Rule changed: made safe and easy refrigeration possible. Raised environmental awareness.
Chlorofluorocarbons (CFC) is a class of compounds, the simplest among them with a structure similar to the one of methane: a tetrahedron. A simple representative is the one pictured below, Dichlorodifluoromethane. It's a molecule made of one carbon atom (in the center, black), two chlorine atoms (on top, green) and two fluorine atoms (on bottom, pea green).
[caption id="" align="aligncenter" width="150" caption="Dichlorodifluoromethane, one compound in the CFC class"]`|Dichlorodifluoromethane| <http://en.wikipedia.org/wiki/File:Dichlorodifluoromethane-3D-vdW.png>`_[/caption]
The class includes slightly more complex molecules, but all of them have one thing in common: they are made of carbon, chlorine and fluorine (and occasionally, hydrogen). In this post, we will briefly examine the history of CFC compounds, why they were so disruptive at the time, why they turned out to be so dangerous, and why their contribute to human knowledge was a strong wake-up call for everybody on this little blue planet.
Why CFCs? A brief history of making cold
Just out of the trees and into the caves, humanity learned how to make heat. Controlling fire was maybe the first important technological advancement of humanity, as it improved quality of food (cooked meat is easier to eat and digest, and cooking kills parasites), safety (dangerous animals don't like fire) and health by warming the cold, humid cave. It took a very low initial technological level to heat things, for a very valid reason: making heat is easily done because the chemical reaction between organic matter and oxygen (that is, burning) is relatively easy to start, produces a lot of heat, and requires components that are easy to find. The opposite operation, cooling, took much longer and way more advanced knowledge.
|Ice man| <http://en.wikipedia.org/wiki/File:Bundesarchiv_Bild_183-47890-0001,_Berlin,_Kinder_mit_dem_Eismann.jpg>`_Before the invention of refrigerators there were basically three techniques to cool down things, typically for preservation purposes. The first technique was storing ice and snow in `ice houses during winter. Good insulation and the mass of accumulated snow allowed to keep low temperatures during the warm months. Later in time, services were built around the need for cold, cutting and transporting ice blocks from cold regions to service warm ones. People went around in carts selling ice blocks, typically giving small chunks of ice to children to have fun with. Your parents may recall being among these children. The blocks of ice were put into ice boxes, together with perishables.
Another solution to make cold was mixing common salt and snow, to create a frigorific mixture due to its eutectic properties. With this technique, temperatures as low as -21 degrees Celsius can be obtained with ease. The same property is used to melt snow in winter. Similar to the previous option, it requires to have solid water to begin with.
Finally, a third option was to dissolve in water some very specific salts, such as sodium, potassium or ammonium nitrate. These salts require heat to dissolve, subtracting it from the environment (the dissolution is said to be endothermic, in opposition to exothermic ones which produce heat). This concept is successfully used in instant cold packs you may find at your local sport shop. This option has two advantages, namely that it does not require something already cold to operate, and that the salt can be restored and reused by evaporating the water.
With a better understanding of thermodynamics and the state of matter, at the beginning of the 19th century the knowledge was available to develop better technology for the production of cold. Through a proper strategy of expansions and compression of well-chosen gases, efficient removal of heat was both feasible and practical, first industrially, then at the consumer level. When the first refrigerators arrived on the market, the choices for the exchange gas was limited to ammonia (highly toxic), sulfur dioxide (also toxic if inhaled in large quantities), and chloromethane (toxic and flammable). Needless to say, leaks occurred, people died, and the general public preferred the old "big ice cube in a box" solution, or they kept the refrigerator outside, where an eventual leak would pose no immediate danger. A better solution was needed.
In 1929 Thomas Midgley, Jr. and Charles Franklin Kettering teamed together to tackle the problem, and they found a good solution in CFCs, unaware of the environmental danger of their discovery. Incidentally, from the efforts of these two guys also came out the gasoline additive tetraethyl lead, another very troublesome compound. It appears they had a special sense for stumbling on ecologically devastating stuff.
In addition to refrigeration, CFCs were found useful for other tasks, such as propellants for aerosols, solvents, and fire fighting equipment. Their stable, inert and non-toxic properties were just perfect, or so it appeared.
The ozone cycle
To understand why CFCs are so dangerous, we first need to know the role of ozone in upper atmosphere. In normal conditions, ozone is a gas made of three oxygen atoms, O3.
[caption id="" align="aligncenter" width="120" caption="Ozone"]`|Ozone| <http://en.wikipedia.org/wiki/File:Ozone-CRC-MW-3D-balls.png>`_[/caption]
In comparison, the "conventional oxygen" we breathe is a molecule made of just two atoms bonded together, O2. Ozone has a characteristic smell we normally call "the smell of electricity", being generated in appreciable quantities by electrical discharges. At ground level is a dangerous pollutant, because it's highly reactive and irritant, but in upper atmosphere is our shield against the intense and carcinogenic ultraviolet radiation emitted by the Sun.
