Abstract
Cyclic volatile methyl siloxanes form secondary organosiloxane aerosol (SOSiA) through atmospheric oxidation, yet the evolution of volatility remains poorly constrained. Here, we investigated SOSiA formation and volatility evolution from OH oxidation of D4, D5, and D6 under controlled relative humidity using a potential aerosol mass oxidation flow reactor. Observed maximum SOSiA yields increased with ring size within the present dataset, reaching approximately 0.15–0.29 for D4, 0.3–0.4 for D5, and up to approximately 0.45 for D6. Despite similar trends, volatility evolution showed clear structure-dependent behavior. D4 exhibited a gradual shift toward low-volatility products, indicating functionalization-dominated aging. D6 showed initial low-volatility formation followed by a decrease at higher oxidation, suggesting fragmentation-driven redistribution. D5 displayed intermediate behavior, with a broad volatility distribution and gradual multigenerational evolution. These results indicate that the observed volatility evolution reflects a changing balance between functionalization and fragmentation, and that this balance varies with precursor structure. Obtained under oxidation flow reactor conditions, these findings provide laboratory-based observational constraints that can support the development of volatility-resolved representations of siloxane-derived SOA.