During the Transport and Chemical Evolution Over the Pacific (TRACE-P) period in the spring of 2001 we launched an ozonesonde at three Chinese sites: Kunming (102.68¿E, 25.03¿N), Hong Kong (114.17¿E, 22.31¿N), and Linan (119.75¿E, 30.30¿N). The sites extend from subtropical southwestern China close to the Southeast (SE) Asian border, to the southeastern Asian coast, and to the edge of the middle latitudes of central eastern China, respectively. The aims of the study are to provide ozonesonde data within the source regions of the Chinese mainland, to investigate the source of tropospheric ozone (O3), and to investigate to what extent SE Asian biomass burning emissions impact both tropospheric O3 over the subtropical Chinese mainland and O3 outflows to the Pacific. The results show that there are substantial variations in vertical O3 distributions over these sites, with low O3 values in the upper troposphere of Hong Kong, high O3 values in the middle and upper troposphere of Linan, and frequent O3 enhancements in the lower troposphere of Hong Kong and Kunming. The low values in the upper troposphere over Hong Kong in the spring of 2001 were not usually observed from 1993 to 2000 and are the result of the transport of O3-depleted air from the intertropical convergence zone of equatorial SE Asian regions following the eastern Asia local Hadley circulation. Such transport processes do not affect the higher latitude at the edge of the middle latitude of Linan, where stratospheric O3 is the major contributing source to middle and upper tropospheric O3. The O3 enhancements over the lower troposphere of Kunming and Hong Kong are caused by SE Asian biomass burning emissions. Such enhancements are frequently observed over Hong Kong, less often over Kunming, and scarcely ever over Linan. Our analysis shows that biomass burning emissions from SE Asia in the spring of 2001 mainly affected the southern parts of the subtropical Chinese region.