Inhibition of odd oxygen production in the carbon bond four and generic reaction set mechanisms

Abstract

Abstract Outdoor smog chamber experiments have been conducted by researchers in Australia at lower initial NOx and VOC-to-NOx ratios than those chamber experiments which were used to formulate photochemical reaction mechanisms such as the Carbon Bond Four (CB4). When the Australians simulated their experiments with CB4, they found that CB4 underpredicted ozone production at low VOC-to-NOx ratios, i.e. in the NOx-rich region of the ozone isopleth diagram above the ridge line. CB4 predicts a significant NOx inhibition in this region whereas the empirical General Reaction Set (GRS) mechanism, which was developed by the Australian researchers to fit their data, does not. In this study, a mass balance and process analysis technique is used to explain the origins of the NOx -inhibition in CB4 simulations. The causes are determined to be in the inorganic reactions of the CB4, which are believed to be both complete and well-known. The primary cause was a strong negative feedback of new radical production from the photolysis of ozone which occurred because of the longer delay needed to oxidize the higher initial NO. The CB4 mechanism could still be incorrect if it is missing a strong radical source, for example, in the poorly understood aromatics chemistry. The GRS, which lacks representation of the inorganic processes responsible for NOx-inhibition, may be achieving its predictive accuracy by compensating errors. If this is true, then the GRS would be unsatisfactory for ambient air use. Additional smog chamber experiments at low VOC-to-NOx ratios and better characterization of the Australian chambers are needed to resolve the discrepancy in CB4 and GRS predictions.