How Does Ozone Form?
Chemists frequently describe a complicated reaction in a step-by-step sequence of events or simpler reactions. There are many combinations of simple reactions that will form ozone. The series of reactions, or steps, presented below describe one scheme for ozone formation from nitrogen oxides and VOCs. This scheme can be separated into two stages: initiation and nitrogen-oxide cycling. Nitrogen-oxide cycling is diagrammatically represented in Figure 5, below.
Initiation
Ironically, the formation of ozone can begin with the destruction of an ozone molecule. When ozone (O3) is photodissociated by ultraviolet light , it can react with water to form two OH radicals (HO•) as shown below in STEP 1, Equation 5. (A radical is a compound with an unpaired electron, which is represented with a dot "•" in the chemical formula.) Radicals are generally very reactive.
| STEP 1 |
O3 + hυ+ H2O → O2 + 2HO• |
(5) |
When volatile organic hydrocarbons are present in the air from car exhaust, or other sources, each OH radical reacts with a volatile organic hydrocarbon (represented as R-H) and O2 to produce peroxy radicals (RO2•, where R represents the rest of the volatile-organic-hydrocarbon molecule). The production of peroxy radicals is shown in STEP 2, Equation 6, below.
| STEP 2 |
2HO• + 2R-H + 2 O2 → 2RO2• + 2H2O |
(6) |
Nitrogen-Oxide Cycling and Ozone Production
Car exhaust also contains nitric oxide (see Box 2, above). Peroxy radicals react with nitric oxide (NO) to form nitrogen dioxide (NO2):
| STEP 3 |
2RO2• + 2NO → 2NO2 + 2RO• |
(7) |
Nitrogen dioxide is photodissociated by sunlight, reforming nitric oxide (STEP 4, Equation 8).
| STEP 4 |
2NO2 + hυ 2NO + 2O |
(8) |
Finally, oxygen atoms formed in Step 4 combine with molecular oxygen in the presence of an inert molecule like N2, to form ozone. (N2 stabilizes the product by removing excess energy through collisions.)
| STEP 5 |
2O2 + 2O → 2O3 |
(9) |
Since NO is produced in Step 4, Steps 3-5 are repeated as long as VOCs are present. This cycling of nitrogen oxides is diagrammatically represented below in Figure 5. An animation of Steps three through five of the mechanism above can be viewed by clicking on the pink button in Figure 5.
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Figure 5
This is a diagrammatic illustration of Steps 3-5 of the mechanism for ozone formation described above. Note: Steps 1 and 2 are not represented in this diagram.
Volatile organic hydrocarbons (VOCs) and NO are both byproducts of fossil fuel combustion (pollution). VOCs form peroxy radicals (RO2• . RO2• and NO react to produce NO2 (Step 3). In sunlight, NO2 dissociates into O and NO (Step 4). The O reacts with O2 to produce ozone (Step 5). Unfortunately, NO2 can be regenerated from NO and RO2 • (Step 3). The cycling of NO2 and
NO(Steps 3 and 4) means that even small concentrations of nitrogen oxides (NO and NO2) can produce large amounts of ozone when VOCs are present.
Click on the pink button below to view an animation of the formation of ozone. Click the blue button below to download QuickTime 6.5 to view the movie.
 
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In summary, nitrogen dioxide, sunlight and oxygen produce ozone and nitric oxide. Nitric oxide (NO) is converted back to nitrogen dioxide (NO2) by peroxy radicals (RO2• ) from VOCs. Notice that even though STEP 1 uses ozone, there is still a net production of ozone: one molecule of ozone produces two hydroxy radicals which, through a series of reactions, form two molecules of nitrogen dioxide, and eventually produce two molecules of ozone. (This is an example of a chain reaction.) Therefore, in order to reduce ozone levels, we need to reduce levels of nitrogen oxides and volatile organic hydrocarbons.
Related Practice Problems
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