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 (PM2.5) 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 PM2.5 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 PM2.5, the most prominent oVMS signal was the disiloxanol which partitioned primarily to the particle phase (averaging 77%). Using the tracer-based approach, D4, D5, and D6-derived SOA was estimated to contribute 14 ± 5 ng m-3 to PM2.5 corresponding to ∼0.10% of PM2.5 organic carbon, making VMS oxidation a minor source of PM2.5 in NYC.