Friday, April 08, 2011

Eli Is Old

Michael Tobis continues to lament that many years of USENET and blogging has been for naught, but Eli has a different take, which was brought home when he recently read Kate's Climate Sight post on ozone depletion. Remember MT, opposition to the Montreal Protocols was furious and involved many of our still favorite characters, S. Fred Singer, the GMI trio and more, but this ship has sailed so far into the sunset that even Kate, let alone her readers make the simplest of mistakes.

They make these mistakes, because outside of a few true dead enders, ozone depletion is history except for the folks involved in monitoring the recovery and the newspaper article in September which tells us about the depth of this year's hole. Of course, the article has moved well back into the depths of most newspapers.

We, the people of the world, have met the challenge and the younger folks think of it as sort of Science War II, before they were born stuff. Eli has good news though, there finally is a worthy successor to Robert Parson's Ozone FAQ (which still ain't bad) with snazzy figures and all: Twenty Questions and Answers About the Ozone Layer. It is accessible to all, for example, this is the answer to the old saw about how can those heavy CFCs make it up to the stratosphere

Heavier-Than-Air CFCs
CFCs and other ozone-depleting substances reach the stratosphere despite the fact that they are “heavier than air.” For example, molecules of CFC-11 (CCl3F) and CFC-12 (CCl2F2) are approximately 4–5 times heavier than the average molecule of air, since air is composed primarily of oxygen and nitrogen. The emissions of long-lived gases accumulate in the lower atmosphere (troposphere). The distribution of these gases in the troposphere and stratosphere is not controlled by the molecular weight of each gas because air is in continual motion in these regions as a result of winds and convection. Continual air motions ensure that new emissions of long-lived gases are horizontally and vertically well mixed throughout the troposphere within a few months. It is this well-mixed air that enters the lower stratosphere from upward air motions in tropical regions, bringing with it ozone-depleting substances emitted from any location on Earth’s surface.

Atmospheric measurements confirm that ozone-depleting substances with long atmospheric lifetimes are well mixed in the troposphere and are present in the stratosphere (see Figure Q8-2). The amounts found in these regions are generally consistent with the emission estimates reported by industries and governments. Measurements also show that gases that are “lighter than air,” such as hydrogen (H2) and methane (CH4), are also well mixed in the troposphere, as expected, and not found only in the upper atmosphere. Noble gases from very light helium to very heavy xenon, which all have very long atmospheric lifetimes, are also uniformly distributed throughout the troposphere and stratosphere. Only at altitudes well above the troposphere and stratosphere (above 85 kilometers (53 miles)), where much less air is present, does the influence of winds and convection diminish to the point where heavy gases begin to separate from lighter gases as a result of gravity.
Notice how the sum of chlorine source gases (the CFCs, methyl chloride, etc), photochemical intermediates and end products (HCl) stays the same with altitude.


Holly Stick said...

Just in the last few days there are reports of an unusual depletion of ozone over the Arctic:

Anonymous said...

The idea that CFCs are heavier than air is kind of funny if you think about it. Why is air air in the first place? Why don't we have a big layer of Nitrogen on top of a big layer of Oxygen? How could it be that the Oxygen and Nitrogen mix but the CFCs don't? The same questions apply to C02 heavier than air arguments as well.


Anonymous said...

I guess Water falls out at higher altitudes not because of molecular weight but because of, uh, freezing?

EliRabett said...

Yep, condensation to either liquid or solid phases

Anonymous said...

Anonymous@12:15 PM: Note that water molecules are lighter that either Nitrogen or Oxygen molecules. As a consequence, humid air (at a specified temperature and pressure) is less dense than dry air.

Anonymous@9:04 PM: It's not that simple. Nitrogen and Oxygen molecules are close in mass (28 vs. 32 amu), whereas CFCs are much heavier (121 for Freon-12, CF2Cl2). If you plug those numbers into a Boltzmann distribution, you end up predicting that at atmospheric temperatures nitrogen and oxygen should be pretty well mixed but the CFC's really should be confined to the very bottom of the atmosphere. In reality, they are all very well (not just pretty well) mixed. The fallacy lies in the implicit assumption that the chemical composition of the lower atmosphere can be calculated by assuming that it is in thermal equilibrium. It's a subtle point, because people routinely assume thermal or local thermal equilibrium to calculate other kinds of atmospheric properties (and are, usually, justfied in doing so.)

Robert Parson

Alexander Harvey said...


Do you recall whether CFCs were known to be GHGs (and considered significant) at the time of the Montreal Protocol process 1987-1988, they certainly were by the time (1990) of the original IPCC AR?

Also I should like to read any thoughts you have on why the protocol is not more lauded as an example of a GHG success story. I am aware that they have had a marginal role in absolute terms but they had been significant in terms of the rate of increase in GHG forcings.


Robert Parson said...

Yes, Ramanathan was writing about the greenhouse gas properties of CFCs in 1975:

Ramanathan, V., 1975: Greenhouse Effect Due to Chlorofluorocarbons: Climatic Implications, Science, 190: 50-51.

