# How can ice crystals with adsorbed chlorine remain over the south pole all winter?

I just saw the Periodic Video Ozone on Bondi Beach - Periodic Table of Videos where the impact of Chlorine from CFCs on Ozone is discussed. At around 04:00 or so, the adsorption of chlorine on to very small atmospheric ice crystals above the poles, especially the south pole is described. The chlorine is "stored" until hemispheric spring, when the sun releases the chlorine from the ice, so its impact on ozone spikes.

My main question is about the ice crystals. What keeps them suspended in the atmosphere for months? Why don't they fall, or participate in precipitation? Are they "hovering" over the pole for the whole winter? How?

Note: The video then goes on to discuss a "recent paper" (as of 2011) where the UV light reaching nearer the surface enabled by the ozone "hole" goes on to affect rainfall patterns over Australia. Is this now believed to be significant?

• The ice crystals are probably quite small: Fine particles ($\approx 1\ \mu m$) have a quite long atmospheric residence times compared to coarse and ultra fine particles. They are not as strong affected by gravitational settling (coarse) and Brownian motion (ultra-fine). Precipitation needs activated droplet nuclei (which means that they can start growing until they reach a relevant size to fall down as precipitation). This in mainly a guess and, hence, just a comment. – daniel.heydebreck Apr 10 '17 at 11:59
• @uhoh, hope you don't mind my thoughts... I'd venture to say that when particles are small, yes, air resistance is significant (as particles get larger, mass goes up by r^3, surface area goes up by r^2, so it is overcome), and the same reason you'll see mist/flurries often meander. – JeopardyTempest Apr 11 '17 at 7:30
• @uhoh, it's certainly not my prime knowledge area (especially in Florida!), but perhaps en.wikipedia.org/wiki/Polar_stratospheric_cloud is a worthy direction to look? Maybe it's not so much that any particular cloud molecule is in existence for the entire 6 months, but that as they form and then dissipate, a continuing % of the stratosphere is frozen, such that a continuing % of CFCs are locked up? And anything falling soon evaporates/submlimates and is available to form more clouds? Hopefully someone more versed on this subject area will give a more expert answer! – JeopardyTempest Apr 11 '17 at 8:05
• @uhoh You are right that gravitational acceleration is equal. However, a drag force acts in the opposite direction. If we consider both forces we get a settling velocity (terminal velocity) that depends on the particle's diameter (see Stokes' Law and Terminal Velocity). Zhang et al. (2001) describes the relevant components of the dry deposition velocity. – daniel.heydebreck Apr 12 '17 at 17:30
• correction of my link to Zhang et al. (2001) above: doi: 10.1016/S1352-2310(00)00326-5. Zhang's work is based on Slinn (1981). – daniel.heydebreck Apr 13 '17 at 8:24