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Ethylene oxide (EtO) is a human carcinogen whose release from sterilization facilities to ambient air has gained recent attention. Measurements have typically relied on canister samples collected over minutes to hours and analyzed using gas chromatography/mass spectrometry (GC/MS). A novel application of tunable infrared laser direct absorption spectrometry (TILDAS) was recently deployed aboard a mobile air quality laboratory in Toronto, Canada, to measure EtO in near real-time. Detection limits comparable to canister-GC/MS methods were achieved while stationary at averaging times of only 100 s, and high frequency (1-s) sampling detected EtO at several locations. EtO was observed consistently near the only local facility reporting on-site releases to Canada's National Pollutant Release Inventory. The maximum 1-s mixing ratio observed nearby was 18 ppb (33 μg m−3), and a mean 1-s mixing ratio of 0.43 ppb (0.78 μg m−3) was observed in the vicinity of the detectable plumes that covered industrial, commercial and residential land uses up to 900 m downwind. EtO in those plumes was not related to common air pollutants such as nitric oxide (NO), carbon monoxide (CO), or methane (CH 4), whereas it was paired with elevated NO or CO and CH 4 at some other locations. EtO was less abundant elsewhere in the city, and no detectable EtO was found near potential sources including hospitals, spice distributors and vehicle exhaust. The EtO background measured in wintertime Toronto air was indistinguishable from zero and substantially lower than that reported in studies using canister-GC/MS methods. The multi-pollutant mobile method described herein marks a significant step forward in the capability to characterize atmospherically relevant concentrations of EtO at high spatiotemporal resolution. Future applications of high-resolution techniques will allow for comprehensive investigations of source impacts and characterization of ambient levels for which deployment of conventional canister/GC-MS methods would be unsuitable or prohibitive. [Display omitted] • First high-resolution ethylene oxide (EtO) in urban air aboard a mobile laboratory. • Levels near sterilization facility suggest exceedance of health-based guidelines. • Measured common air pollutants discern among sources of detected EtO. • Urban background lower than in previous studies using canister-GC/MS methods. [ABSTRACT FROM AUTHOR]