Chlorofluorocarbons rising
Segregation in the lower atmospheric region is prevented by various mechanisms, mainly turbulent mixing of the atmosphere by rising and falling air streams. The main driving force of the large-scale atmospheric circulation is solar heating. The most important circular air flow pattern in the troposphere is the Hadley cell circulation. (Considerable information on the movement and mixing processes in the atmosphere was gained from monitoring following numerous tests of nuclear weapons in the atmosphere mainly during 1951–1962. The radioactive substances served as tracer material.)

Winds mix the atmosphere much faster than gaseous molecules can settle according to their weight. The natural composition of the dry atmosphere remains constant in the lower 80–100 km. Only above 100 km, segregation takes place because the concentration of light gases (e.g. helium) decreases more slowly with altitude than those of the heavy gases.

The same atmospheric mixing processes apply to chlorofluorocarbons. For example, trichlorofluoromethane $(\ce{CCl3F})$ is nearly uniformly distributed throughout the lower 15 km. At higher altititudes, the abundance of trichlorofluoromethane decreases – not because of its density, but because it is decomposed by UV radiation. By way of comparison, the less reactive tetrafluoromethane $(\ce{CF4})$ is uniformly distributed up to at least 50 km.
(Source: Frequently Asked Questions About Ozone (1): How Can Chlorofluorocarbons (CFCs) Get to the Stratosphere If They’re Heavier than Air? in Assessment of Ozone Depletion: 1998 – World Meteorological Organization Global Ozone Research and Monitoring Project – Report No. 44 [html] [pdf])