Intercomparison of Photochemical Mechanisms using Response Surfaces and Process Analysis

Conclusions

As recommended by Kuhn et al. (1998) a comprehensive mechanism evaluation or intercomparison requires that the mechanisms be evaluated for a large number of VOC and NO_x scenarios, and, ideally, for the full response surface. Key difficulties in performing such an evaluation include: the large number of VOC/NO_x scenarios to be performed; the large number of species concentrations and reaction rates to be analyzed; and the temporal variability in the evolution of species concentrations. We have demonstrated here modifications to the widely used OZIPR model that facilitate mechanism evaluations. New model outputs are created allowing the PAVE visualization package to be used to perform sophisticated, time-dependent analyses for multiple data sets. In addition, outputs that describes the budgets of HO_x, NO_y and O_x allows for convenient evaluation of the important component chemical processes. Our example analysis here shows that there are important differences in the CB4.99 and the SAPRC99 mechanisms, particularly in the O_x production efficiency per NO_x for NO_x-limited conditions and in the HO_x budget for radical-limited conditions. It is not possible to conclude which mechanism is more correct, but the model tool demonstrated here will allow mechanism developers to more readily assess the effects of future revisions to these photochemical mechanisms, and to determine which chemical process require better elucidation in future chamber and kinetics experiments.