Ozone production in the troposphere comes from the photolysis of nitrogen dioxide (NO2),

NO2 + light ⇒ NO + O(3P), (1)

which generates nitric oxide (NO) and a species of oxygen atom, O( 3P), that is highly reactive, referred to as the triplet state. In the presence of a spectator molecule like nitrogen (N2) or oxygen (O2)—designated M in this reaction—the triplet oxygen reacts with molecular oxygen to form ozone (O3):


In turn, the ozone can react with the nitric oxide produced in the first step

O3 + NO ⇒ NO2 + O2 (3)

to produce nitrogen dioxide and molecular oxygen, thus circling back to the original atmospheric components. In this cycle of reactions, no net ozone is produced.

Under typical atmospheric conditions, McRae said, the cycle is very rapid, so that the three key species are approximately in equilibrium:


"Since most nitrogen oxide emissions are in the form of nitric oxide (NO), these relationships suggest that ozone levels should be quite low, on the order of 0.02 ppm," according to McRae and Russell. They noted that the levels that are actually observed can be an order of magnitude higher than this. Los Angeles has peak ozone levels above 0.30 ppm, and Mexico City's levels are often higher. Thus, "one of the key questions in atmospheric photochemistry is to understand how ozone can accumulate to these high levels in urban and rural regions. Part of the answer," the authors said, derives from the other primary atmospheric species involved in oxidation, the hydroxyl radical.

The hydroxyl radical is chemically adept at oxidizing organic species present in the urban atmosphere and as a consequence is largely responsible for elevated ozone levels in and around cities. Hydroxyl radicals can be produced in several ways. When an ozone molecule absorbs a relatively high-energy photon, it dissociates to form molecular oxygen and a relatively long-lived, electronically excited atom known as the singlet D state (1D):

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