This defines the major divisions of this course. We can illustrate part of the concept by considering one molecule, ozone. Ozone is a simple triatomic, reactive molecule formed from three oxygen atoms. It is a powerful oxidizing agent. For example, in the smog of Los Angeles a good test for ozone is to stretch a rubber band in the ground level atmosphere and measure the time taken for it to snap.
Ozone reacts with the rubber polymer to weaken it's structure.
Suppose we breath air containing ozone. It would be no surprise that this would be unhealthy. Therefore, ozone in the troposphere is bad for humanity at the street level. After chemical studies we now know that most of the photochemical oxidants, expressed as ozone, come from the reaction of hydrocarbons from automobiles, oxides of nitrogen from combustion and from sunlight. The way in which this works is fundamental chemistry.