Development of revised SAPRC aromatics mechanisms

The representation of the gas-phase atmospheric reactions of aromatic hydrocarbons in the SAPRC-07 chemical mechanism has been updated to give better simulations of recent environmental chamber experiments carried out under lower NOx conditions than used to develop SAPRC-07. SAPRC-07 consistently underpredicted NO oxidation and O3 formation rates observed in most of the newer experiments. The new aromatics mechanisms, designated as SAPRC-11, gave better simulations of O3 formation in almost all experiments, except for higher (>100 ppb) NOx benzene and (to a lesser extent) toluene experiments where O3 formation rates were consistently overpredicted. This overprediction at higher NOx can be corrected if the aromatics mechanism is parameterized to include a new NOx dependence on photoreactive product yields, but that parameterization was not incorporated in SAPRC-11 because it is inconsistent with available laboratory data. The mechanisms for phenolic compounds were also significantly revised to better simulate the new experiments. The base mechanism for SAPRC-11 incorporates a few minor updates, but otherwise is the same as SAPRC-07. The light source used in the chamber experiments had no significant effect on mechanism evaluation results, except possibly for cresols. The mechanisms still underpredict OH radical levels and further studies are needed to reduce uncertainties in aromatics mechanisms.