Vertical exchange and cross-regional transport of lower-tropospheric ozone over Hong Kong

In recent years, the escalating ozone concentrations in urban areas of China have raised significant public concern. Due to the lack of long-term vertical observation, the characteristics of vertical distribution and long-range transport of ozone are still not well understood. This study utilized 27 years (1994–2020) of ozone sounding observations in Hong Kong, in conjunction with the ERA5 reanalysis data and HYSPLIT backward model, to examine the vertical structure and three-dimensional cross-regional transport of tropospheric ozone over Hong Kong. During spring, ozone concentrations above 850 hPa are found to be higher than those observed during other seasons. Conversely, ozone concentrations below 850 hPa are comparatively elevated in autumn. The stratospheric-tropospheric exchange (STE) process is a crucial factor contributing to high ozone concentrations in the upper troposphere, with 46.2% of trajectories crossing the tropopause and becoming concentrated mainly in the eastern Yunnan–Guizhou Plateau. The study identified two obvious peaks in the lower troposphere above Hong Kong, with higher concentrations observed at approximately 700 hPa. The springtime ozone maximum is linked to the long-range cross-regional ozone transport from higher elevations in the Bay of Bengal and the Indo–China Peninsula, which moves eastward and downward under the influence of the southern branch trough. Additionally, transboundary transportation of fire emissions in Southeast Asia, influenced by monsoon circulation, also contributes to the high ozone concentrations during the observation. The autumn ozone maximum occurrs at approximately 925 hPa and is linked to boundary layer dynamics and the southward cross-regional transport of low-altitude ozone from the Pearl River Delta and Yangtze River Delta regions under the influence of the northeasterly Asian monsoon flow. The study advances our knowledge in understanding the vertical profile of ozone and emphasizes the importance of regionally-joint emission control particularly when meteorology-induced cross-regional transportation occurs.