Development and Application of Chemical Mechanisms for Wildfire-Derived Organic Compounds

Chemical mechanisms describe the reactivity of compounds in the gas phase, and include representation of atmospheric oxidants, reactive intermediates, and stable reaction products. Due to computational constraints, most widely used chemical mechanisms have been simplified (i.e., reduced), such that a limited number of reaction pathways, intermediates, and products are considered when applied in air quality models. Emerging sources and evolving chemical conditions necessitate development, reduction, and application of new chemical mechanisms. Here we present new SAPRC-based chemical mechanisms for organic compounds commonly emitted from wildfires, including aromatics, oxygenated aromatics, and the monoterpene, camphene. Details of the mechanisms will be presented, as well as the sensitivities of box model predictions to different mechanisms and model representations. The mechanisms were generated using the SAPRC mechanism generator, MechGen; and box modeling included application of the GECKO-A box model for SAPRC and GECKO-A mechanism comparisons, the SAPRC box model for measurement-model comparisons, and a Lagrangian-plume simulation using the F0AM box model for mechanism-mechanism and measurement-model comparisons. Challenges and opportunities for mechanism reduction will be discussed, including the potentially different requirements for prediction of gaseous pollutants such as ozone, relative to secondary organic aerosol. While mechanism generation and reduction, such as presented here, has been facilitated by the availability of structure-activity relationships, computational chemistry, and machine learning, we will also present needs and opportunities for fundamental laboratory studies that will be required to develop and test chemical mechanisms that are sufficiently robust for air quality modeling applications.