Alexander Harvey said...

Robert, Thanks.

But I must see this as being even more puzzling when it is not commonly seen as a success in AGW terms.

Hank Roberts said...

> not commonly seen as a success in AGW terms.
It was a side benefit, compared to not losing the ozone layer.

You can still find people denying the ozone science (one of the commenters in this thread)

And people denying the CO2 forcing effect (the topic author here)

EliRabett said...

Perhaps the biggest failure was the introduction of fluorocarbons in the 1990s as solvents and heat transfer fluids to replace cfcs. Fluorocarbons have huge global warming potential. (google fluorinert 3M0) They are only now being replaced.

Anonymous said...

Robert Parson - I've found that nothing is simple if you try and use the kinetic theory of gases to explain it. I'm a little unclear how one applies the Boltzmann distribution to a multi-component mixture of gases with gravity. Perhaps you could explain.


Padraig said...

In a recent chat I had with a young fellow on line I was given a rebuttal which used the arguement that carbon dioxide was a heavy gas and as such could not rise into the upper atmosphere and exert an influence on the climate. -- Well in point of fact he did not make his arguement in quite that manner. He said: "CO2 reaching the atmosphere? How can that be possible when not even oxygen can reach that altitude and CO2 is, what, ten times heavier?" -- This sentence is not even coherent. However, one can more or less understand what is being gotten at. The point though is that having made this point he then followed up with a brief message requesting that I not comment back "because (he) really would prefer NOT to go into an argument about this." So as a matter of course I didn't follow up on his comments.

Dependent upon ones sense of humor this might be amusing. Somehow it is not even slightly amusing to me. I find exchanges of this sort food for thought. The thoughts are not happy ones.

Robert Parson said...

Anonymous@6:06 AM: the relevant parameter is the "scale height". In thermal equlibrium, the density of an atmospheric gas is given by rho(z) = rho(0) exp(-(mgz)/(kT)), where m is the molecular mass, g is the gravitational acceleration, z is the altitude, k is Boltzmann's constant, and T is temperature. We can rewrite that as rho(z)/rho(0) - exp(-z/h), where h is the "scale height" = (kT)/(mg) = (1000 RT)/(Mg), where M is the molar mass in grams per mole, R is the "gas constant" (R = kN where N is avogadro's number). The factor of 1000 accounts for the fact that molar masses are measured in grams/mole whereas everything else uses kg for the mass scale. The scale height is the altitude at which the density of a particular gas will have dropped to 1/e (37%) of its value at sea level (z-0), in otherwords 63%
of the gas will be found at lower altitudes. Taking T-280 K, one finds:

Molecule Molar Mass Scale height
Nitrogen 28 g/mol 8.5 km
Oxygen 32 g/mol 7.4 km
CO2 44 g/mol 5.4 km
CF2Cl2 121 g/mol 2.0 km

So if the chemical composition of the lower atmosphere were determined by thermal equilibrium, one would expect only a slight decrease in the O2/N2 ratio as you go up, but that CF2Cl2 (aka Freon-12) would be almost entirely confined to the lowest part of the troposphere, far below the stratosphere (which starts at 12-15 km). In reality, O2, N2, CO2, and CF2Cl2 are well-mixed all the way up to and well into the stratosphere. Which tells us that thermal equilibrium does not apply in this case. It's not hard to understand how this could be true in the troposphere, where a major thunderstorm can carry air from ground to nearly the top of the troposphere in a matter of hours. It's harder to see why it's also the case in the stratosphere, where convection is suppressed. Nevertheless it's true: gases do not segregate by molecular weight in the troposphere, stratosphere, or mesosphere. That's what the data tells us.


Alexander Harvey said...


"It was a side benefit, compared to not losing the ozone layer."

Well that didn't answer the question as to why the protocol is not commonly seen as a success in AGW terms.

It seems to have been rather a large side benefit in terms of AGW forcings.

From data published by Hansen in 2000 I get the following for the rate of increase in forcing due to four primary constituents (W/m^2/yr)

CO2 0.025
CFC 0.009 (just CFC-11 and CFC-12)
CH4 0.008
NO2 0.002

Of that bunch CFCs are ~ 20%

With regard to the perceptable rate of increase after the effects of sulphates are account for the CFCs perhaps constituted ~ 40% as would CH4.

The rate of increase due to CFC-11/12 is now around 0,

If we had had twenty more years with a 0.009 rate we would have 0.18 W/m^2 of unnecessary forcing about 9% of the suspected net rise in forcing since the mid 1800s (taken as 2W/m^2).

In a world not noted for successful efforts to reduce forcings, 9% seems worthy of note, hence the question as to why it is not more commonly mentioned. In terms of the rate of increase in net perhaps we are looking at something in the range 20%-40%. I will defer to any better more recent values.

In case anyone might think that mine is a trick question, may I point out that the bigger the value of the success the bleaker the outlook.


Anonymous said...

Thanks for your tips, Eli - always nice to have readers looking out for accuracy in my posts! Chemistry never was my favourite science...