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Pearl River Estuary (PRE) has been recognized as one of the key areas in South China experiencing severe air pollution. To understand the photochemical pollution over PRE, intensive measurements of O3 and its precursors were simultaneously conducted at a suburban site on the east bank of PRE (Tung Chung, TC) in Hong Kong and a rural site on the west bank (Qi'ao Island, QAI) in Zhuhai from 25 September to 29 November 2016. Three and sixteen O3 episode days (days with the maximum hourly O3 higher than 100 ppbv) were captured at TC and QAI, respectively. On the three days when O3 episodes occurred at TC, QAI also suffered from O3 episodes, which were all associated with tropical cyclones (typhoons). As a downwind site of PRE when northwesterly winds prevailed, TC displayed a higher maximum hourly O3 than QAI on these three days. In contrast, the other thirteen episodes that solely occurred at QAI were generally under northeasterly winds, which brought more coastal air to TC and led to lower levels of O3 at this site. In addition, the stronger titration of the locally emitted NO to O3 also accounted for the lower O3 at TC on these days. The simulations by Weather Research and Forecasting coupled with Chemistry model indicated that the interactions of synoptic winds and mesoscale breezes led to relatively weak winds and long residence time for air pollutants over the PRE, thereby air pollution accumulation. This provided the likelihood of intensive chemical productions of O3 over PRE, with the production rates even higher than those in the surrounding land areas. As such, it was concluded that PRE was a "pool" of O3, where atmospheric dynamics caused the accumulation of air pollutants and subsequently intensive photochemical reactions. The findings in this study may also apply to the other estuaries connecting terrestrial and marine systems over the world. Key Points: O3 pollution prevailed over Pearl River Estuary (PRE), an area connecting terrestrial and marine systems in South ChinaInteractions between local breezes and synoptic winds caused accumulation of air pollutants over PREPRE was a pool of O3, due to accumulation of air pollutants and intensive photochemical reactions there [ABSTRACT FROM AUTHOR]