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1 X 10 cm* molecule

', this suggests that the removal of ROz by HOz will compete with that by NO when the concentrations of these two, H02 and NO, are equal. Typical peak H02 concentrations in polluted areas are believed to be of the order of 109 cm"3, corresponding to ~40 ppt. "Average" HOz concentrations may more typically be (1-2) X 108 cm-3, corresponding to concentrations of 4-8 ppt. Such equivalent small concentrations of NO are found in the remote troposphere and, under such conditions, reactions of R02 with H02 or other R02 can become quite important (see Section J.2b).

In addition, the reaction of R02 with N03 at night can become significant. For example, at HOz of 4 ppt and N03 of 10 ppt, which have been reported in a relatively clean marine region (Carslaw et al., 1997), the lifetimes of ROz relative to reaction with these species are comparable at ~ 103 s. Kirchner and Stockwell (1996) predict that in more polluted areas, the R02 + N03 reactions may be important. For example, under conditions chosen to be representative of aged polluted air masses mixing with rural air masses, they calculate that 77% of the total R02 at night reacts with N03; overall (both day and night), however, only 0.66% is removed by reaction with N03.

The relative magnitude of the R02 + NO reaction compared to the ROz + H02 or R02 reactions is a critical factor in ozone formation in the troposphere. As discussed in more detail in Section J, if R02 + NO predominates, N02 is formed and through its photolysis to 0(3P), o3 is ultimately generated. On the other hand, the R02 + H02/R02 reactions can lead to the formation of stable products such as ROOH, without conversion of an NO to NOz. In this case, no 03 is formed and indeed, through its photolysis to form OH radicals and subsequent reactions with H02 and OH, destruction of 03 occurs.

3. Alkoxy Radicals (RO)

The reactions of R02 with NO and with ROz generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization; as we shall see, reactions with NO and NOz are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals:

• Reaction with 02: If the carbon to which the alkoxy oxygen is attached also has a hydrogen, this H can be abstracted to give H02 and a carbonyl compound:

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