Tracking Oxidized Volatile Methyl Siloxanes in New York City in Summertime

Volatile methyl siloxanes (VMS) are high-production anthropogenic chemicals that can undergo atmospheric oxidation to form oxidized VMS (oVMS) in the atmosphere and contribute to secondary organic aerosol (SOA). Herein, we develop an SOA tracer-based approach to estimate oVMS contributions to ambient fine particulate matter (PM_2.5) organic carbon (OC) in New York City (NYC). oVMS including a series of siloxanols with intact D₄, D₅, and D₆ rings were identified and semiquantified in field and laboratory chamber aerosol samples by liquid chromatography-tandem mass spectrometry. oVMS were persistent in PM_2.5 in NYC during summertime, with oVMS from D₄ and D₆ showing more atmospheric aging than D₅. Statistically significant correlations among these oVMS suggest common atmospheric formation pathways. Daytime concentrations of oVMS were significantly higher than nighttime levels consistent with a photochemical source. In PM_2.5, the most prominent oVMS signal was the disiloxanol which partitioned primarily to the particle phase (averaging 77%). Using the tracer-based approach, D_4, D₅, and D₆-derived SOA was estimated to contribute 14 ± 5 ng m^-3 to PM_2.5 corresponding to ∼0.10% of PM_2.5 organic carbon, making VMS oxidation a minor source of PM_2.5 in NYC.