. We analyze the Asian outflow of CO, ozone, and nitrogen oxides (NO x ) to the Pacific in spring by using the GEOS-CHEM global 3-D model of tropospheric chemistry and simulating the PEM-West B aircraft mission in February-March 1994. The GEOS-CHEM model uses assimilated meteorological fields from the NASA Goddard Earth Observing System (GEOS). It reproduces relatively well the main features of tropospheric ozone, CO and reactive nitrogen species observed in PEM-West B, including the latitudinal and vertical gradients of the Asian pollution outflow over the western Pacific. We use CO as a long-lived tracer to diagnose the processes contributing to the outflow. The highest concentrations in the outflow are in the boundary layer (0-2 km), but the strongest outflow fluxes are in the lower free troposphere (2-5 km) and reflect episodic lifting of pollution over central and eastern China ahead of eastward-moving cold fronts. This frontal lifting, followed by westerly transport in the lower ...