Secondary organic aerosol formation from early-generation oxidation products of decamethylcyclopentasiloxane depends on seed aerosol composition

Abstract

Decamethylcyclopentasiloxane (D₅), a widely used component of personal care products, readily partitions to the atmosphere where it can undergo oxidation, potentially forming secondary organic aerosol (SOA). The mechanism of aerosol formation, particularly at low OH exposure, remains highly uncertain, leaving open questions about the role of multigenerational chemistry, seed aerosol, and oxidation conditions. We performed chamber experiments of D₅ oxidation at low OH exposure to investigate SOA formation from D₅ (SiSOA) and the effect of seed aerosol using dry ammonium sulfate (AS) and dioctyl sebacate (DOS) seeds. We measured gas-phase D₅ and its oxidation products online using chemical ionization mass spectrometry and aerosol size and composition using scanning mobility particle sizing and aerosol mass spectrometry. In select experiments, gas- and particle-phase samples were collected for offline analysis by liquid chromatography with negative electrospray ionization and high-resolution mass spectrometry. The gas-phase products were similar for all experiments, composed primarily of 1-hydroxynonamethylcyclopentasiloxane, a first-generation oxidation product. For AS, the SiSOA was dominated by 1-hydroxynonamethylcyclopentasiloxane, with minor contributions from later-generation products. For DOS, the aerosol was composed of 1-hydroxynonamethylcyclopentasiloxane and an additional unidentified product, and the SiSOA yield was ∼3–10 times more than in AS experiments. For AS-seeded experiments, the timeseries of SiSOA evolution throughout the experiment suggests adsorption as the dominant partitioning mechanism, while for DOS-seeded experiments, absorption appears to be important. We estimated the saturation mass concentration (C*) of the SiSOA to be 1300 μg m⁻³. Overall, our work shows that the SiSOA formation mechanism depends on seed identity and that multiple oxidation steps will be required for significant SiSOA formation.