<p>I just saw the Periodic Video <a href="https://youtu.be/YxsxfsYxA4s" rel="nofollow noreferrer">Ozone on Bondi Beach - Periodic Table of Videos</a> where the impact of Chlorine from CFCs on Ozone is discussed. At around <code>04:00</code> 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.</p>

<p>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?</p>

<p><strong>Note:</strong> 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?</p>

CC BY-SA 3.0
2017-04-10T11:59:42.657Z
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
CC BY-SA 3.0
2017-04-10T13:52:04.837Z
@daniel.neumann Thanks! They still feel the same g of 9.8 m/s^2 ($\mathbf{a}=\mathbf{F}/m$) so what is holding them up? Are they so small that collisions with air molecules keeps them elevated for months? Hopefully a link can be found to address the *How?*
uhoh
CC BY-SA 3.0
2017-04-11T07:30:12.940Z
@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
CC BY-SA 3.0
2017-04-11T07:45:21.993Z
@JeopardyTempest not at all, thanks! In this case it's six months, not six hours. I'm trying to understand if the individual crystals really remain intact, hovering, for a half year, or if I am taking the description to literal. Maybe chlorine is moving around from one crystal to another as one sublimates and the next forms, and the crystals do not just sit up there all that time?
uhoh
CC BY-SA 3.0
2017-04-11T08:05:42.187Z
@uhoh, it's certainly not my prime knowledge area (especially in Florida!), but perhaps https://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
CC BY-SA 3.0
2017-04-12T17:30:22.247Z
@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](https://en.wikipedia.org/wiki/Stokes%27_law#Terminal_velocity_of_sphere_falling_in_a_fluid) and [Terminal Velocity](https://en.wikipedia.org/wiki/Terminal_velocity)). [Zhang et al. (2001)](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.467.1727&rep=rep1&type=pdf) describes the relevant components of the dry deposition velocity.
daniel.heydebreck
CC BY-SA 3.0
2017-04-12T18:45:12.617Z
@daniel.neumann So would an ice crystal so small that it's terminal velocity was of the order of ten kilometers per month even survive six months? It may have to be only a few monolayers thick, and acquire and loose molecules so often that it wouldn't even be the same ice crystal by the time spring comes around. In that case the chlorine is constantly moving from one crystal to another, rather than the simpler picture of it adsorbing and staying stuck to a crystal which then hovers for six months before releasing it in the spring.
uhoh
CC BY-SA 3.0
2017-04-13T08:24:11.563Z
correction of my link to Zhang et al. (2001) above: [doi: 10.1016/S1352-2310(00)00326-5](http://dx.doi.org/10.1016/S1352-2310(00)00326-5). Zhang's work is based on [Slinn (1981)](http://dx.doi.org/10.1016/0004-6981(82)90271-2).
daniel.heydebreck
CC BY-SA 3.0
2017-04-13T08:42:54.010Z
The dry deposition process is controlled by gravitational settling, Brownian motion and wind. For ultra-fine particles the gravitational settling becomes negligible. You may calculate the terminal velocity of a fine particle (d = 1 $\mu m$; simplification: density difference to air = 1000 kg/m3, $\mu$=2 $\mu Pa \times s$)
daniel.heydebreck
CC BY-SA 3.0
2017-04-13T08:44:36.593Z
@daniel.neumann thanks! I'll hit the library this weekend, where I can spend some time and read through carefully. I appreciate the follow-up!
uhoh
CC BY-SA 4.0
2020-03-03T20:38:01.327Z
I wouldn't be surprised if the [polar vortex](https://en.wikipedia.org/wiki/Polar_vortex) over Antarctica might have something to do with it.
Fred
<air-pollution><antarctic><ozone>
How can ice crystals with adsorbed chlorine remain over the south pole all winter?
uhoh
# How can ice crystals with adsorbed chlorine remain over the south pole all winter?

(Asked by: username_0 on 2017-04-09)

I just saw the Periodic Video [Ozone on Bondi Beach - Periodic Table of Videos](https://youtu.be/YxsxfsYxA4s) 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?

**Tags:** air-pollution, antarctic, ozone

> **Comments:**
> **username_1**: 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.  
> **username_0**: @daniel.neumann Thanks! They still feel the same g of 9.8 m/s^2 ($\mathbf{a}=\mathbf{F}/m$) so what is holding them up? Are they so small that collisions with air molecules keeps them elevated for months? Hopefully a link can be found to address the *How?*  
> **username_2**: @username_0, 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.  
> **username_0**: @username_2 not at all, thanks! In this case it's six months, not six hours. I'm trying to understand if the individual crystals really remain intact, hovering, for a half year, or if I am taking the description to literal. Maybe chlorine is moving around from one crystal to another as one sublimates and the next forms, and the crystals do not just sit up there all that time?  
> **username_2**: @username_0, it's certainly not my prime knowledge area (especially in Florida!), but perhaps https://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!  
> **username_1**: @username_0 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](https://en.wikipedia.org/wiki/Stokes%27_law#Terminal_velocity_of_sphere_falling_in_a_fluid) and [Terminal Velocity](https://en.wikipedia.org/wiki/Terminal_velocity)). [Zhang et al. (2001)](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.467.1727&rep=rep1&type=pdf) describes the relevant components of the dry deposition velocity.  
> **username_0**: @daniel.neumann So would an ice crystal so small that it's terminal velocity was of the order of ten kilometers per month even survive six months? It may have to be only a few monolayers thick, and acquire and loose molecules so often that it wouldn't even be the same ice crystal by the time spring comes around. In that case the chlorine is constantly moving from one crystal to another, rather than the simpler picture of it adsorbing and staying stuck to a crystal which then hovers for six months before releasing it in the spring.  
> **username_1**: correction of my link to Zhang et al. (2001) above: [doi: 10.1016/S1352-2310(00)00326-5](http://dx.doi.org/10.1016/S1352-2310(00)00326-5). Zhang's work is based on [Slinn (1981)](http://dx.doi.org/10.1016/0004-6981(82)90271-2).  
> **username_1**: The dry deposition process is controlled by gravitational settling, Brownian motion and wind. For ultra-fine particles the gravitational settling becomes negligible. You may calculate the terminal velocity of a fine particle (d = 1 $\mu m$; simplification: density difference to air = 1000 kg/m3, $\mu$=2 $\mu Pa \times s$)  
> **username_0**: @daniel.neumann thanks! I'll hit the library this weekend, where I can spend some time and read through carefully. I appreciate the follow-up!  
> **username_3**: I wouldn't be surprised if the [polar vortex](https://en.wikipedia.org/wiki/Polar_vortex) over Antarctica might have something to do with it.  

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## 0 answers

