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<p>An Aerodyne tunable infrared laser direct absorption spectrometer with a multipass cell with a 413 m pathlength for the detection of ethylene oxide (EtO) is presented (TILDAS-FD-EtO). This monitor achieves precisions of <span class="inline-formula">&lt;75</span> ppt or <span class="inline-formula">&lt;0.075</span> ppb s<span class="inline-formula">⁻¹</span> and <span class="inline-formula">&lt;20</span> ppt in 100 s (1<span class="inline-formula"><i>σ</i></span>). We demonstrate precisions averaging down to 4 ppt h<span class="inline-formula">⁻¹</span> (1<span class="inline-formula"><i>σ</i></span> 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, SD). 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 to 700 ppb range depending on source identity and distance downwind.</p>