In polar regions, severe marine boundary layer ozone depletion episodes (ODEs) are a yearly recurring phenomenon in the spring. Using 9 years of 10-day three-dimensional trajectory calculations, the origin of ODEs at three Arctic observatories is investigated. The analysis indicates that marginal ice zones are potential source regions of ODEs. Those regions do broadly correspond to areas where increased levels of bromine oxide (BrO), an indicator of ozone depletion chemistry, are observed by the GOME satellite. The source region of ODEs, observed at Barrow, Alaska, is found to be about 1 day's travel upwind, in agreement with expectations based on the rate at which O3 depletion chemistry occurs. In contrast, the likely source region for ODEs observed at Alert, Canada, and Zeppelinfjellet, Norway, appears to be located several days' travel upwind, off the Siberian coast. This result may reflect the absence of favorable ice conditions for O3 depletion chemistry nearer those sites. Assuming that O3 depletion occurs at those regions, this implies that air parcels without O3 remain that way for several days or the depletion is slower than current understanding of the O3 depletion chemistry suggests. Rapid changes in O3 mole fractions at those measurement sites appears not to be an indication of fast chemical destruction of ozone but rather are due to abrupt air mass changes. Data for September indicate a much narrower distribution of ozone mole fractions and no particular pattern linking a preferred area with lower mole fractions.