A modeling study of global distribution and formation pathways of highly oxygenated organic molecules (HOMs) from monoterpenes

Abstract. Highly oxygenated organic molecules (HOMs) derived from monoterpenes are key precursors of secondary organic aerosols (SOA), yet their global-scale formation pathways and climate impacts remain poorly quantified due to uncertainties in autoxidation kinetics and branching ratios of peroxy radicals. Here, we integrate a comprehensive HOMs chemical mechanism into a global climate model, enabling a systematic evaluation of HOMs-derived SOA (HOMs-SOA) contributions and their sensitivity to key chemical parameters. The improved model shows reasonable agreement in the diurnal cycle and average HOM concentrations (normalized mean biases of 69 % and 121 % at the two sites). Sensitivity experiments identify the branching ratio of autoxidation-capable peroxy radicals (MT-bRO2) as the dominant uncertainty source. While the MT-bRO2 branching ratio has limited impact on C10-HOMs concentrations (~60 % formed via NO-terminated autoxidation), it strongly regulates C15/C20-HOM concentrations produced through cross-reactions of biogenic peroxy radicals. The contribution of HOMs-SOA to total monoterpene-derived SOA ranges from 19 % to 41 %, depending on the MT-bRO2 branching ratio used in chamber experiments. C15 and C20 accretion products dominate in pristine regions (e.g., the Amazon, contributing ~50 % of HOMs-SOA), whereas anthropogenic-influenced areas (e.g., southeastern China and India) exhibit higher contributions from NO-mediated formation of C10-ON (nitrate HOMs). Our findings advance the representation of organic aerosols in climate models and provide critical insights to bridge gaps between chamber experiments and global-scale simulations.