Tropospheric ozone enhancements were measured over Hong Kong (22.2 ¿N, 114.3 ¿E) by electrochemical concentration cell ozonesondes during the 1997 period when many forest fires were burning in Indonesia. The enhancements have a maximum ozone concentration of up to 130 ppbv and an ozone-enhanced layer depth of 10 km. We used Total Ozone Mapping Spectrometer, advanced very high resolution radiometer satellite image, and back air rajectory to identify the source region and the transport pattern of ozone. The strong tropospheric ozone enhancements covered all of tropical Southeast Asia and subtropical south China. They were due to photochemical ozone buildup from the biomass burning emissions from the Indonesian fires. The ozone-rich air mass was transported to Hong Kong following the east Asia local Hadley circulation and an abnormal anticyclonic flow related to the El Ni¿o phenomenon in the tropical western Pacific. A rough estimation of the radiative forcing due to the ozone enhancement was carried out for two cases in October and December in 1997 using a normalized tropospheric ozone radiative forcing parameter derived from the Unified Chemistry-Climate model <Mickley et al., 1999>. The ozone enhancements induced an additional radiative forcing of 0.26 and 0.48 Wm-2 compared to the normal total forcing of 0.48 and 0.39 Wm-2 in October and December over the Hong Kong region. Estimation of the associated surface temperature change suggests that enhanced ozone from biomass burning on the scale of the 1997 Indonesian fires may have significant impact on regional surface temperature change. ¿ 2001 American Geophysical Union