Tropospheric ozone (O3) cross sections measured with lidar from a DC-8 aircraft over the western Pacific correspond closely with potential vorticity (PV). Both are transported from the middle latitude stratosphere, although this is not the only source of O3, and both have sinks in the tropical boundary layer. O3 and PV are good indicators of photochemical and transport process interactions. In summer, some Asian pollution, raised by convection to the upper troposphere, passes southward into the tropics and to the Southern Hemisphere. In winter, subsidence keeps the pollution at low altitudes where it moves over the ocean towards the ITCZ, with photochemical destruction and secondary pollutant generation occurring en route. Convection raises this modified air to the upper troposphere, where some may enter the stratosphere. Thus winter Asian pollution may have a smaller direct influence on the global atmosphere than it would if injected at other longitudes and seasons.