Friday, June 18, 2010

It is probable that atmospheric scientists were wrong about CFC's

I have been troubled for many years over the world's decision to stop large scale production of a class of useful chemicals called chlorofluorocarbons (CFCs). As an operational engineer who was assigned the overall responsibility for a half a dozen or so large air conditioning and refrigeration units on board a nuclear powered submarine, I learned that those chemicals were had some very useful properties. They were used as the working fluids in compact, efficient machines that helped to maintain an environment that was healthy for human habitation, even though we were inside a steel tube, surrounded by high pressure sea water with no regular access to outside air or food resupply.

The air conditioning units kept the temperatures comfortable for both humans and sensitive electrical equipment, despite the fact that also sealed up inside that metal tube with 150 humans was a rather large steam power plant. The refrigeration units kept our food at a carefully measured temperature - some refrigerated, some frozen. Those machines were "mission critical" for us since we had to ensure that our food was fit for consumption even after being stored for months at a time.

There were other choices available for air conditioning and refrigeration working fluids, but they were all inferior for various reasons when compared to the R-12 and R-114 that we used. Some of them were wildly inappropriate for use on board a sealed submarine; ammonia, for example, was a potential option for air conditioning units in well-ventilated warehouses, but it would be deadly if it leaked inside a submarine. Vapor absorption air conditioners were an option, but there were no such thing as vapor absorption refrigerators or freezers. Those units could make chilled water as cold as 42 degrees F, but could not bring the temperature any lower due to the physical limits of the technology.

Anyone who has ever operated and maintained large, complex piping systems in the real world will know that there is always a potential for leaks; all systems on board submarines are carefully evaluated for potential health effects under worst case conditions. CFCs had been found to be non-toxic and not harmful to people as long as they did not displace so much oxygen as to cause suffocation.

That hazard was made less of a worry with CFCs because they are about 5-7 times as heavy as air and tend to pool in the very lowest spaces in any environment where they leak. On submarines, as is the case for most ships, there are deckplates installed for people to walk on that are raised above the very lowest parts of the ship - what sailors call the bilges. Those bilges are places where small amounts of water, oil and other liquids tend to fall and accumulate before they can be pumped into holding tanks for later removal from the ship.

During my assignments as an engineering officer, we experienced some significant leaks from our air conditioning units that released a substantial quantity of R-114 into the ship while underwater. Even though submarines are equipped with numerous fans for air recirculation and to ensure that no areas stagnate, we always located the Freon in the bilge, with the concentration being very high within just a few inches of the bottom and falling off rapidly as we raised the detector above that level. When we experienced the leaks, we were able to accept having Freon in our living spaces until it was convenient for us to go up and ventilate. When we did, we had to run blowers with suctions from the bilges to get the R-114 off of the ship.

The behavior of CFCs made sense to us and followed exactly what we had been taught about heavier than air vapors - they tend to sink due to gravity, though there is a certain amount of dispersion and diffusion caused by a material property called vapor pressure. The heavier the vapor, the faster it would sink. Generally speaking, heavier vapors will have less dispersion and diffusion. Since we were operators, not scientists or mathematicians, we did not need to be too concerned with the exact equations, but we did need to understand the physical behavior of these kinds of vapors.

Knowing that heavy vapors tend to sink and that lighter ones tend to rise can be a life saving bit of knowledge. If you are trying to get out of a space where there is smoke and fire, you know you should stay low, where there is better chance of finding breathable air. If you are in a space where there has been a known spill or leak of a heavy vapor that is toxic, explosive or flammable, you have a better chance of surviving if you understand that the vapor will accumulate in the lowest parts of the space and might be at a high enough concentration to burn, explode or kill you from inhalation.

It is with that experience of Freon leaks and safety training that I have never understood how the world's leading atmospheric scientists could have convinced themselves that CFCs tend to rise up through the atmosphere into the stratosphere where they are finally broken down. Even if a few stray molecules of the gas do make it up that high, it would defy gravity to believe that they did so at high enough concentrations to have caused any harm.

At the time that the scientists were convincing themselves that CFC's were destroying the ozone layer, a large portion of the CFC's that had ever been manufactured were still in the systems where they were being used as the working fluid. Though Freon and other CFC's were not terribly expensive (before their production rates were severely limited) and they were not considered to be toxic, the people who designed and manufactured refrigeration and air conditioning systems took care to build tight systems with little leakage.

Most of us take for granted the fact that we rarely, if ever, have to replace the Freon in our refrigerators or central air conditioning systems. Those leak tight systems add to the reliability and protect the rest of the mechanical components of the system; the CFCs are not only the fluid used to compress and expand, but they also serve as lubricants for the compressor and anti-corrosion fluid for the piping system.

Even if the CFCs did leak or were vented in order to perform maintenance or to dispose of the machinery after the end of its useful life, those gases tended to sink into basements, soil, crevasses, ravines, sewers, and any of countless other low spots in the cities and suburbs where most of the chemicals were being used. A small portion of the total will always disperse and diffuse because of the natural property of vapor pressure, but most of the material will sink and spread out.

For some reason, these facts about how heavier than air vapors behave did not seem to be taken into account by the scientists who study the atmosphere. In 1972, James Lovelock found minute quantities of CFCs in samples taken around the world using sensitive instruments capable of detecting levels in units of parts per trillion volume. He wrote about how these relatively inert gases could be used as tracers to show that the atmosphere mixed pretty well around the world - in the horizontal dimension.

