Vertical profile and origin of wintertime tropospheric ozone over China during the PEACE-A period

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 (114.17degreesE, 22.31degreesN), middle latitude northeast China at Beijing (116.47degreesE, 39.81degreesN), and northwest China at Xining (101.45degreesE, 36.43degreesN) in order to study long-range ozone (O-3) transport from Eurasia, tropospheric O-3 sources in China, and O-3 outflow to the Pacific. Tropospheric O-3 showed a complex vertical distribution with average tropospheric O-3 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 O-3 column. The lower tropospheric and boundary layer (BL) O-3 over Xining and especially Beijing exhibited low values, suggesting negligible O-3 formation in middle latitudes of China during the winter season. The results also revealed frequent propagation of enhanced O-3 layers from the lower stratosphere to the upper troposphere over Xining and especially Beijing, suggesting that stratospheric O-3 is an important source of O-3 in the upper troposphere of northern China. This "natural" O-3 is transported downwind by the prevailing westerly wind and acts as a source of O-3 to the east Asian coast and northwestern Pacific. We observed elevated O-3, with a maximum mixing ratio up to 111 ppbv, at 1.5 km in the upper BL over Hong Kong. The elevated O-3 was resulted from transport of pollutants from northwest-central or southwest China and regional O-3 formation and accumulation in south China including the Pearl River Delta and Hong Kong. We also observed enhanced O-3 (>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 O-3 enhancements were most likely due to intrusion of stratospheric O-3 into the troposphere in the Indo-Burmese region of tropical Southeast Asia, where substantial downward motion had been observed.