Widespread occurrence of large molecular methylsiloxanes in ambient aerosols

Abstract. Synthetic pollutants have emerged as a widespread environmental concern. Recently, large molecular methylsiloxanes were identified in traffic emissions. Here, we show that large molecular methylsiloxanes are widely present in atmospheric particulate matter across diverse environments, including urban, coastal, rural, and forest sites in the Netherlands, Lithuania, and Brazil. Overall, methylsiloxanes of varying molecular sizes account for approximately 2.0 %–4.3 % of the non-refractory organic aerosol mass detected by thermal desorption proton transfer reaction mass spectrometry (TD-PTR-MS) analysis. Thermal desorption profiles indicate that over half of the detected methylsiloxanes originate from the depolymerization of large molecular methylsiloxanes, primarily associated with traffic emissions, while the remainder likely arise from the gas-to-particle conversion of volatile methylsiloxanes. Large molecular methylsiloxanes show a distinct correlation with long-chain hydrocarbons characteristic of engine lubricants, suggesting a lubricant-related source. Notably, the mass fraction of methylsiloxanes in organic aerosols does not decrease significantly during atmospheric transport and dilution, and a substantial fraction persists as large molecular methylsiloxanes. This persistence underscores their chemical stability, in contrast to the co-emitted lubricant hydrocarbons that undergo atmospheric oxidation. The substantial mass fraction of methylsiloxanes in particulate matter highlights their role as one of the most concentrated categories of synthetic compounds in the atmosphere, raising concerns about their potential, yet poorly understood, effects on human health and the climate.