Ethylene Oxide Monitor with Part-per-Trillion Precision for In-Situ Measurements

Abstract. An Aerodyne Tunable Infrared Direct Absorption Spectrometer with 413 meter cell for the detection of ethylene oxide (EtO) is presented (TILDAS-FD-EtO). This monitor achieves precisions of <75 ppt or <0.075 ppb in 1 second and < 20 ppt in 100 seconds (1-sigma). We demonstrate precisions averaging down to 4 ppt in an hour (1-sigma precision) when operated with frequent humidity-matched zeroes. A months-long record of 2022 ambient concentrations at a site in the Eastern United States is presented. Average ambient EtO concentration is on the order of 18 ppt (22 ppt standard deviation). Enhancement events of EtO lasting a few hours are observed, with peaks as high as 600 ppt. Back trajectory simulations suggest an EtO source nearly 35 km away. This source along with another are confirmed as emitters through mobile near-source measurements, with downwind concentrations in the 0.5 ppb to 700 ppb range depending on source identity and distance downwind.