As part of the North Atlantic Regional Experiment (NARE), instrumentation for the measurement of O3 and CO was included on research flights conducted by the National Oceanic and Atmospheric Administration WP-3D Orion aircraft from St. John's, Newfoundland, Canada, and Keflavik, Iceland, from February 2 to 25, 1999. These flights sampled the lower troposphere over the western North Atlantic Ocean. One significant feature observed during these flights was the close proximity of air masses with contrasting source signatures: high levels of anthropogenic pollution immediately adjacent to elevated O3 of stratospheric origin. Here we present a case study showing the most pronounced example of this proximity, which was associated with a frontal passage across North America and out into the North Atlantic region. Trajectory analyses and satellite imagery are used to investigate the transport mechanisms that create the interleaving of air masses from the different sources. One important chemical feature was noted: in air masses with differing amounts of anthropogenic pollution admixed, O3 was negatively correlated with CO, which indicates that emissions from surface anthropogenic sources had reduced O3 in this wintertime period, even in air masses transported into the free troposphere. ¿ 2000 American Geophysical Union