The Pacific Exploratory Mission to the Tropics phase B (PEM-Tropics B) aircraft campaign in March--April 1999 surveyed the chemical composition of the Pacific atmosphere from 35 ¿N to 35 ¿S and up to 12 km altitude. We use these observations in combination with a global three-dimensional model driven by assimilated meteorological observations to investigate the transport of northern hemispheric pollutants over the Pacific. We focus on carbon monoxide (CO) and tag it in the model by its region of origin. The model reproduces the observed large-scale latitudinal, longitudinal, and vertical gradients of CO concentrations over the Pacific. Biomass burning in Southeast Asia, which was particularly intense in spring 1999, contributed most of the CO enhancements observed in the free troposphere over the northern tropical Pacific but played only a minor role in the boundary layer. Fossil fuel combustion in Europe and Asia contributed most of the observed CO enhancements in the boundary layer over the North Pacific; the European influence dominated over Asian influence north of 35 ¿N. European influence over the Pacific is particularly strong in spring because of wintertime accumulation of CO at high latitudes. North American pollution made little contribution to CO anywhere over the Pacific. Circulation of Eurasian industrial pollution around the Pacific High and into the trade winds produced a tropical river of pollution flowing in the lower troposphere from the northeastern to the western equatorial Pacific and in the vicinity of the South Pacific Convergence Zone. This pathway, however, made little contribution to interhemispheric exchange. Elevated concentrations observed for CO and other northern hemispheric tracers in the upper troposphere over the southeastern Pacific provide evidence for efficient interhemispheric exchange through a narrow region of upper tropospheric westerlies in the eastern equatorial Pacific (the westerly duct). We find that this westerly duct was the most important pathway for global interhemispheric exchange during PEM-Tropics B. It was particularly well developed because of the La Nina conditions. ¿ 2001 American Geophysical Union