Integrated Experimental and Modeling Investigation of Volatile Methyl Siloxane Oxidation and Secondary Organic Aerosol Formation

Le résumé fourni par la source

Volatile methyl siloxanes (VMS), commonly found in personal care products (PCPs), are persistent environmental contaminants that undergo atmospheric oxidation, primarily with OH, forming a mixture of secondary products, some of which have been found in atmospheric secondary organic aerosols (SOA). The fate of decamethylcyclopentasiloxane (D5), a prominent VMS, is of scientific and ecological interest due to its propensity for long-range atmospheric transport to pristine locations. However, lack of knowledge of the product distribution and aerosol yield resulting from OH oxidation of D5 makes comprehensive knowledge of fate and transport impossible. This study integrates experimental and modeling approaches to better constrain D5 oxidation and extends the results to long-range (global) environmental fate and transport by incorporation of D5 and its gas and aerosol oxidation products into the global Community Earth System Model version 2 (CESM2). An oxidation flow reactor (OFR) was employed to systematically vary OH exposure (5 × 10¹¹ to 1 × 10¹³ molecules·s·cm⁻³). Since the fate of the RO2 radical generated from initial OH attack of the parent compounds varies significantly across the global modeling domain, SOA yields and molecular product distributions were also studied as a function of the RO₂ radical loss pathways (primarily RO2 + HO₂ and RO2 + OH reactions). Particle and gas-phase samples were analyzed using high-resolution liquid chromatography-mass spectrometry (LC-HRMS), revealing siloxanols as major VMS oxidation products that contribute to low-volatility SOA. Results demonstrated that RO2 reactions with HO₂ supported the formation of low-volatility products. A kinetic box model was developed to parameterize these processes, providing critical insights for scaling experimental results to atmospheric conditions.Additionally, we updated the regional refined configuration of CESM2, Multi-Scale Infrastructure for Chemistry Modeling (MUSICA-v0), to investigate the oxidation products of D5. Simulations were conducted for July 2022 and results were compared against field measurements in urban New York City (NYC) during a one-month campaign, demonstrating the ability to capture observed trends in aerosol formation and identifying discrepancies linked to gas-particle partitioning mechanisms. This work bridges laboratory experiments and atmospheric modeling, providing insights into the fate of D5 and its role in SOA formation. Future directions include refining kinetic frameworks to account for multigenerational oxidation products, validating partitioning simulations under diverse environmental conditions, and extending the approach to investigate the biogeochemical implications of VMS-derived SOA on a global scale.

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