The impact of ship emissions on marine boundary layer (MBL) NOx and SO2 levels over the Pacific Ocean has been explored by comparing predictions (with and without ships) from a global chemical transport model (GCTM) against compiled airborne observations of MBLNOx and SO2. For latitudes >15 ¿N, which define that part of the Pacific having the heaviest shipping, this analysis revealed significant model over prediction for NOx and a modest under prediction for SO2 when ship emissions were considered. Possible reasons for the difference in NOx and SO2 were explored using a full-chemistry box model. These results revealed that for an actual plume setting the NOx lifetime could be greatly shortened by chemical processes promoted by ship plume emissions themselves. Similar chemical behavior was not found for SO2. ¿ 2001 American Geophysical Union