Substantial enhancement of ozone was frequently observed in the lower troposphere (2.5--6.0 km) over Hong Kong (22.2 ¿N, 114.3 ¿E). Most of these enhanced ozone layers (with peak concentration up to 138 ppbv) feature moderate and high relative humidity (50--100%) and a temperature inversion at the bottom of the layer. These phenomena predominantly occur in spring (late February to mid April) coinciding with the dry season in Southeast Asia. Back air trajectories showed that these ozone-rich air masses had passed through the Indo-Burma region of Southeast Asia, where large-scale fires were recorded at the time. A case study is presented and evidence from the fire count derived from AVHRR satellite images and carbon monoxide concentration derived from space shuttle during the MAPS experiment was used. We have illustrated with this case study that biomass-burning emissions are the source of the elevated ozone observed. ¿ 2000 American Geophysical Union