I was wondering why FCKWs can destroy the ozone layer. It seems like these gases have a high molecular weight in comparison to $\ce{N2}$ or $\ce{O2}$. How can these gasses come up to the outer layer of our atmosphere? In particular, how long (on average) would it take to get to such an altitude (through the diffusion of such a particle)?

I mean, they are basically several times heavier than all the other components of our atmosphere combined. Hence, it shouldn't be very energetically stable for them to float on the outer layers of space on top of other lighter particles.

  • $\begingroup$ The dominant factor in getting into the upper atmosphere is the atmospheric lifetime of the gas. The atmosphere is fairly well mixed (obviously so since heavy gases like carbon dioxide don't settle out). But compounds that undergo easy reactions don't survive long enough to get fully mixed. CCl2FH is one of those short lived ones unlike its fully halogenated cousins. So it won't be notable in the upper atmosphere, at least compared, for example, to CCl2F2. $\endgroup$ – matt_black Jan 8 at 14:02

This is one of the more frequently asked questions about ozone depletion. I will try to give a short summary.

First of all, it is important to know that Chlorofluorocarbons (CFCs) are extremely stable at room temperature and pressure. It is therefore only natural, that in the lower atmosphere these gases are enriched, therefore they have plenty of time getting in higher altitudes via diffusion.

The earths atmosphere is in constant motion, you can experience that on a macroscopic level via different winds. The sun heats up the earth's surface, causing all sorts of liquids to be evaporated. While evaporating water, these small molecules may form little conglomerates, taking other molecules up to higher heights. This will happen to dust, so it will also happen to CFCs. There will be local areas (or volumes, layers) of the air in different heights of the atmosphere that have different densities, concentrations and composition. As a result they will also have different air pressures. You can experience this on a plane at higher altitudes. Assuming that all reactions tend to react towards equilibrium, there will be forces acting between these areas. In a consequence there will be also upward forces acting on the CFC containing gas mixture. Slowly these molecules will move upwards to the stratosphere.

As you can see, this process is heavily dependent on many different factors and may well require years. I think it is actually very hard to find any accurate numbers for this process. My research on the subject turned up nothing about that. In most of the cases it was just stated, that these gasses are there.

Some further reading:


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