Ozone Layer on the rebound?

[Umfriend]

I heard on the radio today that the Ozon Layer was no longer decreasing and that in fact in some places it's slowly rebuilding already. First estimate was it would be as in the good ole days within 30 yrs.

[Fat Tone]

That would be good, but the BBC has some bad news on the Antarctic hole: http://news.bbc.co.uk/1/hi/sci/tech/4197566.stm

[Brian Ellis]

I heard on the radio today that the Ozon Layer was no longer decreasing and that in fact in some places it's slowly rebuilding already. First estimate was it would be as in the good ole days within 30 yrs.

That is utter nonsense. The 2004 Antarctic ozone depletion was the worst on record, equal to the 2002 one. The ozone levels over Europe this spring were the lowest on record. The Scientific Assessment Panel of the Montreal Protocol forecasts that ozone depletion RISES is slowing down and will reach its peak between 2030 and 2050, assuming that some joker does not release an undue amount of OD gases in the meanwhile. It will take to about the end of this century to reach 1970 levels of Dobson Units and another 150 years before it reaches 1930 levels (pre-industrial CFC production). We are slowly winning the long-term battle but when OD gases have atmospheric residence times of typically 100 to 300 years, you cannot expect miracles.

Just in case you think I'm being alarmist, let it be known that I was professionally involved with this from 1988 to 2003 and have sat on (and chaired) national and international committees set up by the Parties to the Montreal Protocol, as well as being commissioned by the UN on the subject. I'm also one of the few non-US citizens to have received the US EPA Stratospheric Ozone Protection Award. So, please give me a little credit to know what I'm talking about.

[Nowhere]

I wonder Brain, are there some research into getting rid of OD gases, for example by releasing something else to the atmosphere that would destroy them but not much/nothing else? Or is that impossible? (AFAIK the most important reason for using them in the way they were used is because they're somehow stable...). Or is it considered too much risk, releasing another very active compound to the atmosphere?

[Brian Ellis]

CFC gases and halons are very stable and chemically inert. Most organic gases decompose in the atmosphere by hydrolysis. A small number of water vapour molecules split into a hydrogen radical H+ and a hydroxyl group OH-. An OH- group can latch onto a hydrogen atom in an organic molecule and, because the H-C bond is relatively weak, it can pull it off. CFCs and halons are perhalogenated, meaning that the C atoms are covalently bonded to (usually) a F, Cl or Br atom and these bonds are stronger than a C-H bond and an OH radical isn't tough enough to pull the CFC molecule apart. So hydrolysis cannot occur with these perhalogenated substance, which accounts for the long residence time in the atmosphere.

However, the energy levels of the UV light at a given wavelength in the stratosphere above ~12 - 20 km up to ~50 km altitude are strong enough to knock off Br and Cl atoms (but not F) and it is these atoms that cause the ozone depletion in a massive chain reaction (one Cl atom will destroy a mean estimated 700,000 O3 molecules and Br is 50 times worse). This is the only major natural destruction mechanism of CFC and halon molecules, called photolysis, so we have to wait until these drift up to these altitudes to be destroyed by a variety of transport mechanisms, hence also their long life.

There is a third destruction mechanism, which is negligible on a natural scale, pyrolysis. A CFC molecule taken to about 600°C or higher will be destroyed (this is how recovered CFCs from old fridges etc. are destroyed). Pyrolysis results in the release of all sorts of nasty gases such as hydrogen fluoride and chloride, carbonyl fluoride, phosgene etc., so the flue gases from a pyrolytic destruction plant must be well scrubbed clean.

It is physically impossible to pyrolyse or photolyse the totality of the earth's atmosphere to rid something in a pptv concentration and doing so would require more energy than we possess (the sun cannot do it at tropospheric altitudes!). Chemically, it is also impossible to treat the whole of the earth's atmosphere, either, even if we knew of a chemical that would do it. Perhalogenated substances are, by definition, stable, which means unreactive. It is conceivable that decomposition may occur in a very strong electric field under some given chemical conditions, but a) the cure would be worse than the disease and b) the energy requirements would be ginormous.

So the short answer to your question is that, no, we cannot do anything but allow nature to take its photolytic course.

[Nowhere]

Hmm..I think I have some shift of my basic understanding of the whole ozone depletion issue, if I understand your second paragraph correctly.
Are you saying that CFC gases are not directly responsible for the ozone depletion, but instead one of the products of their destruction by UV light is?
And btw, at what altitude is the highest concentration of CFCs? And what happens to Br and Cl atoms knocked off CFCs at high altitudes?

[Brian Ellis]

Roughly speaking, the distribution of CFC and other OD gases is homogeneous throughout the global atmosphere. If a "packet" of gases were released at a given place on earth (except at the zenithal latitude of the sun ±5°), it would circumnavigate the globe at that latitude in 10 - 14 days, then become homogeneous at that latitude ±5° within about a month, then become homogeneous throughout the N or S hemisphere within about a year and globally in about 5 years, up to the altitude of the tropopause. These are very rough approximations as it is weather-dependent, taking into account the three major convection cells, especially the Hadley cell, the jet streams etc. The tropopause is generally roughly at 8 km altitude at the poles, 15 km at the sun's latitude but pushed up to "bubbles" of 18 km by tropical thunderstorms. The anvil head of a thundercloud forms when the updraft of a storm meets the tropopause, lifting it. Mixing of tropospheric air with stratospheric air, through the tropopause, is slower, as there are no very violent mechanisms and it is partially simple diffusion and homogenisation might take 15 or 20 years until the "packet" that was released is uniform throughout the whole atmosphere, above and below the tropopause.

