Dynamic Emissions and Secondary Organic Aerosol Formation from Real Personal Care Products: New Insights into Siloxane-Containing Molecules

In urban atmosohere, volatile methyl siloxanes (VMSs) was treated as the important maker from pesonal care products (PCPs) of volatile chemical products (VCPs), yet the link between their realistic emission profiles and secondary organic aerosol (SOA) formation remains elusive. Moving beyond previous studies using pure standards, we systematically characterized the dynamic emissions and SOA evolution of four typical PCPs using an evaporation chamber coupled with an oxidation flow reactor and other online gas/aerosol mass spectrometry. VMS dominated the emissions (>95%) for most PCP products and exhibited prolonged evaporation time scales compared to coemitted solvent VOCs. Emission factors for VMS ranged from approximately 132 to 300 g kg-1. We quantified the PCP -derived SOA formation yield and saturated and unsaturated molecular composition for the first time and systematically discussed VMS oxidation pathways. Furthermore, molecular fingerprints revealed that while cross-reactions between different VMS precursors occurred readily, cross-reactions between VMS and other VOCs were kinetically inhibited due to mismatches in reactivity and evaporation timing. The evolution of SOA volatility was found to be highly pathway-dependent, with oligomerization driving a volatility reduction of 0.8 or up to several orders of magnitude. These findings provide a mechanistic basis for the atmospheric persistence of VMS and their potential for long-range transport to remote regions, with direct implications for improving source apportionment of VCPs and refining SOA formation in atmospheric models.