Analysis of the seasonal behavior of tropospheric ozone at Hong Kong

Ozonesondes have been launched at Hong Kong (22.2°N, 114.3°E) since 1993. The results of data analysis of the ozone profiles are presented, with a focus on the seasonal cycle in tropospheric ozone. The results show that the tropospheric ozone column has an obvious maximum in spring and a minimum in summer. The former is a common feature at many locations in the Northern Hemisphere. The summer minimum is attributed to the onset of the summer monsoon when air flow from the Asian continent is replaced by air from the South China Sea or the tropical Pacific. The tropospheric ozone has an important influence on the seasonal cycle of total ozone at Hong Kong. The seasonal cycle of ozone mixing ratio below 2 km is bimodal with ozone peaks in spring and autumn. A frequently observed feature in late autumn and winter is a relative minimum of ozone mixing ratio (as low as 30-40 ppbv) in the upper troposphere (from about 9 to 16 km). Trajectory analysis shows this relative minimum of ozone is associated with air masses coming from the tropical region. It is proposed that the East Asia local Hadley circulation is responsible for this feature.