During the Pacific Exploration of Asian Continental Emission (PEACE) phase A mission in January 2002, we launched ozonesondes in subtropical southeast China at Hong Kong (114.17¿E, 22.31¿N), middle latitude northeast China at Beijing (116.47¿E, 39.81¿N), and northwest China at Xining (101.45¿E, 36.43¿N) in order to study long-range ozone (O3) transport from Eurasia, tropospheric O3 sources in China, and O3 outflow to the Pacific. Tropospheric O3 showed a complex vertical distribution with average tropospheric O3 columns of 39 ¿ 4, 23 ¿ 3, and 30 ¿ 6 DU in Hong Kong, Beijing, and Xining, respectively, which accounted for 17 ¿ 2%, 7 ¿ 1%, and 10 ¿ 1% of the total O3 column. The lower tropospheric and boundary layer (BL) O3 over Xining and especially Beijing exhibited low values, suggesting negligible O3 formation in middle latitudes of China during the winter season. The results also revealed frequent propagation of enhanced O3 layers from the lower stratosphere to the upper troposphere over Xining and especially Beijing, suggesting that stratospheric O3 is an important source of O3 in the upper troposphere of northern China. This natural O3 is transported downwind by the prevailing westerly wind and acts as a source of O3 to the east Asian coast and northwestern Pacific. We observed elevated O3, with a maximum mixing ratio up to 111 ppbv, at 1.5 km in the upper BL over Hong Kong. The elevated O3 was resulted from transport of pollutants from northwest-central or southwest China and regional O3 formation and accumulation in south China including the Pearl River Delta and Hong Kong. We also observed enhanced O3 (>95 ppbv) in the middle and upper troposphere over Hong Kong in air masses transported along the subtropical jet from tropical and subtropical East Africa, south Asia, and Southeast Asia. The O3 enhancements were most likely due to intrusion of stratospheric O3 into the troposphere in the Indo-Burmese region of tropical Southeast Asia, where substantial downward motion had been observed.