Modelling of the photooxidation of toluene: conceptual ideas for validating detailed mechanisms

[摘要] Toluene photooxidation is chosen as an example to examine how simulations ofsmog-chamber experiments can be used to unravel shortcomings in detailed mechanisms and to provide information on complexreaction systems that will be crucial for the design of future validation experiments. The mechanismused in this study is extracted from the Master Chemical Mechanism Version3 (MCM v3) and has been updated with new modules for cresol and g-dicarbonyl chemistry. Model simulations are carried out for atoluene-NO_x experiment undertaken at the European Photoreactor(EUPHORE). The comparison of the simulation with the experimental data reveals two fundamental shortcomings in the mechanism: OH production is toolow by about 80%, and the ozone concentration at the end of the experimentis over-predicted by 55%. The radical budget was analysed to identify the key intermediates governing the radical transformation in the toluene system.Ring-opening products, particularly conjugated g-dicarbonyls, wereidentified as dominant radical sources in the early stages of the experiment.The analysis of the time evolution of radical production points to a missingOH source that peaks when the system reaches highest reactivity. First generation products are also of major importance for the ozone production inthe system. The analysis of the radical budget suggests two options to explain the concurrent under-prediction of OH and over-prediction of ozone inthe model: 1) missing oxidation processes that produce or regenerate OH without or with little NO toNO₂ conversion or 2) NO₃ chemistry that sequesters reactive nitrogen oxides into stable nitrogen compounds and at thesame time produces peroxy radicals. Sensitivity analysis was employed to identify significant contributors to ozone production and it is shown howthis technique, in combination with ozone isopleth plots, can be used for thedesign of validation experiments.

[学科分类] 大气科学