One team of two scientists, Sherwood Rowland and Mario Molina wrote a paper that was published in the journal Nature proposing that since CFCs were stable chemicals that did not easily break down, and since they could be measured in the atmosphere around the world, they must be on an inevitable path upward into the stratosphere. According to the paper, there were no other obvious sinks for the chemicals.

As proposed by Rowland and Molina and as accepted by the atmospheric science community, the primary atmospheric sink for the chemicals was stratospheric photolytic dissociation where CFCl3 breaks down into CFCl2 + Cl and where CF2Cl2 breaks down into CF2Cl + Cl. The paper states that the Cl- ions serve as catalysts that extensively destroy O3 (ozone) and O (monatomic oxygen) in the stratosphere through the following two reactions:

Cl + O3 -> ClO + O2

ClO + O -> Cl + O2

The paper goes on to describe the atmospheric modeling used to support the theory that the catalytic reaction was having a significant and dangerous effect on the ozone layer. This 1974 paper and the validation of the proposed reactions by the National Academy of Sciences in 1976 was used as the basis for a world wide effort to ban the use of CFCs as a propellant in aerosol containers and then to severely limit its production and use in refrigeration and air conditioning systems.

According to the theory, the dissociated CFCs were the primary source of Cl- in the stratosphere, despite the fact that solid rocket motors as used for both space exploration and ballistic missiles use a substance called perchlorate (ClO4-, which releases free Cl- ions when burned. (Rockets tend to do at least some of their burning in the stratosphere while passing through.) The effort to ban CFCs moved in fits and starts, but once a 1987 NASA experiment reported that they actually measured CFC in the stratosphere (at concentrations in the pptv range with a rapid reduction as altitude increased) the effort succeeded in the widespread adoption of the Montreal Protocol.
Data from NASA's Airborne Antarctic Ozone Experiment in 1987 "provided the smoking gun measurements that nailed down the cause of the ozone hole being the increase of CFCs combined with the unique meteorology of the Antarctic," Stolarski said. Since then, NASA has sponsored several airborne field campaigns that have furthered understanding of the chemical processes controlling ozone.
What confused me then and still confuses me to this day is the fact that the multi-billion dollar effort to halt production of useful materials was based on a faulty assumption that was stated in a couple of different ways in the original article:
"There are no obvious rapid sinks for their removal and they may be useful as inert tracers of atmospheric motions."

"The most important sink for atmospheric CFCl3 and CF2Cl2 seems to be stratospheric photolytic dissociation to CFCl2 + Cl and CF2Cl + Cl respectively, at altitudes of 20-40 km."

"If the stratospheric photolytic sink is the only major sink for CFCl3 and CF2Cl2 then the 1972 production rates correspond at state to globally averaged destruction rates of about 0.8 x 107 and 1.5 x 107 molecules cm-2s-1 and formation rates of Cl atoms of about 2 x 107 and 3 x 107 atoms cm-2s-1, respectively."

I still do not get it. Heavy gases do not rise, they sink. Winds blow gases around in the horizontal dimension, but they do not often blow things straight up. The earth's surface is full of places where a heavy gas like CFCs can accumulate.

I did a quick computation once of how thick a layer of CFC's would have been if all of the world's cumulative production was simply spread out on the surface of the globe. The thickness of the resulting layer would be on the order of 4 x 10-8 meters, so it would be difficult to measure as that number would put it in the grass, in soils and under the leaf layers on forest floors. (Don't forget, a significant portion of the produced CFCs are still in the machinery where they do their work.) There would be no reason to assert - as some atmospheric scientists have done - that the vapors must dissipate upward, otherwise we would be swimming in a lake of the substance.

Another real world sink that Rowland and Molina overlooked is that CFCs break down under combustion heat - in fact, the EPA lists incineration as one of the ways that CFC's can be destroyed. There are billions of combustion devices at ground level around the world, any CFCs that are pulled in with combustion air into a furnace, an automobile engine, a fire, a boiler, or a jet engine will be destroyed by the process.

So please, help me to understand - why did the world's atmospheric scientists and government leaders agree that heavier than air vapors were rising up 20-40 km into the atmosphere at substantial quantities and destroying the ozone layer? Why did they accept that assumption and use it as the basis for taking useful and easily produced materials that help humans to better control their living conditions and store foods and medicines for longer periods of time off of the market? Why didn't the people who work with heavier than air gases on a daily basis object to this decision with a strong effort questioning the logic and the models used in by the scientists?

Why did the scientists who proposed this theory end up sharing a Nobel Prize in Chemistry for this particular theory about ozone layer destruction? Am I being hopelessly arrogant in questioning this particular episode in scientific history? After all, I have never published a peer reviewed paper in the atmospheric science literature, been employed as a scientist, earned a PhD or been awarded any prizes for my papers on other topics.

Additional Reading

Dynamics of Vapor-Air Mixtures published in American Industrial Hygiene Association Journal, Volume 26, Issue 5 September 1965 , pages 445 - 448 (Note: This link leads to the abstract. I have a copy of the full article that I can send on request to other interested researchers.)

Vapor Density and it’s influence on the Ventilation Decision Making Process

Deflagrations involving heavier-than-air vapor/air mixtures published in Fire Safety Journal 36 (2001) 693-710

Density of Gases Virtual Chembook, Elmhurst College

Sungas Information description of propane safety considerations