SIM-HOM (version 1.0): a Mechanistic Module for the formation of highly oxygenated organic molecules from Isoprene, Monoterpene and Sesquiterpene evaluated with ADCHAM (version 1.0)

Abstract. Biogenic volatile organic compounds (BVOCs), including isoprene, monoterpenes, and sesquiterpenes, are emitted in large quantities and play a critical role in atmospheric chemistry. They contribute to the formation of highly oxygenated organic molecules (HOM), which are essential for new particle formation (NPF) and secondary organic aerosol (SOA) formation. However, current models often oversimplify the oxidation pathways of these compounds, leading to inaccuracies in predicting HOM composition and concentrations. To address this gap, we developed a mechanistic module, SIM-HOM (Sesquiterpene, Isoprene and Monoterpene-derived HOM mechanism), based on Master Chemical Mechanism (MCM), that explicitly incorporates autoxidation processes, detailed fragmentation pathways, and RO₂-RO₂ interactions for isoprene, monoterpene, and sesquiterpenes. The updated module was validated using experimental data from the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, demonstrating substantial improvements in simulating HOM concentrations under various conditions. Specifically, it significantly improves the simulation of highly oxidized isoprene products, resolves discrepancies in monoterpene-derived HOM distributions, and provides the first comprehensive parameterization of sesquiterpene oxidation products. The model also captures the HOM formation under mixed precursor conditions. These advancements underscore the importance of incorporating detailed molecular-level reaction mechanisms into atmospheric models. Future work should focus on refining branching ratios for critical reactions and investigating the influence of temperature and nitrogen oxides on HOM formation, and expanding the mechanism to include additional BVOC classes. Our findings provide a robust foundation for improving global atmospheric simulations of SOA formation and climate interactions.