Ozone is produced through a very slow process from molecular oxygen. The molecule is smashed into individual atoms by the Sun UV radiations
O2 + UV radiation -> 2 O
and each of these oxygen atoms may attach to other oxygen molecules to form ozone
O2 + O -> O3
a reaction that releases heat via intermediary species. Ozone can now adsorb further UV radiation to split back again
O3 + UV radiation -> O2 + O
and start the cycle again. The net effect is the one of a catalyzer, a substance that eases an interconversion (in this case of dangerous UV radiation into heat) without being depleted, as would be a reactant. Instead, a catalyzer is restored in its active state once the interconversion is over, and it is ready to operate again. Please note: a minimal amount of catalyzer is able to promote a huge amount of interconversions.
Thanks to this chemistry, ozone degrades large quantities of dangerous UV radiations into innocuous heat, in a cycle known as the Ozone-Oxygen cycle.
In normal conditions, there are other two important reactions that can occur. Both destroy ozone and restore molecular oxygen
O3 + O -> 2 O2
2 O -> O2
All the reactions given above (creation, catalysis, and destruction, among many others) constantly happen in upper atmosphere. Their final balance leads to an equilibrium of a relatively stable concentration of ozone, dependent on solar irradiation, which in turn depends on seasons, latitude and solar activity.
How CFC disrupt the ozone cycle
Where do CFC enter in the game? It turns out that the biggest advantage of CFCs, their stability, is also their first major problem. CFCs are heavier than air (and thus tend to sink) but this is not preventing them to reach the upper atmosphere, helped by their long life. Diffusion and winds mix up the atmosphere constantly, creating a relevant concentration of CFCs in upper atmosphere. Once there, the second major problem arises: when hit by UV radiations, CFCs release a chlorine atom:
CCl3F + UV radiation -> CCl2F**.** + Cl**.**
The chlorine atom has a lone electron, and in this configuration is highly reactive and combines with ozone, operating as a catalyzer for the ozone distruction. The reactions are complex and numerous (if you want all the gory details, this online book is a start), but the net effect is a reduction of ozone and the creation of molecular oxygen. Remember, a catalyzer emerges unscathed from the reaction it promotes, meaning that a minimal amount of chlorine can promote the destruction of large quantities of ozone, unbalancing the equilibrium previously compensated by the slow reaction of creation O2 + UV -> 2 O. The shielding of UV radiation becomes less and less effective, on par with the decreasing concentration of ozone, and the radiation can now reach the surface.
A swift action is called for: the Montreal protocol
We owe to James Lovelock, Frank Rowland and Mario Molina, among many others if our eyes were open to a dramatic trend. During the 70s, it became clear to the scientific community that CFC were a source of trouble. Confirmation came in the 80s, where incredibly low concentrations of ozone were found over the south polar region, a "ozone hole" of unquestionable evidence.
The Vienna Convention and the Montreal Protocol, enforced on the 1st of January 1989, defined an impressive and immediate worldwide response to the problem, suppressing industrial production and use of CFCs and other ozone-depleting substances. Without this ban, the result would be the one simulated by NASA
[caption id="" align="aligncenter" width="400" caption="Ozone layer simulation by NASA"]`|image4| <http://en.wikipedia.org/wiki/File:Future_ozone_layer_concentrations.jpg>`_[/caption]
Total destruction of the ozone layer, with no chance of recovery, before 2060. NASA also released a movie of the simulation, compared side by side with the projected situation we expect with the ban enforced. You can find it at the NASA page for the simulation, or at this YouTube movie. Without ozone layer, the amount of UV radiation reaching the surface would be so high to cause sunburns in minutes, and occurrences of skin cancer would have soared globally. These, of course, would be just the direct effects on humans. The rest of the biosphere would have had an unpleasant situation as well.
CFCs have been banned from almost any application, from refrigeration to pharmaceutical nebulizers. Some temporary, highly scrutinized exceptions have been defined for those applications where no substitute could be found, such as some fire extinction strategies. The general idea is to use compounds which are either degraded before reaching upper atmosphere, or that don't contain chlorine or bromine atoms, therefore having a reduced impact on the ozone layer. Common substitutes today are R134a (a fluoroethane, also being phased out) and R600a (isobutane, much safer for the environment but highly flammable). There are also strict regulations in force, concerning maintenance, recovery and recycling of currently existing CFC (see, for example, here at US EPA). Although the dangers have been avoided, the legacy of CFCs usage will linger for at least one hundred years.
The aftermath of a potential catastrophe
The Montreal protocol and the avoided catastrophe of ozone depletion have been a huge wake up call for humanity. We realized that our planet has a fragile ecosystem, whose complexity and interdependency is broad and still to be discovered. As humans, we owe us a big pat on the back, but there are still troubles ahead: global warming, oil depletion, overpopulation. It's time we stop wasting time, because swift actions are needed again. With Montreal, we demonstrated that humanity can achieve a common goal and solve a common problem. We need strong leaders, iron-clad reason, proper actions and global effort to face the common goals and problems of tomorrow. We have only one planet, this one: