Q: Are chlorofluorocarbons really lighter than air,...

Q: Are chlorofluorocarbons really lighter than air, as “I Didn’t Know That . . . “ stated last week?

A: No, as many readers have pointed out. The reason that the gases, which were commonly used as refrigerants, are able to reach the upper atmosphere is that they are extremely stable and are not decomposed at ground level. Over time, winds disperse them fairly uniformly throughout the atmosphere. Those that eventually reach the upper atmosphere are destroyed by ultraviolet radiation, forming highly reactive free radicals that, in turn, destroy ozone.

Ozone produced on the ground by the interaction of exhaust gases with other components of the atmosphere in the presence of sunlight is itself extremely reactive. It is destroyed before winds can carry it to the upper atmosphere, and therefore cannot replace the ozone destroyed by chlorofluorocarbons.