| 18 Nov 2025
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.
Liwen Yang et al. “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)”
Paul Wennberg
This is a very welcome effort to build a mechanistically informed module that can capture the formation of highly oxygenated molecules in the oxidation of biogenic alkenes initiated by OH and ozone. The authors have used MCM and other compiled mechanisms that provide detailed gas phase oxidation insights and added pathways that describe HOM formation from autoxidation and RO2+RO2 chemistry. They use a set of simulations from CLOUD to benchmark the compiled mechanism both with single and mixed alkene starting mixtures. The mechanism has significant skill and will substantially advance our ability to simulate the production of HOMs and the associated aerosol in many regions of the world
I recommend publication in close to the current version. I ask the authors, however, to consider providing a more complete description of what chemistry drives the HOM formation – I suspect that perhaps the ‘top ten’ pathways provide most of the HOM production in each system and it would be helpful for future investigations to have this quantified here.
For the RO2 + RO2 chemistry my sense, based on Murphy et al., 2025 (https://pubs.rsc.org/zh-tw/content/articlepdf/2025/ea/d5ea00106d), is that the dimer formation branching ratio is generally ~10%. Even for simple alkanes (e.g. ethane RO2), this seems the case. To the extent that this is true, the major question is just the rate coefficients for these RO2+RO2 reactions. Given that addition of a functional group (e.g. HOCH2CH2OO vs CH3CH2CH2OO) has such a rate enhancement, I am curious whether such enhancements are additive in your mechanism (e.g. does adding a carbonyl and an alcohol or acid yield 100x rate – see for example Ziola and Ziemann - J Phys Chem A. 2025 Feb 13; 129(6):1688-1703). I would also ask the authors to provide evidence for the statement (ln314) “Likewise, reactions between two non-autoxidizable RO2 are not included due to their low propensity to form accretion products”. Finally, in the monoterpene RO2 section, please add a reference to Kenseth, 2023 where you cite Peräkylä et al., 2023. [https://www.science.org/doi/10.1126/science.adi0857 ]
To my knowledge, however, there is no evidence that acylperoxy radicals yield such ROOR and I would like to understand what this mechanism suggests as CH3C(O)OO is both one of the most ubiquitous (PAN decomposition) and reacts at 1/10 the collision rate. This is especially important during nighttime with NO3 addition to biogenic alkenes will produce RO2 in a low radical environment.
Finally, I would suggest that although NOx is not considered in the current model it would be helpful to include in the “future work” some commentary on whether NO3 chemistry with biogenics is likely to be important source of HOMs and which alkenes you would recommend be studied in more detail. For example, I think one of the first demonstrations the ROOR formation from NO3 addition is important for SOA was Sally Ng’s study in 2008 that illustrated that this pathway was likely responsible for all the SOA formed in an isoprene chamber study - https://acp.copernicus.org/articles/8/4117/2008/acp-8-4117-2008.html. I’d recommend citing this study in the introduction as well.