An analysis on abnormally low ozone in the upper troposphere over subtropical East Asia in spring 2004

Abnormally low ozone (O3) mixing ratios were observed by electrochemical concentration cell (ECC) ozonesondes in the upper troposphere over subtropical East Asia in spring 2004, a season when high tropospheric O3 is usually observed in the region. Low O3 with a lowest mixing ratio of 13 ppbv, less than a fourth of the respective seasonal average of 60–100 ppbv, was observed at 11–18 km above ground over Hong Kong (22.31°N, 114.17°E), Sanya (18.23°N, 109.52°E) and Taipei (24.98°N, 121.43°E). The origin of the low O3 was investigated using meteorological evidence, satellite imagery and three-dimensional backward air trajectory. We found for the first time that the low O3 resulted from deep convective pumping of low O3 maritime air masses near the center of typhoon Sudal from the boundary layer of the tropical region to the east of the Philippines to the upper troposphere. The low O3 air masses were then transported to the higher latitudes far ahead of the typhoon following the long-range transport driven by the circulations associated with the typhoon and the northern Hadley cell. The findings of this study highlight that more research efforts are needed to understand the effect of the circulation associated with tropical cyclones on the distribution and budget of O3 and other trace gases in the troposphere.