Yes, it is the free photolysed Cl or Br atoms that cause the depletion, not the CFCs themselves, which are only stable carriers. That is why the Cl in sea salt spray does not cause any substantial ozone depletion; a) it is mostly rained out long before it can reach the stratosphere, being water-soluble (CFCs are not) and b) the bond between the Na and the Cl is ionic and cannot be broken by photolysis.

As a simplification, the stratospheric mechanism is as follows:
some oxygen molecules are split by UV photolysis into two atoms (the energy absorbed in this reaction is at the wavelength that causes so much UV harm where we live and this is why the ozone layer is so valuable for life support on earth):
O2 > 2O
monoatomic oxygen reacts with other oxygen molecules:
O + O2 > O3
occasionally, 2 ozone molecules will collide reforming oxygen:
2O3 > 3O2

This formation and destruction of ozone is a constant process and is in equilibrium.

Let's introduce a chlorine atom and see what happens:
Cl + O3 > ClO + O2 (one molecule of ozone is destroyed)
ClO + O3 > 2O2 + Cl (a second O3 molecule is destroyed but the Cl atom is free to repeat the process 700,000 times, until it diffuses down to the tropopause where hydrolysis could cause a different reaction and the chlorine will eventually be rained out, possibly in the form of HCl or HmClOn)
So the ozone layer becomes ozone-poor and oxygen-rich, but the oxygen cannot photolyse back to ozone fast enough and the natural equilibrium is seriously upset.

I emphasise that this explanation is very much a simplification; there are actually tens of different chemical reactions contributing to ozone depletion, of which this is only one.

Now, here's a question. The ozone layer is not really a layer in the sense that it contains only ozone: it is a diffuse zone of thin air which is richer than normal in ozone. If you could take all the ozone in a column of, say, 1 metre square (or diameter) from the earth's surface to outer space and bring it down to the earth's surface at normal atmospheric temperature pressure, how thick would that layer of ozone be?

The technical answer is 300 - 400 DUs (Dobson Units), averaged over the earth's surface, thinning to well under 100 DUs in the Antarctic in September, rising to ~500 DUs in the tropics. But what is a DU? It is the equivalent of 0.01 mm. Yes, we are being protected by an equivalent layer as thin as 3 - 4 mm thick of ozone, so its fragility becomes obvious, doesn't it? Dobson was one of the pioneers of ozone layer measurement in the 1920s and the Dobson meter is still used today for earthbound thickness measurements (complemented by satellite observations, which are actually less accurate but more informative as to the nature of the depletion).

So, when you see yesterday's Antarctic map with depletion down to 150 DU, knowing that the worst won't happen for 2 - 3 weeks yet, you will understand the severity of the problem and why my colleagues and I have slaved our guts out for so many years to minimise the damage.

[Nowhere]

Thanks for explanation.
The reason I was asking about how the gases are beiing distributed was wonderingabout how feasible it would be to destroy CFCs at ~sea level, and what impact it would have to their number at high attitudes. From your description I gather - negligible, especially when I realized how small in number they essentially are (not to mention problems...energy...and so on)
I also guess that, because of its properties, we could produce ozone only at the alititude of ozone layer, which is utterly impractical? (OTOH...how would that look on one of the poles? Quite deserted areas...but could bring too much instability and unpredicteness to the climate I guess...)

[Fluff]

At how many DU does it cause MAJOR problems <150?. What level do you expect it to be in 2050?

[Nowhere]

Hmm...and I wonder what DU was in late March above PL, when that "ozone hole" above Central Europe formed.
Somehow I connect it with the fact that I had slight sunburns on face after, I don't know, 3h on the Sun...in late March!!!
I never menaged to get sunburns even in May before...

[Brian Ellis]

The DU in itself is not an indication of the problems. It is the UV passing through the weakened ozone layer that causes the problems, combined with its angle of incidence and the reflectivity of the ground (on snow or ice, for ex., you get almost a double dose). It's therefore not possible to be categorical and say >n DUs is safe and <n DUs is dangerous. What we can say is that, all other things being equal, the risk increases as the DU value diminishes.

I vaguely remember hearing that the European depletion this spring dropped to about 250 DUs, but don't take this figure as gospel. I did hear also that some glacier skiers suffered badly, despite the fact that they use near-total sun block.

The DU figure varies from place to place and from hour to hour. If you look at the map above, you will see it is thickest south of Australia, yet S. Australia is one of the places where UV-induced melanomas are on the largest increase, indicating the inhabitants have been exposed to high UV levels, because it is sometimes thin there. It is not even possible to predict more than a few hours in advance, as it is stratospheric weather-driven as well as solar-driven. To predict 45 years hence would therefore be impossible. However, by extrapolation of the OD gas content of the atmosphere, it would be fairly safe to say that the global average ozone layer thickness will probably diminish by 50-100 DU at the peak. I'll not hazard a guess as to where this will be worst or best, other than higher latitudes will be worse hit than lower ones, but to what extent????

[Nowhere]

:/

So...it was about 250, in a sunny day in march, small number of clouds here and there.
What will be in June if it drops one day to 50-100... :/

[Brian Ellis]

You would fry!

However, I said it may drop BY 50-100, not TO 50-100. That means the average in mid European latitudes would drop from 350 - 400 to 250 - 350 DU, possibly lower in spring.

[Nowhere]

Ahh, I misunderstood. Still, not very good perspective...that's average after all - so I suppose we can expect "UV alarms" at some point? (as I understand from previous posts, fluctuations are big enough for this...)

[Sasq]

at least last I was there, in Australia, we already get burn warnings in summer.
the warnings are given in minutes.
ie how many min it takes for a fair skinned person to get sunburned.