Ozone – Stratospheric or Tropospheric

What is the next word you think of if you hear the word ozone? About twenty years ago the answer would invariably have been hole, as in the “ozone hole” over the Antarctic is expanding. Currently if you hear of ozone at all, it is more likely in the form of ozone alerts which occur most commonly on hot summer afternoons in urban environments.

Ozone is another of those Dr. Jekyll, Mr. Hyde molecules. It is at the same time both beneficial and dangerously toxic. It all depends on where it is. Way up in the stratosphere, about 25 kilometers up, it protects us from deadly Ultraviolet rays from the sun, but down near the ground where we breath it is a reactive substance which damages lungs and exacerbates asthma and cardiovascular disease.

There is fairly good evidence for life arising on this planet over 3.5 billion years ago. One hypothesis is that life formed in the seas, as life on the surface of the planet was impossible due to the intense deadly Ultraviolet (UV) rays striking earth. A few billion years later and following the evolution of the chloroplast, oxygen (O2) began to accumulate in the atmosphere. In a cyclic process in the upper atmosphere the more abundant O2 is turned into Ozone (O3). This tiny bit of Ozone absorbs the dangerous UV rays and makes life possible on the surface.

Life then climbed out of the primordial soup and started building air conditioners, refrigerators, and the like which require a refrigerant to work. Compounds know as Chlorofluorocarbons (CFCs) were chosen because they were effective, non toxic and relatively stable. Bear with me here, I’ll get back to Ozone quickly.

Ozone hole over Antarctica

Ozone hole over Antarctica

It’s the stable part got that gets us in trouble. When CFCs escape the refrigerator or air conditioner, they get in the atmosphere. They are stable enough to get all the way up to the Ozone layer without falling apart. There a combination of reactions destroys the Ozone. Enough could literally wipe out life on the planet. Even small amounts of Ozone destruction lead to an increase in retinal damage and skin cancer.

And that is the story of the Ozone hole, a climatically related figure of speech referring to a lessening of the total amount of Ozone in the stratosphere. Luckily we figured this out and by an international agreement known as the Montreal Accords agreed to ban the production and use of these substances. The Ozone hole is slowly shrinking and is by about 2050 expected to be completely healed.

Now back to ground level Ozone, the Ozone we don’t want. Ozone formation at ground level where we breath is not natural. The the fault here is internal combustion engines – Cars and trucks and buses that run of gasoline or diesel.



Because engines aren’t 100 per cent efficient, some uncombusted gases escape the tail pipe. Known as volatile organic carbon (VOCs), they react with Oxygen and sunlight to produce Ozone. Luckily Ozone is unstable and degrades rapidly so if it forms in the afternoon, it is gone by the evening. Toxic ground level Ozone can be controlled by reducing the production of the VOCs. Reducing the number of cars in urban environments through mass transportation is a very effective measure. Alternately reformulating fuels to produce less VOCs works to some degree, but raises the cost. Natural gas fueled vehicles produce less VOCs and electric vehicles none.

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