These reactions are important in a cycle that oxidizes CO and hydrocarbons and produces ozone, in the presence of NOx (N0 + N02). In photochemical smog, ozone can build up to unhealthy levels of several hundred parts per billion (ppb) as a result of these reactions. There are many other reactions that occur, some of which may be significant at various times, including the destruction of 03 by NO, production and loss of HONO (nitrous acid) and peroxyacetyl nitrate (PAN). These reactions, and many more, represent a complex set of chemical interactions. For our purposes here, it is only necessary to note the major features.

1. The oxidation of CO and all hydrocarbons to C02 is indirectly driven by ozone and sunlight via the OH radical.

2. In the presence of sufficient NO* (roughly 30 parts per trillion) this oxidation produces ozone.

3. Most NO and NOz eventually gets removed as HN03.

4. The lifetime of gaseous NO* in the troposphere is on the order of 1-30 days (Soder-lund and Svensson, 1976; Garrels, 1982; Crutzen, 1988).

5. Conversion of NO* to PAN can result in significantly greater disbursement (e.g., Honrath and Jaffe, 1992).

Nitrogen oxides also play a significant role in regulating the chemistry of the stratosphere. In the stratosphere, ozone is formed by the same reaction as in the troposphere, the reaction of 02 with an oxygen atom. However, since the concentration of O atoms in the stratosphere is much higher (O is produced from photolysis of 02 at wavelengths less than 242 nm), the concentration of 03 in the stratosphere is much higher.

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