great deal of flexibility and judgment involved in choosing kinetics and products that are representative of a whole group of organics. For example, the rate constants for 03 reacting with alkenes change by a factor of 40 from propene to 2-methyl-2-butene (Table 6.9). The greater the concentrations of the more reactive species in the air mass, the larger the chosen rate constant for this reaction should be. One approach is to weight the rate constants by the relative number of moles of the individual compounds in this class (e.g., see McRae et al., 1982a, 1982b).

Of course, as the reaction proceeds and the hydrocarbons are consumed, the composition of the remain ing hydrocarbons shifts. If the rate constants for the organics reflect the mix, then they should also change with time. The severity of this problem is reduced somewhat by the fact that there is fresh injection of reactants during the day.

A number of intercomparisons of chemical submodels have been carried out. Figure 16.18, for example, shows some of the results from one such intercompari-son for conditions chosen to be representative of moderately polluted conditions (Kuhn et al., 1998). The average final 03 predicted by the models was 148 ppb, but there are clearly significant differences between the models. Thus the highest, the EMEP model (.European

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