Computer Modeling of Environmental Chamber Measurements of Maximum Incremental Reactivities of Volatile Organic Compounds

A detailed atmospheric photochemical mechanism which had been previously used in model calculations for developing ozone reactivity scales for volatile organic compounds (VOCs) was evaluated by comparing its predictions with measurements of incremental reactivities in an environmental chamber system. An updated version of this mechanism is also described and evaluated. The experiments consisted of determining the effects of adding representative alkanes, alkenes, aromatic hydrocarbons, aldehydes or CO on NO oxidation, ozone formation and radical levels in a simplified model photochemical smog system representing conditions where ozone formation is most sensitive to VOCs. The published mechanism correctly simulated the observed qualitative reactivity trends, but overpredicted the effect of adding formaldehyde early in the experiments, performed poorly in simulating reactivities of branched alkanes, tended to underpredict the reactivities of alkenes, and did not simulate differences in reactivities of aromatic isomers. The updates to the mechanism improved the simulation results for the branched alkanes and the alkanes, but not for formaldehyde and the aromatics. The implications of these results concerning the development of atmospheric mechanisms for VOCs are discussed.