The impact of the East Asian monsoon (EAM) on climatology and interannual variability of tropospheric ozone (O3) over the coastal South China was investigated by analyzing 11 years of ozonesonde data over Hong Kong with the aid of Lagrangian dispersion modeling of carbon monoxide and calculation of an EAM index. It was found that the seasonal cycle of O3 in the lower troposphere is highly related to the EAM over the study region. Ozone enhancements in the free troposphere are associated with the monsoon-induced transport of pollutants of continental anthropogenic and biomass burning origins. Lower tropospheric O3 levels showed high interannual variability, with an annual averaged amplitude up to 61% of averaged concentrations in the boundary layer (0-1 km altitudes) and 49% below 3 km altitude. In spring and autumn, the interannual variability in boundary layer O3 levels was predominately influenced by the EAM intensity, with high O3 mixing ratios associated with northeasterly circulation anomalies. ; Department of Civil and Environmental Engineering