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Ozone Depletion Chemistry

Posted on: April 1, 2018

  4. The Montreal Protocol subsumes that without human intervention the amount of ozone in the stratosphere is invariant and that a decline in ozone over time is a trend and not part of long run cyclical phenomenon. The observed depletion is thus assumed to be man-made and the causative agent is identified as CFC. As a result of these conclusions a global ban on Freon refrigerants was hastily issued. The ban and its rationale are controversial.
  5. The Protocol has caused hundreds of billions of dollars in economic losses worldwide and at the same time it has created a black market for Freon of which the news media have taken note. News reports portray the Montreal Protocol as good and the black market as bad. A close examination of the data raises serious questions as to the validity of this judgement.
  6. The ultraviolet spectrum in incident solar radiation comes in three frequency bands. The high energy band (200-240 nanometers in wavelength) and the medium energy band (240-300 nanometers in wavelength) are harmful to living matter and are absorbed in the ozone layer while the low energy band (300-480 nanometers in wavelength) reaches the earth’s surface and causes tanning.Ozone plays a role in the absorption of harmful UV radiation. It is both created and destroyed in the absorption process.
  7. The high-energy band UV is absorbed by oxygen molecules. The energy absorbed causes the oxygen molecule to break apart into extremely reactive oxygen atoms. A subsequent chance collision of these particles with other oxygen molecules causes the formation of ozone. The ozone thus formed then absorbs the medium-energy UV band and disintegrates back into oxygen.The UV absorption process is a cyclical one that begins and ends with oxygen. Ozone is a transient intermediate product of this process.
  8. The reason that there is any ozone accumulation at all in the stratosphere is that, of the three reactions, the second is the slowest. Sunset finds the stratosphere with an excess of single oxygen atoms still looking for a date with an oxygen molecule. Overnight, with no radiation to destroy their product, these particles build up an inventory of ozone whose destruction will begin anew at sunrise.There is therefore, a diurnal cycle in the ozone content of the stratosphere whose amplitude, incidentally, is of the same order of magnitude as reported ozone depletion that caused Montreal Protocol to be invoked.
  9. A longer but irregular cyclical pattern in stratospheric ozone coincides with the sunspot cycle. The period is approximately eleven years. It has been as long as 17 and as short as 8 years. High-energy band UV increases by 6 to 10% during periods of high sunspot activity but the medium-energy UV emission is largely unaffected. Therefore, high sunspot activity favors ozone accumulation and low sunspot activity is coincident with ozone depletion.
  10. A somewhat similar pattern exists in the case of polar ozone holes. The UV induced reactions described above occur only over the tropics where sunlight is direct. Ozone is formed over the equator and not over the poles. Equatorial ozone is distributed to the poles by the Brewer-Dobson Circulation (BDC). The shape and position of the BDC changes seasonally and also shifts over a longer time cycle. Therefore, the efficiency of the BDC in transporting ozone to the greater latitudes changes seasonally and also over longer time cycles. Brewer, A. W. “Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere.” Quarterly Journal of the Royal Meteorological Society 75.326 (1949): 351-363.
  11. When the distribution of ozone is not efficient, localized “ozone depletion” appears to occur in the extreme latitudes in the form of what has come to be called an ozone hole. These holes come and go in natural cyclical changes and are not the creation of chemical ozone depletion.
  12. Concurrent with the ozone hole scare, climate scientists report that the warming trend has weakened the Brewer Dobson circulation . This connection between climate and ozone appears to indicate that warming can create more frequent and larger ozone holes. Butchart, N., et al. “Simulations of anthropogenic change in the strength of the Brewer–Dobson circulation.” Climate Dynamics 27.7-8 (2006): 727-741. However, the effect of global warming or of changing atmospheric composition on the Brewer Dobson Circulation remains controversial. Garcia, Rolando R., and William J. Randel. “Acceleration of the Brewer–Dobson circulation due to increases in greenhouse gases.” Journal of the Atmospheric Sciences 65.8 (2008): 2731-2739.
  13. The case against CFCs is that when they get to the stratosphere by diffusion, they absorb high-energy band UV and form unstable and reactive chlorine atoms. The chlorine atom particles then participate as catalytic agents to convert ozone back to oxygen. In other words they mediate the reaction between atomic oxygen particles and ozone. It is alleged that the destruction of ozone by this mechanism exposes the surface of the earth to dangerous levels of medium-band UV because there is not enough ozone in the stratosphere to absorb them. Although these reactions can be carried out in the chemistry lab, there are certain rate constraints that make them irrelevant in the stratosphere.
  14. The air up there in the stratosphere is rather thin, containing less than one percent of the molecular density of air at sea level. It is not easy for a molecular particle in random thermal motion to find another particle to react with. Photochemical reactions occur instantaneously while those that require a collision of two particles take much longer. This difference in the reaction rate is the reason that ozone accumulates overnight and why there is an inventory of ozone in the ozone layer.
  15. The atomic oxygen particles that react with oxygen molecules to form ozone could in theory react with an ozone molecule instead and cause its destruction or it could react with another atomic oxygen particle and form oxygen instead of ever forming any ozone. Some of the oxygen atoms do behave in this manner but these reactions proceed too slowly to be important to the chemistry of the stratosphere. The reason is that the stratospheric chemicals in question exist in minute quantities.
  16. One in a million particles is an ozone molecule or an atomic oxygen particle and one in a billion is CFC or chlorine generated from CFC. The accidental collision between chlorine atoms and ozone molecules or between chlorine atoms and oxygen atoms are rarer than those between two oxygen atoms or that between an oxygen atom and an ozone molecule. Therefore the latter collisions are more important to ozone depletion than those mediated by chlorine.
  17. Considering that more than 200,000 out of a million molecular particles in the stratosphere are oxygen molecules it is far more likely that charged oxygen atoms will collide with oxygen molecules rather than with each other or with ozone. Therefore ozone rather than oxygen is formed. Ozone formation is a rate phenomenon.
  18. Since chlorine atoms are a thousand times rarer in the stratosphere than atomic oxygen particles, it is not likely that chlorine’s mediation in short circuiting ozone generation will occur sufficiently fast to be important. Nature already contains an ozone destruction mechanism that is more efficient than the CFC mechanism but ozone forms anyway.
  19. However, the argument can be made that overnight after sunset, as charged oxygen atoms are used up the charged chlorine atoms take on a greater role in ozone destruction and also when these chemicals are distributed to the greater latitudes where sunlight is less direct and too weak to be ionizers of oxygen, the only ozone destruction chemistry left is that of charged chlorine atoms colliding with ozone. The  relative importance of these overnight and greater latitude reactions in making changes to latitudinally weighted mean global ozone can be checked by examining its overall long term trends as well as its trend profiles. These data are shown in the data analysis documents linked below. They do not show the ozone depletion described in the Montreal Protocol.

3 Responses to "Ozone Depletion Chemistry"

Where are the ozone holes located? How can we fill them up?

the south pole
the hole is created by changing wind patterns that distribute ozone from the tropics (the only place where ozone is formed) to the higher latitudes.

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