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This reaction is in the falloff region at 1 atm for R = CH3 and C2H5 but for larger radicals is effectively in the high-pressure limit with a rate constant ~9 X 10"12 cm3 molecule-1 s~'. The reaction is reversible in that the peroxynitrates thermally decompose in the reverse of this reaction. As we shall see, this reaction of ROz with N02 is not significant compared to its reactions with NO, H02, or other R02 under most tropospheric conditions.

e. Fate of R02 under Typical Tropospheric Conditions

To compare the relative importance of these potential atmospheric fates of R02 under typical polluted conditions, and particularly the relative importance of the NO reaction, let us take the C2H502 radical as an example. The lifetime of C2H502 with respect to reaction with NO, H02, or C2H502 at peak concentrations of 20 ppb, 40 ppt, and 40 ppt, respectively, can then be calculated from r= l/k[X\ as 0.2, 1.3 X 102 s and 1.6 X 104 s, respectively. (Note that the NO and H02/R02 peaks will not occur simultaneously.) At night, with an N03 concentration of 100 ppt, the lifetime would be ~ 135 s, assuming a rate constant of 3 X 10~12 cm3 molecule-1 s-1. In short, in areas impacted by anthropogenic emissions, the reaction of R02 with NO will predominate.

One can then ask under what conditions the reaction of R02 with NO will be equivalent in rate to that with H02. To make this calculation, one can estimate the concentration of NO at which /c23[NO] = k2A[HOJ.

Since k

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