This study examines the influence of photochemical processes on tropospheric ozone distributions over the extratropical western North Pacific. The analysis presented here is based on data collected during the Pacific Exploratory Mission-West Phase B (PEM-West B) field study conducted in February--March 1994. Sampling in the study region involved altitudes of 0--12 km and latitudes of 10 ¿S to 50 ¿N. The extratropical component of the data set (i.e., 20--50 ¿N) was defined by markedly different photochemical environments north and south of 30 ¿N. This separation was clearly defined by an abrupt decrease in the tropopause height near 30 ¿N and a concomitant increase in total O3 column density. This shift in overhead O3 led to highly reduced rates of O3 formation and destruction for the 30--50 ¿N latitude regime. Both latitude ranges, however, still exhibited net O3 production at all altitudes. Of special significance was the finding that net O3 production prevailed even at boundary layer and lower free tropospheric altitudes (e.g., ≤4 km), a condition uncommon to Pacific marine environments. These results reflect the strong impact of continental outflow of O3 precursors (e.g., NO and NMHCs) into the northwestern Pacific Basin. Comparisons with PEM-West A, which sampled the same region in a different season (September--October), revealed major differences at altitudes below 4 km, the altitude range most influenced by continental outflow. The resulting net rate of increase in the tropospheric O3 column for PEM-West B was 1--3% per day, while for PEM-West A it was approximately zero. Unique to the PEM-West B study is the finding that even under wintertime conditions substantial column production of tropospheric O3 can occur at subtropical and mid-latitudes. While such impacts may not be totally unexpected at near coast locations, the present study suggests that the impact from continental outflow on the marine BL could extend out to distances of more than 2000 km from the Asian Pacific Rim. ¿ 1997 American Geophysical Union