EVALUATION OF A GAS-PHASE ATMOSPHERIC REACTION MECHANISM FOR LOW NOx CONDITIONS

The ability SAPRC-99 atmospheric chemical mechanism to predict photochemical smog formation under low NO x conditions was evaluated by comparing model predictions to results experiments from three different environmental chamber facilities. These included new experiments from our UCR EPA environmental chamber, and previous experiments at from the Tennessee Valley Authority (TVA) and the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) chambers. The facility and procedures for the new UCR EPA experiments, and the procedures for modeling data from all three chambers, are discussed. The results indicated no apparent low NO x mechanism performance problem for SAPRC-99 for simple chemical systems and for ambient surrogate reactive organic gas (ROG)-NO x experiments with ROG/NO x ratios high enough for maximum ozone formation potentials to be achieved. However, a consistent underprediction bias for NO oxidation and O 3 formation rates was found in simulations ambient surrogate ROG-NO x experiments at low ROG/NO x ratios. The widely used Carbon Bond 4 mechanism was even worse in this regard. Furthermore, new aromatic-CO-NO x experiments indicate problems with current aromatic mechanisms that may be the cause of the low ROG/NO x underpredictions. Integrated reaction rate calculations indicate that increasing the accuracy in representing combination reactions of organic peroxy radicals will probably have an insignificant effect on model predictions. It is concluded that at a minimum new aromatic mechanisms need to be developed for model predictions to be consistent with available data.