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

Abstract 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 (OHexp) 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 OHexp 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 OHexp 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 OHexp. At high OHexp, the product distribution and yield appear to be insensitive to RO2+HO2 vs RO2+OH fate.