Tropospheric ozone (O3) enhancements have been continuously observed over Hong Kong. We studied the O3 enhancement events and assessed their relation to the springtime O3 maximum in the lower troposphere over Hong Kong using a 6-year (1993 to 1999) ozonesonde data set. We identified the source regions of biomass burning emission, and established the chemical and transport characteristics of O3-rich air masses in the enhanced O3 profiles using satellite imagery, air trajectory and trace gas data measured on board the DC-8 aircraft during the PEM-West-B experiment. We identified a total of 39 O3 enhancement events, among which 35 events (90%) occurred from late February to May and 30 events (77%) had O3 enhancement within the 2.0--6.0 km altitude. The excess O3 in the O3-rich layers adds an additional 12% of O3 into the tropospheric O3 column and results in an overall springtime O3 maximum in the lower troposphere. Forward trajectory analysis suggests that the O3-rich air masses over Hong Kong can reach central Pacific and the western coast of North America within 10 days. Back air trajectories show that the O3-rich air masses in the enhanced profiles pass over the Southeast (SE) Asia subcontinent, where active biomass burning occurs in the O3 enhancement period. We identified the Indo-Burma region containing Burma, Laos and northern Thailand, and the Indian-Nepal region containing northern India and Nepal as the two most active regions of biomass burning emissions in the SE Asia subcontinent. Ozone and trace gas measurement on board the DC-8 aircraft revealed that O3-rich air masses are found over many parts of the tropical SE Asia and subtropical western Pacific regions and they have similar chemical characteristics. The accompanying trace gas measurements suggest that the O3-rich air masses are rich in biomass burning tracer, CH3Cl, but not the general urban emission tracers. We thus believe that the springtime O3 enhancement over Hong Kong is as a result of transport of photochemical O3 produced from biomass burning emissions from the upwind SE Asian continent. The large-scale enhancements of O3 in tropical SE Asia and the subtropical western Pacific rim that result from SE Asian biomass burning activities such as presented here thus are of atmospheric importance and deserve further research efforts.