Development and Evaluation of a Detailed Mechanism for Gas-Phase Atmospheric Reactions of Furans

Furan and its alkyl derivatives (furans) are emitted to the atmosphere from multiple sources and can exist in sufficient quantities to affect atmospheric oxidant levels and secondary pollutant formation. Such compounds and their chemical transformations are generally oversimplified in gas-phase chemical mechanisms used for air quality predictions and atmospheric modeling studies. Furans are typically lumped as reactive aromatics, which largely underpredicts their oxidation rates. This work presents a detailed gas-phase mechanism for furans and their major oxidation products. The reactions and rate constants were derived using published data and the Statewide Air Pollution Research Center (SAPRC) mechanism generation system (MechGen). The detailed furans mechanism was implemented in the SAPRC-18 base mechanism to enable evaluation against environmental chamber experiments. A reduced version of the mechanism was developed that maintains consistency and compatibility with the SAPRC-07/-11 base mechanisms. Relative to the lumped SAPRC-11 mechanism, the model skill was improved in predicting furans consumption (21 of 26 experiments) and "([O3]-[NO]). For the latter, mean model bias was reduced to ±10% for 13 experiments (4 for SAPRC-11) and was >±20% for only 3 experiments (11 for SAPRC-11). Sensitivity simulations were performed to evaluate the relative importance of hydroxyl radical (OH)-, nitrate radical (NO3)-, and ozone (O3)-initiated photo-oxidation. While OH-initiated reactions are the major sink for furans, reactions with NO3 and O3 become non-negligible or even of equal importance under dark or high O3 conditions, such as may be expected during biomass burning events. The detailed furans mechanism was developed with no tuning to fit the experimental data used for evaluation and to facilitate the broad application of the mechanism for atmospheric and air quality modeling.