Decamethylcyclopentasiloxane (D5) Oxidation: Product Chemistry, Influence of RO₂ Fate, and Secondary Aerosol...

Volatile methylsiloxanes (VMS), particularly decamethylcyclopentasiloxane (D₅), are recognized anthropogenic precursors of secondary organic aerosol (SOA), yet their formation pathways, product distributions, and yields under atmospherically relevant conditions remain poorly constrained. This study examines D₅-derived SOA across a wide range of OH exposures (OH_exp) and peroxy radical (RO₂) fates (RO₂+HO₂ and RO₂+OH) in an oxidation flow reactor, with comparison to ambient and chamber samples. The siloxanol (1-hydroxynonamethylcyclopentasiloxane, D₄TOH) is consistently observed. Under low OH_exp and HO₂-dominated regimes, samples contained multiple early generation siloxanols accompanied by dimers and high molecular weight products, contrary to expectations that such species require extensive oxidation. As OH_exp and RO₂ + OH reactivity increase, total SOA mass rises, and the fraction of unresolved material increases. The observations were compared to a volatility basis set (VBS) multigenerational oxidation kinetic box model with explicit siloxanols. The model's strongly monotonic progression in volatility, aerosol yield, and degree of OH substitution in the polysiloxanol series diverged from experimental data, aside from the expected increase in yield with OH_exp. At high OH_exp, the product distribution and yield appear to be insensitive to RO₂+HO₂ vs RO₂+OH fate.