We describe the application of cavity ring-down spectroscopy (CRDS) to the detection of trace levels of ethylene in ambient air in a cold storage room of a fruit packing facility over a several month period. We compare these results with those obtained using gas chromatography (GC), the current gold standard for trace ethylene measurements in post-harvest applications. The CRDS instrument provided real-time feedback to the facility, to optimize the types of fruit stored together, and the amount of room ventilation needed to maintain sub-10 ppb ethylene levels for kiwi fruit storage. Our CRDS instrument achieved a detection limit of two parts-per-billion volume (ppbv) in 4.4 minutes of measurement time.