight is much greater than that of air.